Cw#5

Road surface
From Wikipedia, the free encyclopedia

A road being resurfaced

A road surface or pavement is the durable surface material laid down on an area intended to
sustain vehicular or foot traffic, such as aroad or walkway. In the past, gravel
road surfaces, cobblestone and granite setts were extensively used, but these surfaces have mostly
been replaced by asphalt or concrete laid on a compacted base course. Road surfaces are
frequently marked to guide traffic. Today,permeable paving methods are beginning to be used for
low-impact roadways and walkways.

Contents
[hide]

1Asphalt
2Concrete
3Composite pavement
4Recycling
o 4.1In-place recycling
5Bituminous surface
o 5.1Thin membrane surface
o 5.2Otta seal
6Gravel surface
7Other surfaces
8Acoustical implications
9Surface deterioration
10Markings
11See also
12Notes
13External links

Asphalt[edit]
Main article: Asphalt concrete
Closeup of asphalt on a driveway

Asphalt (specifically, asphalt concrete), sometimes called flexible pavement due to the nature in
which it distributes loads, has been widely used since the 1920s. The viscous nature of
the bitumen binder allows asphalt concrete to sustain significant plastic deformation,
althoughfatigue from repeated loading over time is the most common failure mechanism. Most
asphalt surfaces are laid on a gravel base, which is generally at least as thick as the asphalt layer,
although some 'full depth' asphalt surfaces are laid directly on the native subgrade. In areas with
very soft or expansive subgrades such as clay or peat, thick gravel bases or stabilization of the
subgrade with Portland cement or limemay be required. Polypropylene and
polyester geosynthetics have also been used for this purpose[1] and in some northern countries, a
layer of polystyrene boards have been used to delay and minimize frost penetration into the
subgrade.[2]
Depending on the temperature at which it is applied, asphalt is categorized as hot mix, warm mix, or
cold mix. Hot mix asphalt is applied at temperatures over 300 F (150 C) with a free floating screed.
Warm mix asphalt is applied at temperatures of 200250 F (95120 C), resulting in reduced
energy usage and emissions of volatile organic compounds.[3] Cold mix asphalt is often used on
lower-volume rural roads, where hot mix asphalt would cool too much on the long trip from
the asphalt plant to the construction site.[4]
An asphalt concrete surface will generally be constructed for high-volume primary highways having
an average annual daily traffic load greater than 1200 vehicles per day.[5]Advantages of asphalt
roadways include relatively low noise, relatively low cost compared with other paving methods, and
perceived ease of repair. Disadvantages include less durability than other paving methods, less
tensile strength than concrete, the tendency to become slick and soft in hot weather and a certain
amount of hydrocarbon pollution to soil and groundwater or waterways.
In the mid-1960s, rubberized asphalt was used for the first time, mixing crumb rubber from used tires
with asphalt.[6] While a potential use for tires that would otherwise fill landfills and present a fire
hazard, rubberized asphalt has shown greater incidence of wear in freeze-thaw cycles in temperate
zones due to non-homogeneous expansion and contraction with non-rubber components. The
application of rubberized asphalt is more temperature-sensitive, and in many locations can only be
applied at certain times of the year.[citation needed]
Study results of the long-term acoustic benefits of rubberized asphalt are inconclusive. Initial
application of rubberized asphalt may provide 35 decibels (dB) reduction in tire-pavement source
noise emissions; however, this translates to only 13 decibels (dB) in total traffic noise level
reduction (due to the other components of traffic noise). Compared to traditional passive attenuating
measures (e.g., noise walls and earth berms), rubberized asphalt provides shorter-lasting and lesser
acoustic benefits at typically much greater expense.[citation needed]

Concrete[edit]
Concrete roadway in San Jose,California

Further information: Concrete

Concrete surfaces (specifically, Portland cement concrete) are created using a concrete mix
of Portland cement, coarse aggregate, sandand water. In virtually all modern mixes there will also be
various admixtures added to increase workability, reduce the required amount of water, mitigate
harmful chemical reactions and for other beneficial purposes. In many cases there will also be
Portland cement substitutes added, such as fly ash. This can reduce the cost of the concrete and
improve its physical properties. The material is applied in a freshly mixed slurry, and worked
mechanically to compact the interior and force some of the cement slurry to the surface to produce a
smoother, denser surface free from honeycombing. The water allows the mix to combine molecularly
in a chemical reaction called hydration.
Concrete surfaces have been refined into three common types: jointed plain (JPCP), jointed
reinforced (JRCP) and continuously reinforced (CRCP). The one item that distinguishes each type is
the jointing system used to control crack development.

Jointed plain concrete pavements contain enough joints to control the location of all the
expected shrinkage cracks. The concrete cracks at the joints and not elsewhere in the slabs.
Jointed plain pavements do not contain any steel reinforcement. However, there may be smooth
steel bars at transverse joints and deformed steel bars at longitudinal joints. The spacing
between transverse joints is typically about 15 feet (4.6 m) for slabs 7 to 12 inches (180 to
300 mm) thick. Today, a majority of US state agencies build jointed plain pavements.
Jointed reinforced concrete pavements contain steel mesh reinforcement (sometimes called
distributed steel). In jointed reinforced concrete pavements, designers increase the joint spacing
purposely, and include reinforcing steel to hold together intermediate cracks in each slab. The
spacing between transverse joints is typically 30 feet (9.1 m) or more. In the past, some
agencies used a spacing as great as 100 feet (30 m). During construction of the interstate
system, most agencies in the Eastern and Midwestern United States laid jointed-reinforced
pavement. Today only a handful of agencies employ this design, and its use is generally not
recommended[by whom?] as both of the other types offer better performance and are easier to repair.
Continuously reinforced concrete pavements do not require any transverse contraction joints.
Transverse cