Table of Contents

Chapter 1 Road engineering and road traffic safety........................................................................2

1.1 Introduction............................................................................................................................2
1.2 Road Functional Classification and Numbering....................................................................3
1.3 Geometric design...................................................................................................................4
1.4 Factors affecting geometric design........................................................................................5
1.4.1 Design speed...................................................................................................................5
1.4.2 Topography and Terrain.................................................................................................6
1.4.3 Vehicle factors................................................................................................................7
1.4.4 Human factors.................................................................................................................9
1.4.5 Other factors.................................................................................................................11
Chapter 2 Road cross section Elements.........................................................................................13
2.1 Introduction..........................................................................................................................13
2.2 Pavement surface characteristics.........................................................................................14
2.2.1 Friction..........................................................................................................................14
2.2.2 Unevenness...................................................................................................................15
2.2.3 Light reflection.............................................................................................................15
2.2.4 Drainage........................................................................................................................15
2.3 Lane width...........................................................................................................................15
2.4 Shoulders.............................................................................................................................16
2.5 Normal cross fall..................................................................................................................16
2.6 Roadside ditches..................................................................................................................17
2.7 Side Slopes and Back Slopes...............................................................................................18
2.8 Clear Zone...........................................................................................................................18
2.9 Right of way Right of way...................................................................................................19
Chapter 3 Sight distance................................................................................................................21
3.1 Introduction..........................................................................................................................21
3.2 Sight distance classification.................................................................................................22
3.2.1 Stopping Sight Distance...............................................................................................22
3.2.2 Overtaking sight distance.............................................................................................24
 3.3 Decision Sight Distance Decision sight distance (DSD).....................................................25
Chapter 4 Horizontal alignment.....................................................................................................30
4.1 Tangent sections..................................................................................................................30
4.2 Curves..................................................................................................................................30
4.2.1 Reverse Curves, Broken-Back Curves, and Compound Curves...................................31
4.2.2 Isolated Curves.............................................................................................................32
4.3 Transition Curves.................................................................................................................32
4.4 Superelevation.....................................................................................................................33
4.5 Widening on Curves............................................................................................................34
Chapter 5 Vertical alignment.........................................................................................................35
5.1 Introduction..........................................................................................................................35
5.2 Vertical curves.....................................................................................................................35
5.2.1 Crest curve....................................................................................................................37
5.2.2 Sag curve......................................................................................................................44
5.3 Gradient...............................................................................................................................45
5.4 Climbing lanes.....................................................................................................................46
5.5 Vertical Clearances..............................................................................................................49
5.6 Phasing of horizontal and vertical alignment......................................................................49
5.6.1 Types of Mis-Phasing and Corresponding Corrective Action......................................50
5.6.2 The Economic Penalty Due to Phasing........................................................................54
Chapter 6 Intersections..................................................................................................................55
6.1 Introduction..........................................................................................................................55
6.2 Design Requirements...........................................................................................................55
6.3 Control of an intersection....................................................................................................57
6.3.1 Passive control..............................................................................................................57
6.3.2 Semi control..................................................................................................................58
6.3.3 Active control...............................................................................................................58
6.4 Types of intersection............................................................................................................58
6.4.1 At grade intersection control:.......................................................................................59
6.4.2 Grade separated intersection control:...........................................................................59
6.5 Channelized intersection......................................................................................................59
6.6 Priority Intersections............................................................................................................60
6.6.1 Crossroads.....................................................................................................................60
6.6.2 T-Junctions...................................................................................................................61
6.7 Roundabouts........................................................................................................................62
6.7.1 Types of roundabout.....................................................................................................67
6.8 GRADE – SEPARATED JUNCTIONS..............................................................................68
6.8.1 Types of Grade separated junctions..............................................................................69
Interchange................................................................................................................................71
6.8.2 Geometric Standards.....................................................................................................72
6.8.3 Design Principles..........................................................................................................73
6.9 Intersection Sight Distance..................................................................................................74
Chapter 1 Road engineering and road traffic
safety
1.1 Introduction
Growth in urbanization and in the numbers of vehicles has led to increased traffic congestion in
urban centers and increases in traffic accidents on road networks which were never designed for
the volumes and types of traffic which they are now required to carry. In addition, unplanned
urban growth has led to incompatible land-uses, with high levels of pedestrian/vehicle conflicts.
The drift from rural areas to urban centers often results in large numbers of new urban residents
unused to such high traffic levels. As a result, there has often been a severe deterioration in
driving conditions and a significant increase in the hazards to and competition between different
classes of road users of the road system. In addition, the inherent dangers have often been made
worse by poor road maintenance, badly designed intersections and inadequate provision for
pedestrians. All of these have contributed to the serious road safety problems.

Highway design standards in many developing countries tend to ignore pedestrians, other non-
motorized traffic and motor cycles. Unfortunately, such standards may often be too high, costly
or require excessive maintenance for the countries to afford. In such circumstances, the emphasis
tends to focus upon the constructional rather than the operational aspects.

Engineers will typically concentrate on construction details of drainage, for example, rather Than
on how the type of drainage channel chosen may affect road safety. Important operational
elements such as road signs or pedestrian facilities are too often left for later addition "if and
when time or money permits", while the builders move on to the next construction project. It is
rare for the additional time and money ever to be found. As a result, road designs which would
be safe in the operational environment of industrialized countries often become unsafe under the
operational conditions which exist in developing countries.

Little effort is made to modify designs or to add additional features to compensate for the
operational deficiencies likely to occur in the developing world. Few efforts are made to quantity
potential problems which are specific to developing countries. The shortages of trained
professionals and the limited resources devoted to maintenance organizations often means that
overgrown footpaths and damaged traffic control facilities such as road signs and traffic signals
are often left unrepaired.

The result is that roads are often badly in need of maintenance, traffic signing is often
inadequate, facilities for pedestrians are poor and guidance to drivers via channelization or other
control measures is rarely available. These general deficiencies in the operational and control
aspects of the road systems are made worse by the fact that drivers are rarely adequately trained
and tested, traffic law enforcement is ineffective and drivers' behavior in respect of compliance
with regulations is frequently very poor. The net result of these inadequacies is the very high
incidence of road accident casualties and fatalities

Although improvements were achieved through the application of road accident countermeasures
in various sectors, one of the most consistently successful and cost-effective areas of investment
has been the field of road planning and traffic engineering. Gradual elimination of the most
hazardous locations on road networks and the adoption of safety-conscious approaches to the
design and planning of new road networks have contributed greatly towards improving traffic
safety.

By incorporating good design principles from the start it is possible to avoid many problems
simply by planning and designing new roads in a safety-conscious manner. Even where this has
not been done, it may still be possible (although more expensive) to improve existing roads by
subsequent introduction of safety or environment-related measures, e.g. selective road closures
or road humps to reduce speeds, or by prohibitions on heavy goods vehicles in residential areas.
Finally, it is possible to identity hazardous sections of the road network so that appropriate
remedial measures can be undertaken to reduce the likelihood and severity of accidents at those
locations.

There are many characteristics of road design which influence drivers and one of the primary
functions of traffic engineering is to impart information to the driver in a suitable format and in
sufficient time for him to take the necessary action for safety. Road geometry and the many
associated design variables, (such as width or alignment) all influence how and what road users.

1.2 Road Functional Classification and Numbering

The functional classification in Ethiopia includes five functional classes. The following are the
functional classes with their description.

I. Trunk Roads (Class I) Centers of international importance and roads terminating at

international boundaries are linked with Addis Ababa by trunk roads. They are
numbered with an "A" prefix: an example is the Addis-Gondar Road (A3). Trunk
roads have a present AADT ≥1000, although they can have volumes as low as 100
AADT.
II. Link Roads (Class II) Centers of national or international importance, such as
principal towns and urban centers, must be linked between each other by link
roads .A typical link road has over 400 - 1000 first year AADT, although values can
range between 50-10,000 AADT. They are numbered with a "B" prefix. An example
of a typical link road is the WoldiyaDebre Tabor- Woreta Road (B22), which links,
for instance, Woldiya on Road A2 with Bahir Dar of Road A3.
 III. Main Access Roads (Class III) Centers of provincial importance must be linked
between each other by main access roads). First year AADTs are between 30-1,000.
They are numbered with a "C" prefix
IV. Collector Roads (Class IV) Roads linking locally important centers to each other, to a
more important center, or to higher class roads must be linked by a collector road.
First year AADTs are between 25-400. They are numbered with a "D" prefix .
V. Feeder Roads (Class V) Any road link to a minor center such as market and local
locations is served by a feeder road. First year AADTs are between 0-100. They are
numbered with an "E" prefix and.

Roads of the highest classes, trunk and link roads have, as their major function to provide
mobility, while the primary function of lower class roads is to provide access. The roads of
intermediate classes have, for all practical purposes, to provide both mobility and access.

1.3 Geometric design

The geometric design of highways deals with the dimensions and layout of visible features of the
highway. The emphasis of the geometric design is to address the requirement of the driver and
the vehicle such as safety, comfort, efficiency, etc. The features normally considered are the
cross section elements, sight distance consideration, horizontal curvature, gradients, and
intersection. The design of these features is to a great extend influenced by driver behavior and
psychology, vehicle characteristics, traffic characteristics such as speed and volume. Proper
geometric design will help in the reduction of accidents and their severity. Therefore, the
objective of geometric design is to provide optimum efficiency in traffic operation and maximum
safety at reasonable cost. The planning cannot be done stage wise like that of a pavement, but
has to be done well in advance. The main components that will be discussed are:

1. Factors affecting the geometric design,

2. Highway alignment, road classification,

3. Pavement surface characteristics,

4. Cross-section elements including cross slope, various widths of roads and features in the road
margins.

5. Sight distance elements including cross slope, various widths and features in the road
margins.

6. Horizontal alignment which includes features like super elevation, transition curve, extra
widening and set back distance.

7. Vertical alignment and its components like gradient, sight distance and design of length of
curves.
8. Intersection features like layout, capacity

1.4 Factors affecting geometric design

A number of factors affect the geometric design and they are discussed in detail in the following
sections.

1.4.1 Design speed

The geometric design of highways deals with the dimensions and layout of visible features of the
highway. The emphasis of the geometric design is to address the requirement of the driver and
Design speed is the single most important factor that affects the geometric design. It directly
affects the sight distance, horizontal curve, and the length of vertical curves. Since the speed of
vehicles varies with driver, terrain etc. design speed is adopted for all the geometric design.

Design speed is defined as the highest continuous speed at which individual vehicles can travel
with safety on the highway when weather conditions are conducive. Design speed is different
from the legal speed limit which is the speed limit imposed to curb a common tendency of
drivers to travel beyond an accepted safe speed. Design speed is also different from the desired
speed which is the maximum speed at which a driver would travel when unconstrained by either
traffic or local geometry.

Since there are wide variations in the speed adopted by different drivers, and by different types
of vehicles, design speed should be selected such that it satisfies nearly all drivers. At the same
time, a higher design speed has cascading effect in other geometric design and thereby cost
escalation

The Design Speed is used as an index which links road function, traffic flow and terrain to the
design parameters of sight distance and curvature to ensure that a driver is presented with a
reasonably consistent speed environment. In practice, most roads will only be constrained to
minimum parameter values over short sections or on specific geometric elements.

Design elements such as lane and shoulder widths, horizontal radius, superelevation, sight
distance and gradient are directly related to, and vary, with design speed. Thus all of the
geometric design parameters of a road are directly related to the selected design speed. The
design speeds have been determined in accordance with the following guidelines:

(i) Drivers on long-distance journeys are apt to travel at higher speeds than local traffic.
(ii) On local roads whose major function is to provide access, high speeds are undesirable.
(iii) Drivers usually adjust their speeds to physical limitations and prevailing traffic
conditions. Where a difficult location is obvious to the driver, he is more apt to accept a
lower speed of operation.
(iv) Economic considerations (road user savings vs. construction costs) may justify a higher
design speed for a road carrying large volumes of traffic than for a less heavily trafficked
road in similar topography.
(v) Change in design speed, if required due to a change in terrain class, should not be
affected abruptly, but over sufficient distances to enable drivers to change speed
gradually. The change in design speed should not be greater than one design speed step,
and the section with the lower geometric standards should be long enough to be clearly
recognizable by drivers (not, for example, just one single curve).
(vi) It is often the case that the physical terrain changes two steps, i.e. - from mountainous to
flat terrain. Where possible in such circumstances, a transition section of road shall be
provided with limiting parameters equivalent to the rolling terrain type. Where this is not
possible, i.e. - a Departure from Standards, special attention shall be given to the
application of warning signs and/or rumble strips to alert the driver to the changing
conditions.

1.4.2 Topography and Terrain

The next important factor that affects the geometric design is the topography. It is easier to
construct roads with required standards for a plain terrain. However, for a given design speed,
the construction cost increases multiform with the gradient and the terrain. Therefore, geometric
design standards are different for different terrain to keep the cost and time of construction under
control. This is characterized by sharper curves and steeper gradients.

The geometric design elements of a road depend on the transverse terrain through which the road
passes. Transverse terrain properties are categorized into four classes as follows:

Flat or gently rolling country: which offers few obstacles to the construction of a road, having
continuously unrestricted horizontal and vertical alignment (transverse terrain slope up to 5
percent).

ROLLING: Rolling, hilly or foothill country where the slopes generally rise and fall moderately
and where occasional steep slopes are encountered, resulting in some restrictions in alignment
(transverse terrain slope from 5 percent to 25 percent).

MOUNTAINOUS: Rugged, hilly and mountainous country and river gorges. This class of terrain
imposes definite restrictions on the standard of alignment obtainable and often involves long
steep grades and limited sight distance (transverse terrain slope from 25 percent to 50 percent).

ESCARPMENT: In addition to the terrain classes given above, a fourth class is added to cater to
those situations whereby the standards associated with each of the above terrain types cannot be
met. We refer to escarpment situations inclusive of switchback roadway sections, or side hill
transverse sections where earthwork quantities are considerable, with transverse terrain slope in
excess of 50 percent).
In general, construction costs will be greater as the terrain becomes more difficult and higher
standards will become less justifiable or achievable in such situations than for roads in either flat
or rolling terrain. Drivers accept lower standards in such conditions and therefore adjust their
driving accordingly, so minimizing accident risk. Design speed will therefore vary with
transverse terrain.

1.4.3 Vehicle factors

Vehicles Highway systems accommodate a wide variety of sizes and types of vehicles, from
smallest compact passenger cars to the largest double and triple tractor-trailer combinations.
According to the different geometric features of highways like the lane width, lane widening on
curves, minimum curb and corner radius, clearance heights etc. some standard physical
dimensions for the vehicles has been recommended. Road authorities are forced to impose limits
on vehicular characteristics mainly:

• To provide practical limits for road designers to work to,

• To see that the road space and geometry is available to normal vehicles,

• To implement traffic control effectively and efficiently,

• Take care of other road users also.

Taking the above points into consideration, in general, the vehicles can be grouped into
motorized two wheeler’s, motorized three wheeler’s, passenger car, bus, single axle trucks, multi
axle trucks, truck trailer combinations, and slow non-motorized vehicles.

Vehicle dimensions

The vehicular dimensions which can affect the road and traffic design are mainly: width, height,
length, rear overhang, and ground clearance. The width of vehicle affects the width of lanes,
shoulders and parking facility. The capacity of the road will also decrease if the width exceeds
the design values. The height of the vehicle affects the clearance height of structures like over-
bridges, under-bridges and electric and other service lines and also placing of signs and signals.
Another important factor is the length of the vehicle which affects the extra width of pavement,
minimum turning radius, safe overtaking distance, capacity and the parking facility. The rear
overhang control is mainly important when the vehicle takes a right/left turn from a stationary
point. The round clearance of vehicle comes into picture while designing ramps and property
access and as bottoming out on a crest can stop a vehicle from moving under its own pulling
power.
Wight axle configuration

The weight of the vehicle is a major consideration during the design of pavements both flexible
and rigid. The weight of the vehicle is transferred to the pavement through the axles and so the
design parameters are fixed on the basis of the number of axles. The power to weight ratio is a
measure of the ease with which a vehicle can move. It determines the operating efficiency of
vehicles on the road. The ratio is more important for heavy vehicles. The power to weight ratio is
the major criteria which determines the length to which a positive gradient can be permitted
taking into consideration the case of heavy vehicles.

Turning radius and turning path

The minimum turning radius is dependent on the design and class of the vehicle. The effective
width of the vehicle is increased on a turning, an intersection, roundabout, terminals, and parking
areas.

Visibility

The visibility of the driver is influenced by the vehicular dimensions. As far as forward visibility
is concerned, the dimension of the vehicle and the slope and curvature of wind screens,
windscreen wipers, door pillars, etc should be such that:

• Visibility is clear even in bad weather conditions like fog, ice, and rain;

• It should not mask the pedestrians, cyclists or other vehicles;

• During intersection maneuvers.

Equally important is the side and rear visibility when maneuvering especially at intersections
when the driver adjusts his speed in order to merge or cross a traffic stream. Rear vision
efficiency can be achieved by properly positioning the internal or external mirrors.

Acceleration characteristics

The acceleration capacity of vehicle is dependent on its mass, the resistance to motion and
available power. In general, the acceleration rates are highest at low speeds, decreases as speed
increases. Heavier vehicles have lower rates of acceleration than passenger cars. The difference
in acceleration rates becomes significant in mixed traffic streams. For example, heavy vehicles
like trucks will delay all passengers at an intersection. Again, the gaps formed can be occupied
by other smaller vehicles only if they are given the opportunity to pass. The presence of upgrades
makes the problem more severe. Trucks are forced to decelerate on grades because their power is
not sufficient to maintain their desired speed. As trucks slow down on grades, long gaps will be
formed in the traffic stream which cannot be efficiently filled by normal passing maneuvers.
Braking performance

As far as highway safety is concerned, the braking performance and deceleration characteristics
of vehicles are of prime importance. The time and distance taken to stop the vehicle is very
important as far as the design of various traffic facilities is concerned. The factors on which the
braking distance depends are the type of the road and its condition, the type and condition of tire
and type of the braking system.

The main characteristics of a traffic system influenced by braking and deceleration performance
are:

• Safe stopping sight distance: The minimum stopping sight distance includes both the reaction
time and the distance covered in stopping. Thus, the driver should see the obstruction in time to
react to the situation and stop the vehicle.

• Clearance and change interval: The Clearance and change intervals are again related to safe
stopping distance. All vehicles at a distance further away than one stopping sight distance from
the signal when the Yellow is flashed is assumed to be able to stop safely. Such a vehicle which
is at a distance equal or greater than the stopping sight distance will have to travel a distance
equal to the stopping sight distance plus the width of the street, plus the length of the vehicle.
Thus the yellow and all red times should be calculated to accommodate the safe clearance of
those vehicles.

• Sign placement: The placement of signs again depends upon the stopping sight distance and
reaction time of drivers. The driver should see the sign board from a distance at least equal to or
greater than the stopping sight distance.

It is clear that the braking and reaction distance computations are very important as far as a
transportation system is concerned. Stopping sight distance is a product of the characteristics of
the driver, the vehicle and the roadway. and so this can vary with drivers and vehicles. Here the
concept of design vehicles gains importance as they assist in general design of traffic facilities
thereby enhancing the safety and performance of roadways.

1.4.4 Human factors

Road users can be defined as drivers, passengers, pedestrians etc. who use the streets and
highways. Together, they form the most complex element of the traffic system - the human
element - which differentiates Transportation Engineering from all other engineering fields. It is
said to be the most complex factor as the human performances varies from individual to
individual.

Thus, the transportation engineer should deal with a variety of road user characteristics. For
example, a traffic signal timed to permit an average pedestrian to cross the street safely may
cause a severe hazard to an elderly person. Thus, the design considerations should safely and
efficiently accommodate the elderly persons, the children, the handicapped, the slow and speedy,
and the good and bad drivers.

Variability

The most complex problem while dealing human characteristics is its variability. The human
characteristic like ability to react to a situation, vision and hearing, and other physical and
psychological factors vary from person to person and depends on age, fatigue, nature of stimuli,
presence of drugs/alcohol etc. The influence of all these factors and the corresponding variability
cannot be accounted when a facility is designed. So a standardized value is often used as the
design value.

Critical characteristics

The road user characteristics can be of two main types, some of them are quantifiable like
reaction time, visual acuity etc. while some others are less quantifiable like the psychological
factors, physical strength, fatigue, and dexterity.

Reaction time

The road user is subjected to a series of stimuli both expected and unexpected. The time taken to
perform an action according to the stimulus involves a series of stages like:

• Perception: Perception is the process of perceiving the sensations received through the sense
organs, nerves and brains. It is actually the recognitions that a stimulus on which a reaction is to
happen exists.

• Intellection: Intellection involves the identification and understanding of stimuli.

• Emotion: This stage involves the judgment of the appropriate response to be made on the
stimuli like to stop, pass, move laterally etc.

• Volition: Volition is the execution of the decision which is the result of physical actions of the
driver.

For e.g., if a driver approaches an intersection where the signal is red, the driver first sees the
signal (perception), he recognizes that is is a red/STOP signal, he decides to stop and finally
applies the brake(volition). This sequence is called the PIEV time or perception-reaction time.
But apart from the above time, the vehicle itself traveling at initial speed would require some
more time to stop. That is, the vehicle traveling with initial speed u will travel for a distance, d =
vt where, t is the above said PIEV time. Again, the vehicle would travel some distance after the
brake is applied.
Visual acuity and driving

The perception-reaction time depends greatly on the effectiveness of driver’s vision in

perceiving the objects and traffic control measures. The PIEV time will be decreased if the vision
is clear and accurate. Visual acuity relates to the field of clearest vision. The most acute vision is
within a cone of 3 to 5 degrees, fairly clear vision within 10 to 12 degrees and the peripheral
vision will be within 120 to 180 degrees. This is important when traffic signs and signals are
placed, but other factors like dynamic visual acuity, depth perception etc. should also be
considered for accurate design. Glare vision and color vision are also equally important. Glare
vision is greatly affected by age. Glare recovery time is the time required to recover from the
effect of glare after the light source is passed, and will be higher for elderly persons. Color vision
is important as it can come into picture in case of sign and signal recognition.

Walking

Transportation planning and design will not be complete if the discussion is limited to drivers
and vehicular passengers. The most prevalent of the road users are the pedestrians. Pedestrian
traffic along footpaths, sidewalks, crosswalks, safety zones, islands, and over and under passes
should be considered. On an average, the pedestrian walking speed can be taken between 1.5
m/sec to 2 m/sec. But the influence of physical, mental, and emotional factors need to be
considered. Parking spaces and facilities like signals, bus stops, and over and under passes are to
be located and designed according to the maximum distance to which a user will be willing to
walk.

Hearing

Hearing is required for detecting sounds, but lack of hearing acuity can be compensated by usage
of hearing aids. Lot of experiments was carried out to test the drive vigilance which is the ability
of a drive to discern environmental signs over a prolonged period. The results showed that the
drivers who did not undergo any type of fatiguing conditions performed significantly better than
those who were subjected to fatiguing conditions. But the mental fatigue is more dangerous than
skill fatigue. The variability of attitude of drivers with respect to age, sex, knowledge and skill in
driving etc. are also important.

1.4.5 Other factors

There are various other factors that affect the geometric design and they are briefly discussed
below

• Traffic: It will be uneconomical to design the road for peak traffic flow. Therefore a reasonable
value of traffic volume is selected as the design hourly volume which is determined from the
various traffic data collected. The geometric design is thus based on this design volume, capacity
etc.
 • Environmental: Factors like air pollution, noise pollution etc. should be given due
consideration in the geometric design of roads.

• Economy: The design adopted should be economical as far as possible. It should match with
the funds allotted for capital cost and maintenance cost.

• Others: Geometric design should be such that the aesthetics of the region is not affected.
Chapter 2 Road cross section Elements
2.1 Introduction
A cross-section will normally consist of the carriageway, shoulders or curbs, drainage features,
and earthwork profiles.

 Carriageway- the part of the road constructed for use by moving traffic, including traffic
lanes, auxiliary lanes such as acceleration and deceleration lanes, climbing lanes, and
passing lanes, and bus bays and lay-byes. ƒ
 Roadway- consists of the carriageway and the shoulders, parking lanes and viewing areas
 Earthwork profiles- includes side slopes and back slopes

For urban cross-sections, cross-section elements may also include facilities for pedestrians,
cyclists, or other specialist user groups. These include curbs, footpaths, and islands. It may also
provide for parking lanes. For dual carriageways, the cross-section will also include medians.

Lane and shoulder widths should be adjusted to traffic requirements and characteristics of the
terrain. The cross-section may vary over a particular route because these controlling factors vary.
The basic requirements are, however, that changes in cross-section standards shall be uniform
within each sub-section of the route and that any changes of the cross section shall be effected
gradually and logically over a transition length. Abrupt or isolated changes in cross-section
standards lead to increased hazards and reduced traffic capacity and complicate construction
operations.

In certain cases, however, it may be necessary to accept isolated reductions in cross-section

standards, for example when an existing narrow structure has to be retained because it is not
economically feasible to replace it. In such cases a proper application of traffic signs and road
markings is required to warn motorists of the discontinuity in the road. However, all such narrow
structures must be widened or replaced however when the width across the structure is less than
the adjacent carriageway width

Few rural cross-sections cater for what may be substantial pedestrian, cyclist and animal drawn
traffic, and these user groups tend to have to share the carriageway with fast moving motorized
traffic.

Adding extra width to cross-sections can be very expensive, and many safety benefits can be
obtained simply by careful design of the cross-sectional profile. The need to maintain the
designed cross-sectional profile is particularly important.

The development of cross-sections to cater safely for all road users is a most important aspect of
road design and this can be done as follows: Open channel drains should be covered where
possible or have some physical barrier to separate them from the carriageway. The physical
barrier could consist of a raised curb or similar structure. In some situations the drainage ditch or
curb will effectively segregate vehicular and pedestrian movements. The incorporation of a
shoulder will give room for parked or stopped vehicles and for maneuvering in an emergency.

2.2 Pavement surface characteristics

For a safe and comfortable driving four aspects of the pavement surface are important; the
friction between the wheels and the pavement surface, smoothness of the road surface, the light
reflection characteristics of the top of pavement surface, and drainage to water.

2.2.1 Friction
Friction between the wheel and the pavement surface is a crucial factor in the design of
horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and
deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of
vehicles.

Skidding happens when the path traveled along the road surface is more than the circumferential
movement of the wheels due to friction. Slip occurs when the wheel revolves more than the
corresponding longitudinal movement along the road.

The frictional force that develops between the wheel and the pavement is the load acting
multiplied by a factor called the coefficient of friction and denoted as f. The choice of the value
of f is a very complicated issue since it depends on many variables .Various factors that affect
friction are:

• Type of the pavement (like bituminous, concrete, or gravel),

• Condition of the pavement (dry or wet, hot or cold, etc.),

• Condition of the tire (new or old), and

• Speed and load of the vehicle.

2.2.2 Unevenness
It is always desirable to have an even surface, but it is seldom possible to have such one. Even if
a road is constructed with high quality pavers, it is possible to develop unevenness due to
pavement failures. Unevenness affects the vehicle operating cost, speed, riding comfort, safety,
fuel consumption and wear and tear of tires.

Unevenness index is a measure of unevenness which is the cumulative measure of vertical

undulation of the pavement surface recorded per unit horizontal length of the road. An
unevenness index value less than 1500 mm/km is considered as good, a value less than 2500
mm.km is satisfactory up to speed of 100 km/h and values greater than 3200 mm/km is
considered as uncomfortable even for 55 km/h.

2.2.3 Light reflection

• White roads have good visibility at night, but caused glare during day time.

• Black roads has no glare during day, but has poor visibility at night

• Concrete roads have better visibility and fewer glares.

It is necessary that the road surface should be visible at night and reflection of light is the factor
that answers it.

2.2.4 Drainage
The pavement surface should be absolutely impermeable to prevent seepage of water into the
pavement layers. Further, both the geometry and texture of pavement surface should help in
draining out the water from the surface in less time.

2.3 Lane width

Road width should be minimized so as to reduce the costs of construction and maintenance,
whilst being sufficient to carry the traffic loading efficiently and safely.

The following factors need to be taken into account when selecting the width of a road:

1. Classification of the road. A road is normally classified according to its function in the
road network. The higher the class of road, the higher the level of service expected and
the wider the road will need to be.
2. Traffic. Heavy traffic volumes on a road mean that passing of oncoming vehicles and
overtaking of slower vehicles are more frequent and therefore the paths of vehicles will
be further from the center-line of the road and the traffic lanes should be wider.
3. Vehicle dimensions. Normal steering deviations and tracking errors, particularly of heavy
vehicles, reduce clearances between passing vehicles. Higher truck percentages require
wider traffic lanes.
4. Vehicle speed. As speeds increase, drivers have less control of the lateral position of
vehicles, reducing clearances, and so wider traffic lanes are needed.

A feature of a highway having great influence on safety and comfort is the width of the
carriageway. Lane widths of 3.65m are used for Design Classes DS1 and DS2. The extra cost of
3.65 m above that for 3.0 m is offset to some extent by a reduction in cost of shoulder
maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the
pavement edges. The wider 3.65m lane also provides desired clearances between large
commercial vehicles on two-way rural highways. Narrower lanes are appropriate on lower
volume roads.

On rural roads, narrow lanes are likely to experience higher rates of run-off-road and head-on
collisions. Wider roads increase the time needed to walk across, and increase storm water runoff.

2.4 Shoulders
A shoulder is the portion of the roadway contiguous to the carriageway for the accommodation
of stopped vehicles; traditional and intermediate non-motorized traffic, animals, and pedestrians;
emergency use; the recovery of errant vehicles; and lateral support of the pavement courses.
They vary from no shoulder on minor rural roads where there is no surfacing, to a 1.5-3.0m or
even greater sealed shoulder on major roads depending on the terrain and design classification.
Wider configurations cater to the need for a parking lane in urban areas where paved
carriageways exist.

The sealed shoulder width may increase to 3.5 meters in urban areas where a provision for a
parking lane is required. In urban areas, the shoulders should be paved rather than sealed. The
actual shoulder width provided shall be determined from an assessment of the total traffic flow
and level of non-motorized traffic for each road section.

In cases where terrain is severe, the existing roadway width is narrow, and where the shoulder
width could only be maintained through an excessive volume of earthwork – e.g. at escarpment
conditions, standards can be reduced.

2.5 Normal cross fall

Cross slope / cross fall describes the slope of a roadway perpendicular to the centerline. If a road
were completely level, water would drain off it very slowly. This would create problems
with hydroplaning, and ice accumulation in cold weather.

Normal cross fall (or camber, crown) should be sufficient to provide adequate surface drainage
whilst not being so great as to make steering difficult. The ability of a surface to shed water
varies with its smoothness and integrity. On unpaved roads, the minimum acceptable value of
cross fall should be related to the need to carry surface water away from the pavement structure
effectively, with a maximum value above which erosion of material starts to become a problem.
Figure 3:1 Different types of camber

2.6 Roadside ditches

Drainage ditches are an essential part of any road which is not on an embankment and must be
incorporated into most highways. These are designed to accommodate the expected rainfall but
can often be hazardous to vehicles that run off the road. Adequate attention must therefore be
given to the safety considerations of drainage facilities when designing and upgrading highways.
Drainage ditches collect and disperse the water from the roadway pavement and the run off from
the uphill side of the carriageway.

Inadequate maintenance and clearing of debris from drainage channels, especially on the uphill
side of the carriageway where large volumes of solid material are often washed down into the
ditch, can result in water and debris overflowing onto the carriageway. This results in potential
danger of collision or aquaplaning of traffic on the road.

Drainage ditches must, first and foremost, be designed to accommodate the expected rainfall and
run-off and are required to prevent structural damage to the road. In areas where flash or floods
occur these channels may need to be very deep. Unfortunately deep, steep-sided drainage
channels can result in increased danger to vehicles which accidentally run off the road.

Where expected volumes of run-off permit L- type and J-type drainage channels should be used
in preference to open U or V-type to minimize danger to vehicles which run off the road and to
provide, during dry periods, a safe area for pedestrians to walk. The depth and width of the
channel gap on a typical U-type ditch offers no opportunity for the motorist to recover if he
should temporarily lose.
Slopes on the side of the ditch nearest the road preferably shallower if feasible as this will
minimize damage and injury control and no possibility for pedestrians to walk along it.

2.7 Side Slopes and Back Slopes

Side slopes should be designed to insure the stability of the roadway and to provide a reasonable
opportunity for recovery of an out-of-control vehicle. Three regions of the roadside are important
when evaluating the safety aspects: the top of the slope (hinge point), the side slope, and the toe
of the slope (intersection of the fore slope with level ground or with a back slope, forming a
ditch).rounding at the hinge point can significantly reduce the hazard potential. Similarly,
rounding at the toe of the slope is also beneficial.

Steep slopes on fills and inner slopes of side drains create a serious accident hazard. If one wheel
of a vehicle goes over the shoulder break point, the driver hazard loses control and may overturn
the vehicle. With flat slopes the car can often be directed back on to the road or continue down
the slope without overturning.

Figure 3:2 : Designation of Roadside Regions

2.8 Clear Zone

Once a vehicle has left the roadway, an accident may occur. The end result of an encroachment
depends upon the physical characteristics of the roadside environment. Flat, traversable, stable
slopes will minimize overturning accidents, which are usually severe. Elimination of roadside
furniture or its relocation to less vulnerable areas is options in the development of safer
roadsides. If a fixed object or other roadside hazard cannot be eliminated, relocated, modified, or
shielded, for whatever reason, consideration should be given to delineating the feature so it is
readily visible to a motorist.

For adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as
wide as practical on a specific highway section. The cleared width should be a minimum of 15
meters each side from the edge of the roadway for the higher road standards.
For lower standard roads, the clear zone can be reduced as practical. It should extend beyond the
toe of the slope. Lateral clearances between roadside objects and obstructions and the edge of the
carriageway should normally be not less than 1.5 meters. At existing pipe culverts, box culverts
and bridges, the clearance cannot be less than the carriageway width; if this clearance is not met,
the structure must be widened. New pipe and box culvert installations, and extensions to same,
must be designed with a 1.5-meter clearance from the edge of the shoulder.

Horizontal clearance to road signs, marker posts, etc. shall be a minimum of 1.0m from the edge
of the carriageway.

2.9 Right of way Right of way

Right of way or land width is the width of land acquired for the road, along its alignment. It
should be adequate to accommodate all the cross-sectional elements of the highway and may
reasonably provide for future development. To prevent ribbon development along highways,
control lines and building lines may be provided. Control line is a line which represents the
nearest limits of future uncontrolled building activity in relation to a road. Building line
represents a line on either side of the road; between which and the road no building activity is
permitted at all. The right of way width is governed by:

 Width of formation: It depends on the category of the highway and width of roadway and
road margins.
 Height of embankment or depth of cutting: It is governed by the topography and the
vertical alignment.
 Side slopes of embankment or cutting: It depends on the height of the slope, soil type etc.
 Drainage system and their size which depends on rainfall, topography etc.
 Sight distance considerations: On curves etc. there is restriction to the visibility on the
inner side of the curve due to the presence of some obstructions like building structures
etc.
 Reserve land for future widening: Some land has to be acquired in advance anticipating
future developments like widening of the road.

The importance of reserved land is emphasized by the following Extra width of land is available
for the construction of roadside facilities. Land acquisition is not possible later, because the land
may be occupied for various other purposes like buildings, business etc.
Figure 3:3 A typical Right of way (ROW)
Chapter 3 Sight distance
3.1 Introduction
Sight distance is a fundamental criterion in the design of any road or street. It is essential for the
driver to be able to perceive hazards on the road, with sufficient time in hand to initiate any
required action safely. On a two-lane two-way road it is also necessary for him or her to be able
to enter the opposing lane safely while overtaking.

Sight distance is the distance visible to the driver of a passenger car. For highway safety, the
designer must provide sight distances of sufficient length that drivers can control the operation of
their vehicles. Two-lane highways should also have sufficient sight distance to enable drivers to
occupy the opposing traffic lane for passing maneuvers, without risk of accident

Sufficient stopping sight distance must always be available for drivers to stop their vehicles
when faced with an unexpected obstruction in the carriageway. Trucks and buses, because of
their greater weight, generally require a greater distance to stop than cars. However, bus and
truck drivers are approximately one meter higher above the road than car drivers and can thus
often see further ahead. Therefore, extended stopping sight distances for buses and trucks are not
required except, perhaps, when horizontal sight distance restrictions occur at the end of a long
downgrade, or where the inside edge of a horizontal curve is bounded by a high vertical barrier
such as a hedge or a fence.

Sight distances may be substantially reduced due to growth of untended vegetation. Suitable
sight distance may be achieved by increasing the radii of horizontal and vertical curves, to allow
visibility outside the road width.

There will normally be some sections of road, such as on bends and summit curves, where there
is insufficient sight distance for safe overtaking; these may be designated 'non-overtaking'
sections and should be clearly marked as such. Adding an overtaking lane at hillcrests may be a
cheaper solution than increasing vertical curve radius. If sharp, low-radius bends are replaced
with longer bends in rolling terrain, the remaining overtaking opportunities may be inadequate.
To increase the opportunities, shorter sharper curves can be used, provided this is done
consistently along the route and does not result in approach speeds which are too high.

The most important consideration in all these is that at all times the driver travelling at the design
speed of the highway must have sufficient carriageway distance within his line of vision to allow
him to stop his vehicle before colliding with a slowly moving or stationary object appearing
suddenly in his own traffic lane. The computation of sight distance depends on:
 1. Reaction time: - of the driver Reaction time of a driver is the time taken from the instant
the object is visible to the driver to the instant when the brakes are applied. The total
reaction time may be split up into four components based on PIEV theory. In practice, all
these times are usually combined into a total perception- reaction time suitable from
design purposes as well as for easy measurement. Many of the studies show that drivers
require about 1.5 to 2 sec under normal conditions. However taking into consideration the
variability of driver characteristics, a higher value is normally used in design. For
example, IRC suggests a reaction time of 2.5 sec.
2. Speed: - the speed of the vehicle very much affects the sight distance. Higher the speed,
more time will be required to stop the vehicle. Hence it is evident that, as the speed
increases, sight distance also increases.
3. Efficiency of brakes:-The efficiency of the brakes depends upon the age of the vehicle,
vehicle characteristics etc. If the brake efficiency is 100%, the vehicle will stop the
moment the brakes are applied. But practically, it is not possible to achieve 100% brake
efficiency. Therefore it could be understood that sight distance required will be more
when the efficiency of brakes are less. Also for safe geometric design, we assume that the
vehicles have only 50% brake efficiency.
4. Frictional resistance: -The frictional resistance between the tire and road plays an
important role to bring the vehicle to stop. When the frictional resistance is more, the
vehicles stop immediately. Thus sight required will be less. No separate provision for
brake efficiency is provided while computing the sight distance. This is taken into
account along with the factor of longitudinal friction. IRC has specified the value of
longitudinal friction in between 0.35 to 0.4.
5. Gradient: - While climbing up a gradient, the vehicle can stop immediately. Therefore
sight distance required is less. While descending a gradient, gravity also comes into
action and more time will be required to stop the vehicle. Sight distance required will be
more in that case.

3.2 Sight distance classification

Sight distance available from a point is the actual distance along the road surface, over which a
driver from specified height above the carriage way has visibility of stationary or moving
objects. Three sight distance situations are considered for design:

• Stopping sight distance (SSD) or the absolute minimum sight distance

• Intermediate sight distance (ISD) is the defined as twice SSD

• Overtaking sight distance (OSD) for safe overtaking operation

3.2.1 Stopping Sight Distance
Stopping Sight Distance is the minimum sight distance available on a highway at any spot
having sufficient length to enable the driver to stop a vehicle traveling at design speed, safely
without collision with any other obstruction. It is the distance a vehicle travels from the point at
which a situation is first perceived to the time the deceleration is complete. Drivers must have
adequate time if they are to suddenly respond to a situation. Thus in a highway design, a sight
distance at least equal to the safe stopping distance should be provided.

Stopping distance involves the ability of the driver to bring the vehicle safely to a standstill and
is thus based on speed, driver reaction time and skid resistance .The stopping sight distance is the
sum of lag distance and the braking distance. Lag distance is the distance the vehicle traveled
during the reaction time t and is given by vt, where v is the velocity in m/sec 2. Braking distance
is the distance traveled by the vehicle during braking operation.

Visibility distance Situations frequently exists where something on the inside of a curve, such as
vegetation, a building or the face of a cutting, obstructs the line of sight. Where it is either not
feasible or economically justified to move the object, a larger radius curve will be required to
ensure that stopping sight distance is available.

Stopping sight distance should always be provided because any road location can become a
hazard. Examples of conditions, which are high priority with respect to the need for stopping
sight distance, are the following:

● Change in lane width;

● Reduction in lateral clearance;
● beginning of hazardous fill slope;
● Crest vertical curve;
● Horizontal curve;
● Driveway;
● Narrow Bridge;
● Roadside hazards (e.g., fixed objects at driveways);
● Unmarked pedestrian crossings;
● Unlit pedestrian crosswalks;
● High volume pedestrian crosswalks;
● Frequent presence of parked vehicles very near the through lane;
● Slow moving vehicles; and
● frequents pedestrian or bicycle presence

3.2.2 Overtaking sight distance

The overtaking sight distance is the minimum distance open to the vision of the driver of a
vehicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite
direction. OSD is limited to 2-lane, 2-way highways. On these facilities, vehicles may overtake
slower moving vehicles, and the passing maneuver must be accomplished on a lane used by
opposing traffic.

The overtaking sight distance or passing sight distance is measured along the center line of the
road over which a driver with his eye level 1.2 m above the road surface can see the top of an
object 1.2 m above the road surface.

Passing Sight Distance is the minimum sight distance on two-way single roadway roads that
must be available to enable the driver of one vehicle to pass another vehicle safely without
interfering with the speed of an oncoming vehicle traveling at the design speed. Within the sight
area the terrain should be the same level or a level lower than the roadway. Otherwise, for
horizontal curves, it may be necessary to remove obstructions and widen cuttings on the insides
of curves to obtain the required sight distance. Care must be exercised in specifying passing/no-
passing zones in areas where the sight distance may be obscured in the future due to vegetative
growth.

The factors that affect the OSD are:

• Velocities of the overtaking vehicle, overtaken vehicle and of the vehicle coming in the
opposite direction.

• Spacing between vehicles, which in-turn depends on the speed

• Skill and reaction time of the driver

• Rate of acceleration of overtaking vehicle

• Gradient of the road

Overtaking sight distance consists of three parts:-

d1= the distance traveled by overtaking A vehicle during the reaction time.

d2= the distance traveled by the vehicle A during the actual overtaking operation.

d3 =is the distance traveled by on-coming vehicle C during the overtaking operation.

Therefore:

OSD = d1 + d2 + d3

Overtaking zones are provided when OSD cannot be provided throughout the length of the
highway. These are zones dedicated for overtaking operation, marked with wide roads. The
desirable length of overtaking zones is 5 times OSD and the minimum is three times OSD.
Potentially unsafe overtaking on curves with inadequate sight distances should be prevented by
signs, road markings or physical barriers. Additionally, positive signing or markings may be
introduced to inform drivers of safe overtaking opportunities.

3.3 Decision Sight Distance Decision sight distance (DSD)

Decision sight distance (DSD) is the length of road a driver needs to receive and interpret
information, select an appropriate speed and path and begin and complete an action in a safe
maneuver. This distance is greater than the distance needed to simply bring a vehicle to a stop,
and provides for a reasonable continuity of traffic flow.

If possible, provide decision sight distance in advance of any feature requiring increased driver
awareness and action. This includes intersections, lane changes, congested areas, pedestrian
crossings, turnouts, pullouts or other features. When decision sight distance is unavailable and
relocation of the feature is not possible, provide suitable traffic control devices.

Potential alternatives or mitigation for limited DSD include the separation of decision points to
different locations for the driver, simplify the decisions to be made, reduce the operating speed to
provide additional time for decision-making and to provide additional advance information.

Examples of roadway geometric elements that are high priority to provide decision sight distance
include:

● First intersection in a sequence;

● Isolated rural intersections; and
● A change in cross-section (i.e., two-lane to four-lane, four-lane to two-lane, passing lane,
climbing lane, lane drop, deceleration lane, channelization).

Geometric or visual complexity combined with any of the above elements increases the needs for
decision sight distance. Frequent truck or recreational vehicle traffic, that block the view of
traffic control devices and road geometric elements may be mitigated by increased sight distance
for the specific area.

Potential mitigation for limited ISD includes:

● Clear sight triangles of all obstructions,

● Provide additional traffic control devices or restrictions,
● Adjust stop line placement,
● Use offset median left-turn lanes,
● Adjust length or offset of turn lanes to minimize potential obstruction by turning vehicles,
● Adjust roadway alignment, and
● Adjust intersection configuration
The available decision sight distances for avoidance maneuvers C, D, and E are
 Determined as follows
DSD = 0.278VT
Where:
DSD = decision sight distance, m;
V = design speed, km/h;
T = Maneuver time, seconds.
For maneuvers A and B
v2
DSD=0.278 VT +0.039
a

Decision Sight Distance, meters

design speed, km/h avoidance maneuvers

A B C D E
50 70 155 145 170 195
60 95 195 170 205 235
70 115 235 200 235 275
80 140 280 230 270 315
90 170 325 270 315 360
100 200 370 315 355 400
110 235 420 330 380 430
120 265 470 360 415 470
130 305 525 390 450 510

Table3. AASHTO recommended decision sight distance

2.1. Driver’s Eye Height for SSD

The driver eye height of 1.08 m that is commonly recommended is based on re- search that
suggests average vehicle heights have decreased to 1.30 m (4.25 ft) with a comparable decrease
in average eye heights to 1.08 m (3.50 ft). For large trucks, the driver eye height ranges from
1.80 m to 2.40 m (3.50 ft to 7.90 ft). The recommended height for a truck driver for design is
2.33 m (7.60 ft) above the road surface.

2.2. Object’s Height for SSD

An object height of a 0.6 m (2.0 ft) is commonly selected based on studies that have indicated
that objects less than 0.60 m in height are less likely to cause crashes. Therefore, an object height
of 0.6 m is considered the smallest object that could pose risk to drivers. In addition, an object
height of 0.60 m is a good representative of the height of automobile headlights and taillights [1].

3.4 Comparison between SSD and DSD

The distinction between stopping sight distance and decision sight distance must be well
understood. Stopping sight distance is applied where only one obstacle must be seen in the
roadway and dealt with. Decision sight distance applies when traffic conditions are complex, and
driver expectancies are different from normal traffic situation. The difference between stopping
in the context of decision sight distance and stopping sight distance is that the vehicle should
stop for some complex traffic condition, such as a queue of vehicles or hazardous conditions,
rather than an object in the roadway. The values of decision sight distance are greater than the
values of stopping sight distance because they provide the driver an additional margin for error
and afford sufficient length to maneuver at the same or reduced speed rather than to stop. The
added complexity in DSD requires additional perception-reaction time prior to applying the
brakes to begin to slow the vehicle to a stop or change the speed or travel path. This allows the
driver additional time to detect and recognize the roadway or traffic situation, identify alternative
maneuvers, and initiate a response on the highway. AASHTO Greenbook (2018 and 2011)
suggest that about 3.0 to 9.0 seconds are required for detecting and understanding the unexpected
traffic situation with an additional 5.0 to 5.5 seconds required to perform the appropriate
maneuver compared to only 2.5 seconds as perception reaction time in stopping sight distance
calculations. Similar to the stopping sight distance, AASHTO Greenbook(2018 and 2011)
recommends assuming the driver’s eye height at 1.08 m (3.5 ft), and the object height as 0.60 m
(2.0 ft) for decision sight distance calculations.

Conclusions

Sight distance is the length of highway a driver needs to be able to see clearly.

Sight distance is one of the important areas in highway geometric design. For safety of highway
operations, the designer must provide sight distances of sufficient length along the highway that
most drivers can control their vehicles to avoid collision with other vehicles and objects that
conflict with their path.

Three types of sight distances are to be considered in the design of highway alignments and
segments: stopping, decision, and passing sight distance. The American Association of State
Highway and Transportation Officials (AASHTO) has defined acceptable limits for stopping,
decision, and passing sight distances based on analysis of safety requirements. Although greater
length is desirable, sight distance at every point along the highway should be at least that
required for a below-average driver or vehicle to stop in this distance. Stopping sight distance is
the sum of two distances: the distance traversed by the vehicle from the instant the driver sights
an object necessitating a stop to the instant the brakes are applied and the distance required to
stop the vehicle from the instant brake application begins. Mostly, the stopping sight distance is
an adequate sight distance for roadway design. However, there are cases where it may not be
appropriate. In areas where information about navigation or hazards must be observed by the
driver, or where the driver’s visual field is cluttered, the stopping sight distance may not be
adequate. In addition, there are avoidance maneuvers that are safer than stopping, but require
more reaction time by the driver. These may not be possible if the minimum stopping sight
distance is used for design. In these instances, the proper sight distance to use is the decision
sight distance. Various design values for the decision sight distance have been developed from
research by AASHTO. The design engineer will decide when to use the decision sight distance.
Providing the extra sight distance will probably increase the cost of a project, but it will also
increase safety. The decision sight distance should be provided in those areas that need the extra
margin of safety, but it isn’t needed continuously in those areas that don’t contain potential
hazards. Passing sight distance is a critical component of two-lane highway design. The capacity
of a two-lane roadway is greatly increased if a large percentage of the roadway’s length can be
used for passing. However, providing a sufficient passing sight distance over large portions of
the roadway can be very expensive. Determining the passing sight distance required for a given
roadway is best accomplished using a simplified AASHTO model.

The passing sight distance can be divided into four distance portions:

d1: The distance the passing vehicle travels while contemplating the passing maneuver, and
while accelerating to the point of encroachment on the left lane.

d2: The length of roadway that is traversed by the passing vehicle while it occupies the left lane.

d3: The clearance distance between the passing vehicle and the opposing vehicle when the
passing vehicle returns to the right lane.

d4: The distance that the opposing vehicle travels during the final 2/3 of the period when the
passing vehicle is in the left lane.
Chapter 4 Horizontal alignment
The design elements of the horizontal alignment are the tangent, or straight section, the circular
curve, the transition curve (spiral) and the super elevation section.

The horizontal alignment should always be designed to the highest standard consistent with the
topography and be chosen carefully to provide good drainage and to minimize earthworks. The
alignment design should also be aimed at achieving a uniform operating speed. Therefore the
standard of alignment selected for a particular section of road should extend throughout the
section with no sudden changes from easy to sharp curvature. Where sharper curvature is
unavoidable, a sequence of curves of decreasing radius is recommended.

4.1 Tangent sections

From an aesthetic point of view, tangent sections may often be beneficial in flat country but are
less so in rolling or mountainous terrain. From a safety standpoint, they provide better visibility
and more passing opportunities. However, long tangent sections increase the danger from
headlight glare and usually lead to excessive speeding. In hot climate areas, such as on the
Awash- Djibouti Road, long tangents have been shown to increase driver fatigue and hence
cause accidents. This issue needs to be addressed in the course of the horizontal design. The
maximum length of a tangent section should not exceed 4.0 kilometers.

Long straights should be avoided as they are monotonous for drivers and cause headlight dazzle
on straight grades. A more pleasing appearance and higher road safety can be obtained by a
winding alignment with tangents deflecting some 5-10 degrees alternately to the left and right.
Short straights between curves in the same direction should not be used because of the broken
back effect. In such cases where a reasonable tangent length is not attainable, the use of long,
transitions or compound curvature should be considered. The unfavorable broken back effect
may also be improved by the introduction of a sag curve.

4.2 Curves
The presence of horizontal curve imparts centrifugal force which is reactive force acting outward
on a vehicle negotiating it. Centrifugal force depends on speed and radius of the horizontal curve
and is counteracted to a certain extent by transverse friction between the tire and pavement
surface. On a curved road, this force tends to cause the vehicle to overrun or to slide outward
from the center of road curvature.

Various forces acting on the vehicle negotiating curve are the centrifugal force acting outward,
weight of the vehicle acting downward, and the reaction of the ground on the wheels.

Where possible, the horizontal curvature of a road should be consistent with speed requirements.
If a relaxation in standard is necessary for economic or environmental reasons, clear signs,
markings and other warning devices should be introduced to make the driver aware of the
potential problem ahead and to guide him through the hazard obstruction from the center-line

In general, horizontal curves should either be designed geometrically so that they can be safely
negotiated by the driver of an approaching vehicle, or the driver should be adequately warned of
the need to reduce speed. At the design stage, the geometric solution to reduce the hazard of an
unexpectedly tight horizontal curve is to increase the radius of the curve.

Unexpectedly tight horizontal curves can lead to accidents as drivers try to negotiate them at too
high a speed. A similar situation may occur on horizontal curves at other hazardous situations,
such as on steep gradients or long straights where drivers are encouraged or misled by the
approach geometry to be travelling at excessive speeds. The sight distances associated with
larger radius curves may also encourage drivers to overtake when it is unsafe.

4.2.1 Reverse Curves, Broken-Back Curves, and Compound Curves

Curves are more frequent in rugged terrain. Tangent sections are shortened, and a stage may be
reached where successive curves can no longer be dealt with in isolation.

Three cases of successive curves are

• Reverse Curve: a curve followed by another curve in the opposite direction

• Broken-Back Curve: a curve followed by another curve in the same direction

• Compound curve: curves in the same direction, but without any intervening tangent section

The occurrence of abrupt reverse curves (having a short tangent between two curves in opposite
directions) should be avoided. Such geometrics make it difficult for the driver to remain within
his lane. The "broken-back" arrangement of curves (having a short tangent between two curves
in the same direction) should be avoided except where very unusual topographical or right-of
way conditions dictate otherwise. Drivers do not generally anticipate successive curves in the
same direction.

The use of compound curves affords flexibility in fitting the road to the terrain and other
controls. Caution should however be exercised in the use of compound curves, because the driver
does not expect to be confronted by a change in radius once he has entered a curve. Their use
should also be avoided where curves are sharp. Compound curves with large differences in
curvature introduce the same problems as are found at the transition from a tangent to a small-
radius curve. Where the use of compound curves cannot be avoided, the radius of the flatter
circular arc should not be more than 50 percent greater than the radius of the sharper arc; i.e. R1
should not exceed 1.5 R2. A compound arc on this basis is suitable as a form of transition from
either a flat curve or a tangent to a sharper curve, although a spiral transition curve is preferred.
4.2.2 Isolated Curves
Long tangent roadway segments, joined by an isolated curve designed at or near the minimum
radius; result in unsafe operations, as a driver will anticipate derivable speeds in excess of the
design speed. Good design practice is to avoid the use of minimum standards in such conditions.
For isolated curves, the minimum horizontal curve radius shall be increased by 50 percent. This
will result, generally, in the ability to negotiate the curve at a speed approximately 10 km/h
higher than the design speed.

4.3 Transition Curves

Transition curve is provided to change the horizontal alignment from straight to circular curve
gradually and has a radius which decreases from infinity at the straight end (tangent point) to the
desired radius of the circular curve at the other end (curve point) .

Transition curves inserted between tangents and circular curves to reduce the abrupt introduction
of lateral acceleration. They may also be used between two circular curves. In order to facilitate
the gradual transition of steering from straight sections of road to the curves, transition curves are
often provided. The Long transition curves can be deceptive and drivers may enter such curves at
speeds that they are unable to sustain safely as the radius reduces.

Transition curves provide a useful role in enabling drivers to moue safely from straight-ahead to
circular motion round a curve. The transition length is also useful in introducing super elevation,
the removal of adverse camber and lane widening.

The length of a transition curve should be the sum of the length required to remove adverse
camber and the length needed to increase this cross fall to the full super elevation requirement.

The provision of transition curves between tangents and circular curves has the following
principal advantages:-

● Transition curves provide a natural easy-to-follow path for drivers, such that the
centripetal force increases and decreases gradually as a vehicle enters and leaves a
circular curve, avoiding sudden jerk on the vehicle. This increases the comfort of
passengers.
● the transition between the normal cross-slope and the fully super elevated section on
the curve can be affected along the length of transition curve in a manner closely fitting
the speed–radius relationship for the vehicle. Where the pavement is to be widened
around a sharp circular curve, the widening can conveniently be applied over the
transition curve length.
● the appearance of the road is enhanced by the application of transition curves.
4.4 Super elevation
Super-elevation or cant or banking is the transverse slope provided at horizontal curve to
counteract the centrifugal force, by raising the outer edge of the pavement with respect to the
inner edge, throughout the length of the horizontal curve. When the outer edge is raised, a
component of the curve weight will be complimented in counteracting the effect of centrifugal
force.

Super elevation is often applied over the length of a circular curve to reduce the sideways
frictional requirements between the tires and road surface and to increase comfort. In such
situations, the transition curve length may be used to introduce the super elevation.

The road has to cater for mixed traffic. Different vehicles with different dimensions and varying
speeds ply on the road. For example, in the case of a heavily loaded truck with high center of
gravity and low speed, super elevation should be less; otherwise chances of toppling are more.

For fast moving vehicles, providing higher super elevation without considering coefficient of
friction is safe, i.e. centrifugal force is fully counteracted by the weight of the vehicle or super
elevation. For slow moving vehicles, providing lower super elevation considering coefficient of
friction is safe, i.e. Centrifugal force is counteracted by super elevation and coefficient of friction

Too high a super elevation will result in the possibility of stationary, slow moving vehicles
sliding sideways or, in extreme cases, overturning. Too low a super elevation may result in
standing water on the carriageway.

Factors limiting selection of higher values are as follows. Low friction values may prevail with
thin layers of mud on the pavement surface, with oil spots, and with high speeds and sufficient
depth of water on pavement surface to permit hydroplaning. Account has to be taken of the
situation in Ethiopia where truck and heavily and/or badly loaded vehicles move slowly due to
poor mechanical condition.

In urban areas where traffic congestion or extensive marginal development acts to curb top
speeds, it is common practice to utilize a low maximum rate of super elevation, usually 4
percent. Similarly, either a low maximum rate of super elevation or no super elevation is
employed within important intersection areas or where there is a tendency to drive slowly
because of turning and crossing movements, warning devices, and signals. Super elevation is a
requirement for all standards of roads.

Without adequate super elevation or removal of adverse camber, the friction required between
the tire and road surface will be much greater, and the risk of an accident higher. Such a situation
will encourage drivers to use the center of the road, or the inside lane, irrespective of direction. A
maximum super elevation of eight or ten per cent will generally eliminate overturning and
sliding problems.
4.5 Widening on Curves
The widening of traffic lanes is often necessary on lower radius curves to allow for the offset of
the rear axles of heavy vehicles following a smaller radius curve than the steering axle.

Extra widening

Extra widening refers to the additional width of carriageway that is required on a curved section
of a road over and above that required on a straight alignment. This widening is done due to two
reasons: the first and most important is the additional width required for a vehicle taking a
horizontal curve and the second is due to the tendency of the drivers to ply away from the edge
of the carriageway as they drive on a curve. The first is referred as the mechanical widening and
the second is called the psychological widening. These are discussed in detail below.

Mechanical widening

The reasons for the mechanical widening are: When a vehicle negotiates a horizontal curve, the
rear wheels follow a path of shorter radius than the front wheels as shown in figure 15.5 this
phenomenon is called off tracking, and has the effect of increasing the effective width of a road
space required by the vehicle. Therefore, to provide the same clearance between vehicles
travelling in opposite direction on curved roads as is provided on straight sections, there must be
extra width of carriageway available. This is an important factor when high proportion of
vehicles is using the road. Tailor trucks also need extra carriageway, depending on the type of
joint. In addition speeds higher than the design speed causes transverse skidding which requires
additional width for safety purpose.

Psychological widening

Widening of pavements has to be done for some psychological reasons also. There is a tendency
for the drivers to drive close to the edges of the pavement on curves
Chapter 5 Vertical alignment
5.1 Introduction
The two major aspects of vertical alignment are vertical curvature, which is governed by sight
distance criteria, and gradient, which is related to vehicle performance and level of service. We
discuss features of the vertical curve; gives values for maximum and minimum gradients;
indicates gradient requirements through villages; develops the criteria for incorporation of a
climbing lane; and provides vertical clearance standards.

5.2 Vertical curves

Vertical curves are used to provide smooth transitions between consecutive gradients. A simple
parabola is normally used to provide a constant rate of change of curvature, and hence visibility,
along its length Lane and edge markings are critical on the vertical curves of surfaced roads. Or
Vertical curves are used in highway and street vertical alignment to provide a gradual change
between two adjacent grade lines. Some highway and municipal agencies introduce vertical
curves at every change in grade line slope, whereas other agencies introduce vertical curves into
the alignment only when the net change in slope direction exceeds a specific value (e.g., 1.5
percent or 2 percent).

In figure below, vertical curve terminology is introduced: g1 is the slope (percent) of the lower
station grade line. g2 is the slope of the higher station grade line. BVC is the beginning of the
vertical curve. EVC is the end of the vertical curve. PVI is the point of intersection of the two
adjacent grade lines. The length of vertical curve (L) is the projection of the curve onto a
horizontal surface and as such corresponds to plan distance.
The design of vertical curves is based on comfort or visibility criteria and a parabolic function is
usually used to connect gradients in the profile alignment .There are two types of vertical curve:
crest curves, which occur on hills, and sag curves, which occur in valleys.

Design of Vertical Curves

A parabolic curve is the most common type used to connect two vertical tangents.
2
y=a x +bx +c

y = roadway elevation at distance x from the PVC

x = distance from the PVC

c = elevation of PVC

b = G1

G1−G 2
a=
2L

The slope of this curve at any point is given by the first derivative,

dy/dx = 2ax + b

The rate of change of slope is given by the second derivative,

D2y/dx2 = 2a

2a is a constant.

The rate of change of slope (2a) can also be written as A/L.

 * Typically horizontal distances expressed in station format.

Two types of vertical curves:

 Crest
 Sag

5.2.1 Crest curve

The minimum required lengths of crest curves are normally designed to provide stopping sight
distance during daylight conditions. Longer lengths would be needed to meet the same visibility
requirements at night on unlit roads. Even on a level road, low meeting-beam headlight
illumination may not show up small objects at the design stopping sight distances. However,
vehicle tail lights and taller objects will be illuminated at the required stopping sight distances on
crest curves.

Drivers are likely to be more alert at night and/or travelling at reduced speed, so longer lengths
of curve are not justified. If full overtaking sight distance cannot easily be obtained, the design
should aim to reduce the length of crest curves to provide the minimum stopping sight distance,
thus increasing overtaking opportunities on the gradients on either side of the curve.

Sight distance requirements for safety are particularly important on crest curves. Vertical curves
are usually designed as parabolas .The minimum lengths of crest curves are designed so as to
provide sufficient sight distances for safe stopping during daylight conditions.
Figure 6:4 Stopping Sight Distance at Crest
It may be difficult for a driver to appreciate the sight distance available on a crest curve and he
may overtake when it is insufficient for him to do so safely. It can be extremely expensive to
provide safe overtaking sight distances on crest curves. However, a complete ban on overtaking
would be difficult to enforce because of the presence of very slow moving vehicles, the lack of
driver discipline in selecting stopping places, and poor maintenance of road markings and signs.
Successive short vertical curves on a straight section of road may produce misleading forward
visibility.

The major control for safe operation on crest vertical curves is the provision of ample sight
distances for the design speed. Minimum stopping sight distance should be provided in all cases.
Overtaking opportunities can be maximized by using small vertical curves allowing longer
tangential gradient sections.

Profiles with successive short vertical curves (i.e. 'roller coaster' profiles), should be avoided as
they are potentially dangerous. Sections of highway composed of two vertical curves in the same
direction separated by a short tangent length (i.e. 'broken back' profiles), should also be avoided.

To ensure adequate night-time visibility by taking account of the upper limit of headlamp beams
Successive short vertical curves should be avoided, particularly on straight sections of road.
1. The difference in elevation between the BVC and a point on the g1

Grade line at a distance X units (feet or meters) is g1X (g1 is expressed as a decimal).

2. The tangent offset between the grade line and the curve is given by ax2, where x is the
horizontal distance from the BVC; (that is, tangent offsets are proportional to the squares of the
horizontal distances).

3. The elevation of the curve at distance X from the BVC is given (on a crest curve) by:

BVC + g1x - ax2

(The signs would be reversed in a sag curve).

4. The grade lines (g1 and g2) intersect midway between the BVC and the EVC. That is, BVC to
V = 1/2L = V to EVC.

5. Offsets from the two grade lines are symmetrical with respect to the PVI.

6. The curve lies midway between the PVI and the midpoint of the chord; that is, Cm = mV.
Definitions:

PVI = Point of vertical intersection of tangent lines

PVC = Point of vertical curvature

PVT = Point of vertical tangency

L = Length of curve

G1 = initial roadway grade in percent

G2 = final roadway grade in percent

A = absolute value of difference in grades

Procedure for Computing a Vertical Curve

1. Compute the algebraic difference in grades: A

2. Compute the chain age of the BVC and EVC. If the chain age of the PVI is known,

1/2L is simply subtracted and added to the PVI chain age.

3. Compute the distance from the BVC to the high or low point (if applicable):

x = -g1L/A. And determine the station of the high/low point.

4. Compute the tangent grade line elevation of the BVC and the EVC.

5. Compute the tangent grade line elevation for each required station.

6. Compute the midpoint of chord elevation {elevation of BVC + elevation of EVC}/2

7. Compute the tangent offset (d) at the PVI (i.e., distance Vm):

d = {elevation of PVI - elevation of midpoint of chord}/2

8. Compute the tangent offset for each individual station.

Tangent offset = {x/(L/2)}2d where x is the distance from the BVC or EVC (whichever is closer)
to the required station.

9. Compute the elevation on the curve at each required station by combining the tangent offsets
with the appropriate tangent grade line elevations. Add for sag curves and subtract for crest
curves.

Example

Given that L = 300 ft, g1= -3.2%, g2= +1. 8%, PVI at 30 + 30, and elevation =

465.92. Determine the location of the low point and elevations on the curve at even stations, as
well as at the low point

Solution

A = g2 - g1 = 1.8 - (-3.2) = 5.0

PVI - 1/2L = BVC

30+30 - 150 = 28+80.00

PVI + 1/2L = EVC

30+30 + 150 = 31+80.00

EVC - BVC = L

31+80 - 28+86 = 300….Check

Elevation of PVI = 465.92

150 ft at 3.20% = 4.80ft

Elev. BVC = 465.92 + 4.80 = 470.72

Elevation PVI = 465.92

150 ft at 1.8% = 2.70

Elevation EVC = 465.92 + 2.70 = 468.62

Location of low point

x = -g1L/A

{3.2 x 300}/5 = 192.00 ft

Tangent grade line computations

elevation at 29 + 00 = 470.72 - (0.032 x 20)

= 470.72 - 0.64 = 470.08

Mid-chord elevation:

{470.72 (BVC) + 468.62 (EVC)}/2 = 469.67

 Tangent offset at PVI (d):

d = {elevation of PVI - elevation of mid-chord}/2

{469.67 - 465.92}/2 = 3.75/2 = 1.875ft

Point Station Tangent Elevation Curve

Tangent Offset {x/(L/2)}  d
2
Elevation

BVC 28 + 80 470.72. 2 470.72

(0/150) X 1.875 = 0
29 + 00 470.08 2 470.11
(20/150) X 1.875 = 0
30 + 00 466.88 2 468.08
(120/150) X 1.875 = 0
PVI 30 + 30 465.92 2 467.80
(150/150) X 1.875 = 0

Low Point 30 + 72 466.68 2 467.65

(108/150) X 1.875 = 0

31 + 00 467.18 2 467.71

(80/150) X 1.875 = 0

EVC 31 + 80 468.62 2 468.62

(0/150) X 1.875 = 0

Parabolic Curve Elevations by Tangent Offsets

5.2.2 Sag curve
The maximum vertical accelerations at the top of a crest curve and at the bottom of a sag curve
also need consideration. The comfort criterion for sag curves as a result of vertical acceleration is
often taken as the critical design factor extends beyond the vertical curve.

Sag curves should be designed according to comfort criteria, in which a vertical acceleration of
0.05g would be an appropriate maximum on major roads, although this may be relaxed to 0.10g
on other roads.

Long sag curves connecting shallow gradients can lead to drainage problems. When sag curves
are associated with highway underpasses, curve lengths must be chosen to ensure the necessary
vertical clearances and to maintain adequate sight distances into the underpass.

Care is needed with drainage, especially on long, shallow sag curves.

A sharp horizontal curve shortly after a pronounced crest curve is a serious hazard that must be
avoided. The corrective action is either to separate the curves, apply a gentler horizontal curve,
or make the horizontal curve start well before the summit of the crest curve. In the sag curve
situation an apparent kink is produced, creating an aesthetically unpleasant appearance of the
road Stopping Sight Distance for Sag Vertical Curve:

- Sight distance is governed by nighttime conditions.

- We are concerned with height of headlight above roadway and inclined angle of

headlight beam.

Figure 6:2 Stopping Sight Distance at Sag

where:

H = height of headlight above road surface

β= inclined angle of headlight beam.

5.3 Gradient
Vehicle operations on gradients are complex and depend on a number of factors: severity and
length of gradient; level and composition of traffic; and the number of overtaking opportunities
on the gradient and in its vicinity.

For very low levels of traffic flow represented by only a few four-wheel drive vehicles other
references advocate a maximum traversable gradient of up to 18 percent. Small commercial
vehicles can usually negotiate an 18 per cent gradient; whilst two-wheel drive trucks can
successfully manage gradients of 15-16 per cent except when heavily laden.

However, the vehicle fleet in Ethiopia is composed of a high percentage of vehicles that are
underpowered and poorly maintained. Certain existing roads in fact are avoided and
underutilized by traffic due to an inability to ascend the existing grades.

Maximum vertical gradient is therefore and extremely important criterion that greatly effects
both the serviceability and cost of the road.

When gradients of 10 percent or greater are reached, consideration should be given to the
possibility of paving these steep sections to enable sufficient traction to be achieved, as well as
for pavement maintenance reasons.

Gradients need to be considered from the standpoint of both length and steepness, and the speed
at which heavy vehicles enter the gradient. They should be chosen such that any marginal
increase in construction cost is more than offset by the savings in operating costs of the heavy
vehicles ascending them over the project analysis period. For access roads with low levels of
traffic (less than about 20 vehicles per day), it is appropriate to use the maximum gradient that
the anticipated type of vehicle can climb safely. Maximum gradient is limited ultimately by
traction requirements.

As traffic flow increase, the economic dis-benefits of more severe gradients, measured as
increased vehicle operating and travel time costs, are more likely to result in economic
justification for reducing the severity and/or length of a gradient. On the higher design classes or
road, the lower maximum recommended gradients reflect these economics.

However, a separate economic assessment of alternatives to long or severe gradients should be

undertaken where possible or necessary. Standards for desirable maximum gradients were set to
assure user comfort and to avoid severe reductions in the design speed. If the occasional terrain
anomaly is encountered that requires excessive earthworks to reduce the vertical alignment to the
desirable standard an absolute maximum gradient can be used. Employment of a gradient in
excess of the desirable maximum can only be authorized through the employment of a Departure
from Standard.

Critical length may be defined at the point at which a truck reaches a certain speed or the point at
which it has lost a certain amount of speed. Critical length of gradient is considered to be the
maximum length of a designated upgrade upon which a loaded truck can operate without
unreasonable reduction in speed. Critical length of gradient is, to some extent, dependent on the
gradient of the approach; a downhill approach will allow vehicles to gain momentum and
increase the critical length.

The critical length of gradient decreases, a gradient increases. Where it is necessary to exceed the
critical length of gradient on heavily trafficked roads, it is desirable to provide either with safe
passing distances on the rise, or a climbing lane for heavy vehicles.

The cost of construction of a road generally increases as the terrain gets steeper, so the use of
higher gradients is justified Sustained grades steeper than about 3 per cent slow down heavy
vehicles significantly. Gradient design should aim at grades that will not reduce the speeds of
heavy vehicles by more than 25km/h. Climbing lanes enable faster vehicles to overtake more
easily, resulting in shorter average journey times and reduced vehicle-operating costs Their use
may be a cost-effective alternative where terrain or other physical features prevent shortening or
flattening of gradients, and where the length of critical grade is exceeded.

5.4 Climbing lanes

Restricted overtaking opportunities and the presence of slow moving vehicles can result in
substantial congestion and high accident rates through injudicious overtaking. Congestion effects
are greatest on long steep gradients. The situation is worse in many developing countries because
of the presence of overloaded trucks and buses with very low power-to-weight ratios

In such circumstances, the provision of an auxiliary climbing lane can be extremely beneficial to
enable vehicles travelling up the gradient to overtake safely and efficiently. However, the criteria
for introducing and evaluating climbing lanes are complex and involve length and severity of
gradient, traffic composition, level of flow and an estimation of the speed differences between
the various vehicle grin heavy flows, there may be merging and accident problems when the
climbing lane ends and the overtaking and overtaken flows merge.

Although the major benefits of a climbing lane are in terms of values of travel time saving, there
is some evidence to suggest that they also result in a reduction in the accident rate. Accident
savings may also occur on adjacent sections of road, if the climbing lane reduces levels of
frustration and injudicious overtaking on these approach sections.
Auxiliary lanes, climbing lanes are particularly effective as they operate at locations where the
maximum overtaking advantage can be obtained from the relative power-to-weight ratios of light
and heavy vehicles. The case for their introduction is largely economic, and relatively high
traffic flows are usually needed for their justification

Climbing lanes should start before the gradient, and end after it, to ensure as small a speed
differential as possible between overtaking and overtaken streams to aid safe and efficient
overtaking and merging.

A climbing lane is an effective means of reducing the impact of a steep gradient. A climbing lane
is an auxiliary lane added outside the continuous lanes and has the effect of reducing congestion
in the through lanes by removing slower moving vehicles from the traffic stream. It also
enhances road safety by reducing the speed differential in the through lane. The requirements for
climbing lanes are therefore based on road standard, speed and traffic volume.

Benefits from the provision of a climbing lane accrue because faster vehicles are able to overtake
more easily, resulting in shorter average journey times, reduced vehicle-operating costs, and
increased safety. Benefits will increase with increases in gradient, length of gradient, traffic flow,
the proportion of trucks, and reductions in overtaking opportunities. The effect of a climbing lane
in breaking up queues of vehicles held up by a slow moving truck will continue for some
distance along the road.

Climbing lanes must be considered for roads when present traffic volumes are greater than 400
ADT. Thus the application of climbing lanes is limited particularly to trunk and link roads.

Climbing lanes must be clearly marked and, where possible, should end on level or downhill
sections where speed differences between different classes of vehicles are lowest to allow safe
and efficient merging maneuvers. The introduction and termination of a climbing lane shall be
effected by tapers of lengths of 100 meters. The tapers shall not be considered as part of the
climbing lanes.

There is a problem in the application of a climbing lane in escarpment terrain. Here the
carriageway and shoulder widths may have been reduced, and thus a climbing lane will increase
the roadway width. Consideration must be given to a balance between the benefits to traffic and
the initial construction cost. In sections requiring heavy side cut, the provision of climbing lanes
may be unreasonably high in relation to the benefits. Reduced level of service over such sections
is an alternative.

The performance characteristics of a heavy vehicle are such that, for a particular gradient, the
vehicle speed will reduce to final ambient speed that can be maintained by that vehicle on that
grade. This limits, in most references, any discussion on the maximum length allowable at a
given grade even considering the employment of a climbing lane.
Figure 6:5 Layouts for Climbing Lane
5.5 Vertical Clearances
Bridges over water shall normally have a minimum clearance height. The standard minimum
headroom or clearance under bridges or tunnels shall be 5.1m for all classes of roads. This
clearance should be maintained over the roadway(s) and shoulders. Where future maintenance of
the roadway is likely to lead to a rising of the road level, then an additional clearance of up to
0.1m may be provided. Light superstructures (i.e. - timber, steel trusses, steel girders, etc.) over
roadways shall have a clearance height of at least 5.3m.

Underpasses for pedestrians and bicycles shall not be less than 2.4m. For cattle and wildlife,
underpasses shall be designed as the normal height of the actual kind of animal plus 0.5m, and
for horse-riding the clear height shall be not less than 3.4m. Bridges above railways shall have a
clearance height of at least 6.1m- if not otherwise stated- to facilitate possible future
electrification.

5.6 Phasing of horizontal and vertical alignment

Phasing of the vertical and horizontal curves of a road implies their coordination so that the line
of the road appears to a driver to flow smoothly, avoiding the creation of hazards and visual
defects. It is particularly important in the design of high-speed roads on which a driver must be
able to anticipate changes in both horizontal and vertical alignment well within the safe stopping
distance. It becomes more important with small radius curves than with large.

Defects may arise if an alignment is mis-phased. Defects may be purely visual and do no more
than present the driver with an aesthetically displeasing impression of the road. Such defects
often occur on sag curves. When these defects are severe, they may create a psychological
obstacle and cause some drivers to reduce speed unnecessarily. In other cases, the defects may
endanger the safety of the user by concealing hazards on the road ahead. A sharp bend hidden by
a crest curve is an example of this kind of defect.

Horizontal and vertical alignments should not be considered independently. They complement
each other and poor design combinations can confuse drivers and lead to potentially dangerous
situations. It is extremely difficult and costly to correct alignment deficiencies after the highway
has been constructed. Evidence suggests that initial cost savings may be more than offset by the
subsequent economic loss to the public in the form of accidents and delays.

Poor coordination of the horizontal and vertical alignments of a road can result in visual effects
which contribute to accidents and are detrimental to the appearance of the road.

An appearance likely to be misinterpreted by a driver may result when horizontal and vertical
curves of different length occur at the same location. For example, drivers who judge their
approach speeds and lateral locations on the expectation of a single vertical crest curve may be
surprised by the later appearance of a short horizontal curve contained within the vertical curve.
These situations are particularly dangerous.

Where possible, horizontal and vertical curvature should be so combined that the safety and
operational efficiency of the road is enhanced. If horizontal and vertical curves cannot be entirely
separated, they should be combined with common changes for intersection points and where
possible, should be of the same or similar length

The presentation of misleading information to drivers can be avoided by making coincident all
the points where horizontal and vertical curvatures change. Where this is not possible and the
curves cannot be separated entirely, the vertical curves should be either contained wholly within,
or wholly outside the horizontal curves. Also, horizontal and vertical curves should be of the
same length and the chainage of their centers should coincide

A logical design is a compromise between the alignment, which offers the most in terms of
safety, capacity, ease and uniformity of operation, and pleasing appearance, within the practical
limits of the terrain and area traversed

Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest
vertical curve as drivers will not be able to perceive the horizontal change in alignment,
especially at night.

Expenditure is often justified to increase the radii of horizontal curves at the bottom of steep
grades to allow for vehicles running out of control. Alternative measures include 'escape' lanes
where vehicles travelling too fast to turn can be safely stopped

Horizontal alignment and profile should be made as flat as possible at interchanges and
intersections where sight distance along both highways is important. Sight distances well above
minimum should be provided at these locations, where possible.

Defects may arise if an alignment is mis-phased. Defects may be purely visual and do no more
than present the driver with an aesthetically displeasing impression of the road. Such defects
often occur on sag curves. When these defects are severe, they may create a psychological
obstacle and cause some drivers to reduce speed unnecessarily. In other cases, the defects may
endanger the safety of the user by concealing hazards on the road ahead. A sharp bend hidden by
a crest curve is an example of this kind of defect.

5.6.1 Types of Mis-Phasing and Corresponding Corrective Action

When the horizontal and vertical curves are adequately separated or when they are coincident, no
phasing problem occurs and no corrective action is required. Where defects occur, phasing may
be achieved either by separating the curves or by adjusting their lengths such that vertical and
horizontal curves begin at a common station and end at a common station. In some cases,
depending on the curvature, it is sufficient if only one end of each of the curves is at a common
station.

Cases of mis-phasing fall into several types. These are described below together with the
necessary corrective action for each type.

1. Vertical Curve Overlaps One End of the Horizontal Curve

If a vertical curve overlaps either the beginning or the end of a horizontal curve, a driver’s
perception of the change of direction at the start of the horizontal curve may be delayed because
his sight distance is reduced by the vertical curve. This defect is hazardous. The position of the
crest is important because the vehicles tend to increase speed on the down gradient following the
highest point of the crest curve, and the danger due to an unexpected change of direction is
consequently greater. If a vertical sag curve overlaps a horizontal curve, an apparent kink may be
produced, as indicated in Figures 10-1b and c.

The defect may be corrected in both cases by completely separating the curves. If this is
uneconomic, the curves must be adjusted so that they are coincident at both ends, if the
horizontal curve is of short radius, or they need be coincident at only one end, if the horizontal
curve is of longer radius

INSUFFICIENT SEPARATION BETWEEN THE CURVES

If there is insufficient separation between the ends of the horizontal and vertical curves, a false
reverse curve may appear on the outside edge-line at the beginning of the horizontal curve. This
is a visual defect, illustrated in Figure 10-1d. Corrective action consists of increasing the
separation between the curves, or making the curves concurrent, as in Figure 10-1a.

BOTH ENDS OF THE VERTICAL CURVE LIE ON THE HORIZONTAL CURVE

If both ends of a crest curve lie on a sharp horizontal curve, the radius of the horizontal curve
may appear to the driver to decrease abruptly over the length of the crest curve. If the vertical
curve is a sag curve, the radius of the horizontal curve may appear to increase. An example of
such a visual defect is shown in Figure10-1e. The corrective action is to make both ends of the
curves coincident as in Figure 10-1a, or to separate them.

2. VERTICAL CURVE OVERLAPS BOTH ENDS OF THE HORIZONTAL CURVE

If a vertical crest curve overlaps both ends of a sharp horizontal curve, a hazard may be created
because a vehicle has to undergo a sudden change of direction during the passage of the vertical
curve while sight distance is reduced. The corrective action is to make both ends of the curves
coincident. If the horizontal curve is less sharp, a hazard may still be created if the crest occurs
off the horizontal curve. This is because the change of direction at the beginning of the
horizontal curve will then occur on a downgrade (for traffic in one direction) where vehicles may
be increasing speed. The corrective action is to make the curves coincident at one end so as to
bring the crest on to the horizontal curve. No action is necessary if a vertical curve that has no
crest is combined with a gentle horizontal curve. If the vertical curve is a sag curve, an illusory
crest or dip, depending on the “hand” of the horizontal curve will appear in the road alignment.
The corrective action is to make both ends of the curves coincident or to separate them.

3. OTHER MIS-PHASING

Other types of mis-phasing are also indicated in Figure 10-1: A sag curve occurs between two
horizontal curves in the same direction in Figure 10-1g. This illustrates the need to avoid broken
back curves in design. A double sag curve occurs at one horizontal curve in Figure 10-1h. This
illustrates the effect in this case of a broken back vertical alignment on design (see Chapter 9:
Vertical Alignment). Figure 10-1i shows a lack of phasing of horizontal and vertical curves. In
this case, the vertical alignment has been allowed to be more curvilinear than the horizontal
alignment.
Figure 6:6 Phasing of Horizontal and Vertical Curves
5.6.2 The Economic Penalty Due to Phasing
The phasing of vertical curves restricts their movement and fitting to the ground so that the
designer is prevented from obtaining the lowest cost design. Therefore, phasing is usually bought
at the cost of extra earthworks and the designer must decide at what point it becomes
uneconomic. He will normally accept curves that have to be phased for reasons of safety. In
cases when the advantage due to phasing is aesthetic, the designer will have to balance the costs
of trail alignments against their elegance.
Chapter 6 Intersections
6.1 Introduction
Intersection is the general area where two or more roads join. A disproportionate amount of
traffic accidents occur at junctions, and thus from a traffic safety aspect junctions require
attention and careful design. Good junction design should allow transition from one route to
another or through movement on the main route and intersecting route with minimum delay and
maximum safety. To accomplish this, the layout and operation of the junction should be obvious
to the driver, with good visibility between conflicting movements.

Conflicting vehicle movements at junctions are the largest cause of accidents in many
developing countries. A small number of well-designed junctions on a route are preferable to a
large number of low standard junctions. Simple crossroads have the worst accident record.
Staggered crossroads, consisting of two successive T-junctions on opposite sides of the road, can
reduce the accident rate. The use of roundabouts, traffic lights and channelization may be
appropriate to improve vehicle flow and safety. Conflicts can be largely eliminated by the
expensive solution of grade separation.

It is the most complex location on any highway. Conflicts are common at the intersections. This
is because vehicles moving in different direction want to occupy same space at the same time. In
addition, the pedestrians also seek same space for crossing.

Intersection is an area shared by two or more roads. It is some area designated for the vehicles to
turn to different directions to reach their desired destinations. Its main function is to provide
channelization of route direction.

Drivers have to make split second decision at an intersection by considering his route.
Intersection geometry, other vehicles, their speed, direction etc. A small error in judgment can
cause severe accidents. It also causes delay and it depends on type, geometry and type of control.
Overall traffic flow depends on the performance of intersection. It also affects the capacity of the
road. Especially in the case of an urban scenario, both from the accident perspective and the
capacity perspective.

6.2 Design Requirements

The design of intersection must take account of the following basic requirements:

• Safety

• Operational comfort

• Capacity
• Economy

Intersection is considered safe when it is visible, comprehensible, and maneuverable. These three
requirements can generally be met by complying with the following guidelines

VISIBILITY

The intersection should be sited so that the major road approaches are readily visible. The angle
of skew of the junction should be no more than 20 degree from perpendicular.

COMPREHENSION

(i) The right of way should follow naturally and logically from the junction layout.
(ii) The types of junctions used throughout the whole road network should be similar.
(iii) The use of road signs is necessary. Road markings and other road furniture may also be
required.

MANEUVERABILITY

(i) All traffic lanes should be of adequate width and radius for the appropriate vehicle
turning characteristics. To accommodate truck traffic, turn radii shall be 15 meters
minimum.
(ii) The edges of traffic lanes should be clearly indicated by road markings. The operation
of the junction depends principally upon the frequency of gaps that naturally occur
between vehicles in the main road flow. These gaps should be of sufficient duration
to permit vehicles from the minor road to merge with, or cross, the major road flow.
In consequence junctions are limited in capacity, but this capacity may be optimized
by, for example, canalization or the separation of maneuvers

CHECKLIST FOR INTERSECTION DESIGN

1. Will the junction be able to carry the expected/future traffic levels without becoming
overloaded and congested?

2. Have the traffic and safety performance of alternative junction designs been considered?

3. Is the route through the junction as simple and clear to all users as possible?

4. Is the presence of the junction clearly evident at a safe distance to approaching vehicles for all
directions?

5. Are warning and information signs placed sufficiently in advance of the junction for a driver
to take appropriate and safe action given the design speeds on the road?

6. On the approach to the junction, is the driver clearly aware of the actions necessary to
negotiate the junction safely?
7. Are turning movements segregated as required for the design standard?

8. Are drainage features sufficient to avoid the presence of standing water?

9. Is the level of lighting adequate for the junction, location, pedestrians, and design standard?

10. Are the warning signs and markings sufficient, particularly at night?

11. Have the needs of pedestrian and noon-motorized vehicles been met?

12. Are sight lines sufficient and clear of obstructions including parked and stopped vehicles?

13. Are accesses prohibited a safe distance away from the junction?

14. Have adequate facilities such as footpaths, refuges, and crossings, been provided for
pedestrians?

15. Does the design, road marking and signing clearly identify rights of way and Priorities?

16. Is the design of the junction consistent with road types and adjacent junctions?

17. Are the turning lanes and tapers where required of sufficient length for speeds and storage

6.3 Control of an intersection

The control of an intersection can be exercised at different levels. They can be either passive
control, semi control, or active control. In passive control, there are no such direct strict rules on
the driver. In semi control, some amount of control on the driver is there from the traffic agency.
Active control means the movement of the traffic is fully controlled by the traffic agency and the
drivers cannot simply maneuver the intersection according to his choice.

6.3.1 Passive control

Some of the intersection controls that are classified under passive control are as follows:

1. No control: If the traffic coming to an intersection is low, then by applying the basic rules of
the road like driver on the left side of the road must yield and that through movements will have
priority than turning movements, the driver itself can manage to traverse the intersection.

2. Traffic signs: With the help of warning signs, guide signs etc. it is able to provide some level
of control at an intersection.

3. Traffic signs plus marking: In addition to the traffic signs, road markings also complement the
traffic control at intersections. Some of the examples are stop line marking, yield lines, arrow
marking etc.
4. GIVE WAY control: This control requires the driver in the minor road to slow down to a
minimum speed and allow the vehicle on the major road to proceed.

5. Two way stop control: In this case, the vehicle drivers on the minor streets should see that the
conflicts are avoided.

6. All-way stop control: This is usually used when it is difficult to differentiate between the
major and minor roads in an intersection. In such a case, STOP sign is placed on all the
approaches to the intersection and the driver on all the approaches are required to stop the
vehicle. The vehicle at the right side will get priority over the left approach

6.3.2 Semi control

Channelization and traffic rotaries come under semi control.

1. Channelization: The traffic is separated to flow through definite paths by raising a portion of
the road in the middle usually called as islands distinguished by road markings. The conflicts in
traffic movements are reduced to a great extent in such a case.

2. Traffic rotaries: It is a form of intersection control in which the traffic is made to flow along
one direction round a traffic island. This completely avoids through-conflicts.

6.3.3 Active control

Active control means the road user will be forced to follow the path suggested by the traffic
control agencies. He cannot maneuver according to his wish. Traffic signals and grade separated
intersections come under this classification.

1. Traffic signals: This control is based on time sharing approach. At a time with the help of
appropriate signals, certain traffic movements are restricted where as certain other movements
are permitted. Two or more phase signals may be provided depending upon the traffic conditions
of the intersection.

2. Grade separated intersections: It is an intersection where crossing movements at different

levels is permitted. It is very expensive and is usually used on high speed facilities like
expressways, freeways etc.

6.4 Types of intersection

The intersections are of two types. They are at-grade intersections and grade-separated
intersections. In at grade intersections, all roadways join or cross at the same vertical level.
Grade separated intersections allows the traffic to cross at different vertical levels. Sometimes
the topography itself may be helpful in constructing such intersections. Otherwise the initial
construction cost required will be very high. But it increases the road capacity because vehicles
can flow with high speed and accident potential is also reduced by such vertical separation of
traffic.

6.4.1 At grade intersection control:

Different types of intersection control provided at ’at-grade’ are as follows:

1. Uncontrolled or passive control: The traffic control at ’at-grade’ intersection may be

uncontrolled in cases of low traffic. Here the road users are required to obey the basic rules of
the road. Passive control like traffic signs, road markings etc. are used to complement the
intersection control.

2. Semi controlled or channelized intersections: In channelized intersections, as the name

suggests, the traffic is directed to flow through different channels and this physical separation is
made possible with the help of some barriers in the road like traffic islands, road markings etc.

3. Rotary/Round about: It is a form of ’at-grade’ intersection laid out for the movement of traffic
such that no through conflicts are there. Free-left turn is permitted whereas through traffic and
right-turn traffic is forced to move around the central island in a clock-wise direction in an
orderly manner. Merging, weaving and diverging operations reduces the conflicting movements
at the rotary.

4. Signalized control: When the vehicles traversing the intersection are very large, then the
control is done with the help of signals.

6.4.2 Grade separated intersection control:

Grade-separated intersections are provided to separate the traffic in the vertical grade. But the
traffic need not be those pertaining to road only. When a railway line crosses a road, then also
grade separators are used. Different types of grade-separators are flyovers and interchange.
Flyovers itself are subdivided into overpass and underpass. When two roads cross at a point, if
the road having major traffic is elevated to a higher grade for further movement of traffic, then
such structures are called overpass. Otherwise, if the major road is depressed to a lower level to
cross another by means of an under bridge or tunnel, it is called under-pass.

6.5 Channelized intersection

The vehicles approaching the intersection are directed to definite paths by islands, marking etc.
and this process is called channelization. There can be channelized or unchannalized intersection
but channelized intersection provides more safety and efficiency. It reduces the number of
possible conflicts by reducing the area of conflicts available in the carriageway. If no
channelizing is provided, the driver will have fewer tendencies to reduce the speed while
entering the intersection from the carriageway. The presence of traffic islands, markings etc.
makes the driver to reduce the speed and becomes more cautious while maneuvering the
intersection. A channelizing island serves as a refuge for pedestrians and makes pedestrian
crossing safer.

6.6 Priority Intersections

A priority intersection occurs between two roads; one termed the ‘major’ road and the other the
‘minor’ road. The major road is the one assigned a permanent priority of traffic movement over
that of the minor road. The minor road must give priority to the major road with traffic from it
only entering the major road when appropriate gaps appear. The principal advantage of this type
of junction is that the traffic on the major route is not delayed.

The principle at the basis of the design of priority intersections is that it should reflect the pattern
of movement of the traffic. The heaviest traffic flows should be afforded the easiest paths.
Visibility, particularly for traffic exiting the minor junction, is a crucial factor in the layout of a
priority intersection. Low visibility can increase the rate of occurrence of serious accidents as
well as reducing the basic capacity of the intersection itself.

Priority intersections can be in the form of simple T-junctions, staggered junctions or crossroads;
though the last form should be avoided where possible as drivers exiting the minor road can
misunderstand the traffic priorities. This may lead to increased accidents.

Priority Intersections are the most common form of intersection. Control is by a “Give way “or
“stop” sign on the minor road with no restriction on the major road. Priority intersections fall
into two basic categories namely T junctions and crossroads.

6.6.1 Crossroads
Crossroads often have a poor safety record because of minor road traffic failing to stop for main
road traffic either because of driver indiscipline or because the driver is not aware that there is a
major road ahead. If the stop-line is in the dip at the edge of the major road camber it can be
invisible from a distance on the minor road. The major accident types at priority intersections are
accidents where side road vehicles fail to stop, implying inadequate visibility of the intersection
from the minor road, and accidents with emerging vehicles, which suggests inadequate sight
lines along the major road.

For all types of priority intersection, the problem of delay exists for minor road traffic which has
to give way. If the delays are excessive, emerging drivers may take undue risks in order to enter
or cross the main stream. Multiple lane approaches place greater demands on the emerging driver
and tend to be more hazardous locations.

Slow moving or stationary vehicles turning into a side road across a main road stream of traffic
are often the cause of serious accidents, particularly at night problems can also be caused in
urban areas by inadequate curbs which give an unclear layout and make little or no provision for
pedestrians.

Priority intersections should only be used where flows are relatively low (up to around 5,000
AADT on the major road and only 3,000 AADT on the minor road).

Where space permits, staggered intersections are preferable to crossroads on safely grounds. The
same effect may be achieved by the use of offset central islands at the entries. The stagger or
offset should always allow minor road crossing vehicles to enter the intersection by crossing the
nearest traffic stream and to leave the intersection using an unopposed turn. ‰

Multiple lane approaches should be avoided where possible. On fast dual carriageways the
median should be built wide enough to accommodate the longest heavy vehicle found, otherwise
part of the vehicle will overhang into an overtaking lane as the vehicle is waiting in the center of
the carriageway to complete the maneuver.

Ghost island width should be a minimum of 3.5m in width. Where space allows and the major
road has high flows and/or speeds, then an offside diverging lane can be of use. The length and
taper of these depend on the design speed of the major road. Local widening may be required to
create such facilities. ‰

The minor road approach must be designed to show clearly that a major road is ahead and that
drivers must yield priority. Deflection islands, bollards and clear signing are necessary to achieve
this. If visibility is in any way inadequate additional advance warning signs must be used. Sight
distances must not be blocked by vegetation growth.

6.6.2 T-Junctions
The basic junction layout for rural roads is the T-junction with the major road traffic having
priority over the minor road traffic. Applications of T-junctions include staggered T junction,
which caters to cross-traffic.

There are three distinct types of geometric layout for a single-carriageway priority intersection

Simple junctions: A T-junction or staggered junction without any ghost or physical islands in the
major road and without channeling islands in the minor road approach.

Ghost island junctions: Usually a T-junction or staggered junction within which an area is
marked on the carriageway, shaped and located so as to direct traffic movement.

Single lane dualling: Usually a T-junction or staggered junction within which central reservation
islands are shaped and located so as to direct traffic movement.
Staggered intersections should be used where information on the traffic volumes, or relative
importance of two crossing roads, enables the designer to distinguish between the major road and
the minor road.

Staggered intersections are referred to as either ‘left-right’ or ‘right-left’ according to turning

movements of a vehicle on the minor road crossing the major road. The staggering should be
designed so that traffic does not have to cross the opposing main road traffic stream before
exiting. This avoids the need for traffic to wait in the middle of the main road. Consequently,
right-left stagger is preferred for left-hand driving, and left-right is preferred for right-hand
driving. The minimum spacing between the legs of a staggered intersection should be 100m.
Where intersections have tapered deceleration/acceleration lanes, the minimum spacing should
be increased to 200m.

Staggered T-junctions are often the result of a realignment of the minor route to improve the
angle of the skew of the crossing,. Where such staggered T-junctions are used, the left-right
stagger is preferred to the right-left stagger (see Figure 12-3). The reason for this is that, in the
latter case, a crossing vehicle must re-enter the minor road by making a left turn on the major
roadway. In such cases, the inclusion of a left-turning lane between the staggers should be
considered.

6.7 Roundabouts
Rotary intersections or roundabouts are special form of at-grade intersections laid out for the
movement of traffic in one direction round a central traffic island. Essentially all the major
conflicts at an intersection namely the collision between through and right-turn movements are
converted into milder conflicts namely merging and diverging. The vehicles entering the rotary
are gently forced to move in a anticlockwise direction for left hand traffic countries.

A roundabout is a one-way circulatory system around a central island, entry to which is

controlled by 'give-way' markings and signs. Roundabouts provide a high capacity, cause little
delay in the off-peak period and require no technical maintenance. Roundabouts are particularly
suitable where there are more than four arms to the intersection, although three or four arm
roundabouts are generally used.

Poor visibility on the approaches or across the central island can result in drivers making unwise
entry decisions. High entry speeds can lead to accidents between entering and circulating
vehicles. Poor enforcement of priority rules can lead to high accident rates and inefficiencies in
operation.

Long delays may result when there are substantial differences in entering flows. Flows on one
arm may dominate at the expense of others and the resulting long delays may lead to unwise
entry decisions Roundabouts can quickly become blocked if circulating traffic is not given right-
of-way.
The central island may contain concrete and other structures. These substantially increase
accident severity for those vehicles which fail to negotiate the roundabout through too high an
approach speed. Sources of danger in the geometry of roundabouts include: very acute merging
angles, roundabouts which are not circular, poorly designed or positioned signing and steep
gradients or poor skidding resistance on approaches. Accidents between motorized and non-
motorized vehicles can be a particular problem because of the speed differences as they move
through the roundabout, especially if it is large.

Advantages and disadvantages of rotary

The key advantages of the rotary intersection are listed below:

1. Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts
between crossing movements.

2. All the vehicles entering the rotary are gently forced to reduce the speed and continue to move
at slower speed. Thus, more of the vehicles need to be stopped.

3. Because of lower speed of negotiation and elimination of severe conflicts, accidents and their
severity are much less in rotaries.

4. Rotaries are self-governing and do not need practically any control by police or traffic signals.

5. They are ideally suited for moderate traffic, especially with irregular geometry, or
intersections with more than three or four approaches.

Although rotaries offer some distinct advantages, there are few specific limitations for rotaries
which are listed below.

1. All the vehicles are forced to slow down and negotiate the intersection. Therefore the
cumulative delay will be much higher than channelized intersection.

2. Even when there is relatively low traffic, the vehicles are forced to reduce their speed.

3. Rotaries require large area of relatively flat land making them costly at urban areas.

4. Since the vehicles are not stopping, and the vehicles accelerate at rotary exits, they are not
suitable when there is a high pedestrian movement.

Traffic operations in a rotary

The traffic operations at a rotary are three; diverging, merging and weaving. All the other
conflicts are converted into these three less severe conflicts.

1. Diverging: It is a traffic operation when the vehicles moving in one direction is separated into
different streams according to their destinations.
 2. Merging: Merging is the opposite of diverging, when traffic streams coming from various
places and going to a common destination are joined together into a single stream it is referred to
as merging.

3. Weaving: Weaving is the combined movement of both the merging and diverging movements
in the same direction.

Design elements

The design elements include design speed, radius at entry, exit and the central island, weaving
length and width, entry and exit widths.

All the vehicles are required to reduce their speed at a rotary. Therefore, the design speed of a
rotary will be much lower than the roads leading to it. Although it is possible to design
roundabout without much speed reduction, the geometry may lead to large size incurring huge
cost of construction. The normal practice is to keep the design speed as 30 and 40 mph for urban
and rural areas respectively.

The radius at the entry depends on various factors like design speed, super elevation, and
coefficient of friction. The entry to the rotary is not straight, but a small curvature is introduced.
This will force the driver to reduce the speed. The speed range of about 20 mph and 25 mph is
ideal for an urban and rural design respectively.

The exit radius should be higher than the entry radius and the radius of the rotary island so that
the vehicles will discharge from the rotary at a higher rate. A general practice is to keep the exit
radius as 1.5 to 2 times the entry radius. However, if pedestrian movement is higher at the exit
approach, then the exit radius could be set as same as that of the entry radius.

The radius of the central island is governed by the design speed, and the radius of the entry
curve. The radius of the central island, in practice, is given a slightly higher reading so that the
movement of the traffic already in the rotary will have priority of movement. The radius of the
central island which is about 1.3 times that of the entry curve is adequate for all practical
purposes.

The entry width and exit width of the rotary is governed by the traffic entering and leaving the
intersection and the width of the approaching road. The width of the carriageway at entry and
exit will be lower than the width of the carriageway at approaches to enable reduction of speed.
Low entry speeds may be achieved by deflecting entering traffic with road markings, islands and
by channelization. The radius of the entry path should not exceed 100 meters.

The width of the weaving section should be higher than the width at entry and exit. Normally this
will be one lane more than the average entry and exit width. Weaving length determines how
smoothly the traffic can merge and diverge. It is decided based on many factors such as weaving
width, proportion of weaving traffic to the non-weaving traffic etc. This can be best achieved by
making the ratio of weaving length to the weaving width very high.

Traffic rotaries reduce the complexity of crossing traffic by forcing them into weaving
operations. The shape and size of the rotary are determined by the traffic volume and share of
turning movements. Capacity assessment of a rotary is done by analyzing the section having the
greatest proportion of weaving traffic.‰

Visibility for entering drivers must be sufficient to allow circulating drivers a stopping sight
distance at the circulating speed. Enforcement of priority is important and additional
enforcement resources may be required where driving behavior is poor.

Facilities for pedestrians to cross the arms of the intersection safely should be provided. ‰

Other measures which have been found to help safety include: improved provision and siting of
signs, making the 'give-way' line more conspicuous, additional chevron signs, the provision of
yellow bar markings on fast dual carriageway approaches, improved skid resistance (micro-
texture is important at low speeds) and the reduction of excessive entry widths by hatching or
physical means.

Figure 7:7 Roundabout Layout

Figure 7:8 Roundabout Dimensions
The following factors influence the choice of selecting a roundabout over some other form
of intersection control:

SAFETY

Roundabouts should not be introduced on rural roads where the design speeds of adjacent
sections are 90 km/h or greater. For design speeds approaching this value, consideration should
be given to the use of rumble strips and warning signs at the approaches to warn the driver to
anticipate the roundabout. Roundabouts are usually more difficult for pedestrians to cross than
normal junctions.

TRAFFIC FLOW

High proportions of turning movements favor roundabouts. Roundabouts should generally be

used if the minor road flow is greater than one third of the major road flow. Roundabouts are
also an advantage where peak flows are 50 percent greater than the average flows.

SITE CONDITIONS

Roundabouts generally take up more land than fully canalized junctions do. The additional land
acquisition costs for roundabouts should be balanced against the increased capacity offered.

DRIVER BEHAVIOR
Roundabouts regularize traffic flow and should reduce accidents as well as increase capacity.

THE GENERAL LAYOUT

The general layout of a roundabout should provide for the following (see Figure 12-12):

• Adequate entry widths

• Adequate circulation space compatible with entry widths

• Central islands of diameter sufficient only to give drivers guidance on the maneuvers expected.

• Deflection of the traffic to the right on entry to promote movement and ensure low traffic
speeds.

• A simple and clear layout

• Suitable visibility at any entry of each adjacent entry

• Entry and exit deflection angles and central island radius should prevent through speeds in
excess of 50 km/h. This is accomplished by maximizing the difference between the shortest
tracks a driver can take through the roundabout, vs. the straight-line distance from an entry to the
opposite exit. No vehicle path should allow a vehicle to traverse the roundabout at a radius
greater than 100 meters (see Figure 12-11)

6.7.1 Types of roundabout

Mini-roundabout

(Fig. 5.15)

Mini-roundabouts can be extremely successful in improving existing urban junctions where side
road delay and safety are a concern. Drivers must be made aware in good time that they are
approaching a roundabout. Mini-roundabouts consist of a 1-way circulatory carriageway around
a reflectorized, flush/slightly raised circular island less than 4 m in diameter which can be
overrun with ease by the wheels of heavy vehicles. It should be domed to a maximum height of
125 mm at the centre for a 4 m diameter island, with the height reduced pro rata for smaller
islands. The approach arms may or may not be flared. Miniroundabouts are used predominantly
in urban areas with speed limits not exceeding 48 km/h (50 mph).

They are never used on highways with high speed limits. In situations where physical deflection
of vehicle paths to the left may be difficult to achieve, road markings should be employed in
order to induce some vehicle deflection/speed reduction. If sufficient vehicle deflection cannot
be achieved, the speed of the traffic on the approach roads can be reduced using traffic calming
techniques.
Because of the short distance between the entry points to the roundabout, drivers arriving at the
intersection must monitor very closely the movements of other vehicles both within the junction
and on the approaches in order to be in a position to react very quickly when a gap occurs.

Normal roundabout (Fig. 5.16)

A normal roundabout is defined as a roundabout having a 1-way circulatory carriageway around

a curbed central island at least 4 m in diameter, with an Inscribed circle diameter (ICD) of at
least 28 m and with flared approaches to allow for multiple vehicle entry. The number of
recommended entry arms is either three or four. If the number is above four, the roundabout
becomes larger with the probability that higher circulatory speeds will be generated. In such
situations double roundabouts may provide a solution.

Double roundabout (Fig. 5.17)

A double roundabout can be defined as an individual junction with two normal/mini-roundabouts

either contiguous or connected by a central link road or curbed island. It may be appropriate in
the following circumstances:

For improving an existing staggered junction where it avoids the need to realign one of the
approach roads

In order to join two parallel routes separated by a watercourse, railway or motorway

At existing crossroads intersections where opposing right-turning movements can be separated

Catering for junctions with more than four entries and overloaded single roundabouts where
overall capacity can be increased by reducing the circulating flow travelling past critically
important entry points.

In situations where the double roundabout is composed of two miniroundabouts, the speed limit
on the approaches must not exceed 48 km/h (30 mph).

Other forms

Other roundabout configurations include two-bridge roundabouts, dumbbell roundabouts, ring

junctions and signalized roundabouts

6.8 GRADE – SEPARATED JUNCTIONS

The circumstances in which the use of a grade separated junction is warranted are usually as
follows: ƒ

 An at-grade junction has insufficient capacity

  The junction is justified economically from the savings in traffic delays and accident
costs ƒ
 Grade separation is cheaper on account of topography or on the grounds that expensive
land appropriation can be avoided by its construction ƒ
 For operational reasons ƒ
 Where roads cross motorways

In deciding on the location of a grade-separated junction, the following factors should be taken
into account:

 Trip length (travel distance)

 Size of urban areas
 Predicted traffic volumes
 Cost of junction
 Congestion control

The use of grade separation results in the separation of traffic movements between the
intersecting roads so that only merging and diverging movements remain. The extent to which
individual traffic movements should be separated from each other depends mainly upon capacity
requirements and traffic safety aspects; it also depends upon the extent to which important traffic
movements should be given free flow conditions.

From a study of conflicting traffic movements, it will generally be apparent which traffic streams
must be grade separated, leaving the other streams to be dealt with by junctions at grade; the
choice of these will depend upon the capacities needed. A study of the characteristics of various
types of grade-separated junctions is necessary, and a number of alternative designs should be
prepared. The final choice of scheme must satisfy capacity requirements, geometric standards,
and operational needs, and represent an economical design. In some instances the choice of a
particular design will be determined by the adoption of two-stage construction, e.g. constructing
an at-grade junction first and providing grade separation later.

6.8.1 Types of Grade separated junctions

Grade separated junctions generally fail into four categories depending upon the number of roads
involved and their relative importance. These categories are as follows:

 Three-way junctions;
 Junctions of major/minor roads;
 Junctions of two major roads; and
 Junctions of more than two major roads.
Each category is discussed briefly below with reference, where appropriate, to the basic line
diagram layouts shown in Figure 13-1. 13.5.1

THREE-WAY JUNCTIONS (LAYOUTS A AND B)

For some Y-junctions where grade separation of only one traffic stream is required, Layout A
may be appropriate. The movements associated with the missing leg would have to be channeled
to another location. This would only be appropriate of the traffic volumes on the missing leg
were slight and were capable of being served by an at-grade junction elsewhere. Layout B shows
a typical three-leg junction. This configuration is appropriate for traffic volumes of up to 30,000
AADT on the four-lane major road (3,000 vehicles per hour). With a single loop lane, it is
appropriate for loop traffic of 1,000 vehicles per hour. Higher loop traffic would require multiple
loop lanes.

JUNCTIONS OF MAJOR/MINOR ROADS (LAYOUTS C AND D)

Layouts C and D are the most simple for major/minor road junctions and both transfer the major
traffic conflicts to the minor road. These configurations are appropriate for traffic volumes of up
to 30,000 AADT on the four-lane major road (3,000 vehicles per hour), with traffic of up to
10,000 ADT on the minor road. They are appropriate for traffic where the major road is DS1 and
the minor road is DS2- DS6. With a single loop lane, it is appropriate for loop traffic of 1,000
vehicles per hour. Higher loop traffic would require multiple loop lanes. Layout C shows the
‘half cloverleaf’ type of junction, which has the advantage of being easily adapted to meet
difficult site conditions. Layout D shows the normal ‘diamond’ junction, which requires the least
land appropriation. The choice between these options is generally dependent on land
requirements.

JUNCTIONS OF TWO MAJOR ROADS (LAYOUTS E AND F)

Layouts E and F show the two basic junction layouts use where high traffic flows would make
the use of simpler layouts unsatisfactory. They are appropriate for traffic volumes on both
crossing roads of between 10,000 and 30,000 AADT (3,000 vehicles per hour). . Layout E shows
a ‘full cloverleaf’ junction involving only one bridge but requiring a large land appropriation.
Layout F shows a typical roundabout interchange involving two bridges. This layout would only
be suitable if the secondary road containing the roundabout was of a low design speed but carried
a comparatively higher volume of traffic.

JUNCTIONS OF MORE THAN TWO MAJOR ROADS

These junctions are difficult to design, operationally difficult, occupy large areas of land and,
requiring numerous bridges, are extremely expensive. This type of junction, although unlikely to
be required on rural roads in Ethiopia, can often be reduced by changes in the major road
alignments, which will simplify the traffic pattern, to a combination of the more simple and
economic layouts described above

Figure 7:9 Typical Layouts for Grade-Separated Junctions

Interchange
Interchange is a system where traffic between two or more roadways flows at different level.

Different types are

1. Diamond interchange: It is the most popular form of four-leg interchange found in the urban
locations where major and minor roads crosses. The important feature of this interchange is that
it can be designed even if the major road is relatively narrow.
2. Clover leaf interchange: It is also a four leg interchange and is used when two highways of
high volume and speed intersect each other. The main advantage of cloverleaf intersection is that
it provides complete separation of traffic. Also high speed at intersections can be achieved. But
the disadvantage is that large area of land is required.

3. Trumpet interchange: It is a three leg interchange. If one of the legs of the interchange meets a
highway at some angle but does not cross it, then the interchange is called trumpet interchange.

6.8.2 Geometric Standards

The geometric standards given in this module for roads and at-grade junctions also apply to
grade separated junctions. However, the low design speeds of loops and other ancillary roads
necessitate further standards to be given. These are described below፡-

DESIGN SPEED

The design speed for the through traffic movements shall be determined. Stopping sight
distances appropriate for the design speed should always be provided. Where a dual carriageway
intersects with another dual carriageway, the junction between the facilities shall be affected in
such a manner that the loop roads do not entail any significant reduction in the design speeds of
the crossing carriageways.

ACCELERATION AND DECELERATION LANES

The minimum standards to be applied for right turn deceleration lanes are the same as for atgrade
junctions. The total length of the acceleration lane (i.e. not including the merging taper) shall
never be less than 150 meters or more than 400 meters.

HORIZONTAL CURVES AND SUPERELEVATION

The maximum superelevation for loops shall be e = 8% which, at a design speed of 50 km/h,
leads to a minimum radius of 80 meters. Where smaller radii are unavoidable, warning signs will
be necessary. It is important where transitions occur from high to low speeds that the curves
should be compound or transitional, the radius at any point being appropriate for the vehicle
speed at that point.

VERTICAL CURVES

To ensure reasonable standards of visibility, comfort and appearance, vertical curves should be
introduced at all changes in gradient. Vertical curve lengths should be determined in accordance
with to provide safe stopping sight distances.

GRADIENTS
Maximum gradient shall be 8%.

WIDTHS OF LOOPS

The minimum carriageway width for loops on straight sections and horizontal radii greater than
150m shall be 4.0m with shoulders of 1.5 meters on the near side and 1.0 meter on the far side
(widened by 0.5 meter where guard rail is required).

GRADIENTS

For loops, an up gradient of 5% and a down gradient of 7% should normally be regarded as

maximal.

CLEARANCES

The required vertical and horizontal clearances shall be in accordance with

CAPACITY

Grade-separated junctions are generally designed using traffic volumes given in Daily High
Volume (DHV) rather than Annual Average Daily Traffic (AADTs). A detailed traffic study and
analysis can be made to determine these values. In the absence of such a study, it can be assumed
that DHV, in an urban area, is 10% of AADT. It is also a good estimate of vehicles per hour. The
capacity of each traffic lane, in DHV, is usually given as 1000 vehicles per hour.

Dual carriageway Design Standard DS1 indicates a design traffic flow of 10,000 to 15,000
AADT. The capacity of this facility would be exceeded at more than 1000 vehicles per hour per
lane, which equates to 4000 vehicles per hour for all four lanes, and approximately is 40,000
AADT. In practice, this volume is undesirable, and the volumes of between 10,000-15,000 are
appropriate for design. These DHV values are necessary in choosing the number of lanes for the
loops corresponding to the junction.

MINIMUM SPACING

The distance between two successive grade-separated junctions is an element of great importance
in ensuring the desired level of service. In suburban zones, therefore, it is necessary to establish a
minimum distance between successive grade-separated junctions. The recommended minimum
distance is 2.0km.

6.8.3 Design Principles

Special design principles apply to grade separated junctions and must be considered when
comparing the characteristics of alternative designs. The main principles and described below:
1. The high speeds normally met with on roads where grade separation is required and the low
design speeds of ancillary roads make it necessary to pay particular attention to the transitions
between high and low speed. This not only influences the use of long speed-change lanes and
compound curves but also the choice of types of interchange which do not result in abrupt
changes in vehicle speeds.

2. Weaving between lanes on the main roadway within the interchange is undesirable and can be
avoided by arranging for diverging points to precede merging points.

3. On a road with a large number of grade-separated junctions, a consistent design speed is

desirable for loops. This speed shall be not less than 65% of the speed of the adjoining major
road.

4. As a general rule, left-turning movements that are grade separated should be made through a
right-hand loop.

5. Unexpected prohibited traffic movements, especially where traffic is light, are difficult to
enforce and cause danger. If possible the geometric layout should be designed to make
prohibited movements difficult, e.g. on one-way loops entry contrary to the one way movement
can be restricted by the use of suitably shaped traffic islands to supplement the traffic signs.

6.9 Intersection Sight Distance

Intersection sight distance (ISD) is the minimum sight distance needed by drivers to safely
negotiate intersections, including intersections with or without stop controls or traffic signals.

Provide sufficient sight distance to allow drivers to perceive the presence of potentially
conflicting vehicles. If possible, provide decision sight distance (DSD) for the approach to
intersections, if it is greater than the ISD.

Determine the sight distances required for vehicles to turn left from a stop onto a two-lane
highway and attain an average running speed without being overtaken by a vehicle going the
same direction.

Where practical, avoid using sight distances less than that required for the design vehicle, which
will require the through traffic to reduce speed. For approach to at-grade intersection provide
sufficient sight distance for an unobstructed view of the entire intersection and sufficient length
of the intersecting roadway to discern the movements of vehicles.

For intersections with stop signs on the minor road provide sight distance of the major highway
to safely cross before a vehicle on the major highway reaches the intersection. Under some
conditions, if it is impractical to provide adequate site distance for cross road traffic to safely
enter the main road, it may be necessary to install traffic signals.
Provide sight triangles along the intersection approach legs that are clear of obstructions that can
block driver’s view of oncoming traffic. The dimensions of the triangle are based on the design
speed of the intersecting roadways and the type of traffic control used at the intersection, grades
on the roadways, and the roadway width.

Figure 7:10 Sight distance at intersections

Within the sight triangle, remove, adjust or lower cut slopes, hedges, trees, signs, utility poles or
anything large enough to constitute a sight obstruction. Eliminate parking and offset signs to
prevent sight distance obstructions.

Determine ISD for all applicable intersection maneuvers, including situations described in the
Green Book for through, left and right-turning maneuvers at intersections with no control,
fourway stop control, two-way stop control, yield control and signal control from the minor road;
and for a left-turning maneuver from the major road.

Provide additional intersection sight distance wherever significant visual distractions, messages
or driver workload exists, for example where there are:

 High traffic volumes on the major road;

 Complex signs (e.g. multiple destinations, route shield assemblies);
 Complex pavement markings (e.g. multiple turn lanes);
 Complex or unusual intersection geometry;
 Visual distractions in urban areas due to commercial signs and lighting; and
 A high percentage of unfamiliar or older drivers.

Also provide additional intersection sight distance wherever drivers are less likely to be
expecting to respond to an intersection, such as for:

 A stop condition after having the right-of-way on previous road sections;

  An isolated stop or signal-controlled intersection; and
 Intersections with high traffic volume, but signals are not yet warranted.

For these situations, ISD is a minimum and it is preferable to provide DSD. For the following
conditions, the sight distance for cross traffic to enter the roadway may need to be lengthened:

 Turning right through the minor angle of skew intersection (i.e., where drivers must turn
their heads through a greater angle to assess the presence of oncoming vehicles);
 Crossing or turning at an intersection on a horizontal curve, especially where the main
road curves behind the driver gap, may be more difficult to assess; and
 Crossing at an offset or skewed intersection, and Trucks turning.

Consider the need for additional sight distance where:

 The major road has complex signing, lane drops or other driver-attention demands prior
to the intersection,
 Traffic conditions or site information indicates problems accommodating entering traffic,
and
 At left-turn lanes where the decision to initiate the turn may occur significantly in
advance of intersection.

At intersections, consider the driver’s view of the intersection from all approaches. Of major
concern are intersections where a driver may fail to recognize a potential conflict location. An
example may be where an approach road intersects a divided roadway and the driver perceives
the intersection across the median as the primary concern, and does not recognize the initial
intersection. Evaluation of the driver’s viewpoint with respect to the signing and pavement
markings should be considered during the layout of the intersection.

Reference

[1] AASHTO (2018) A Policy on Geometric Design of Highways and Streets. 7th Edition, the
American Association of State Highway and Transportation Officials,

Washington DC.
[2] AASHTO (2011) A Policy on Geometric Design of Highways and Streets. 6th Edition, The
American Association of State Highway and Transportation Officials,

Washington DC.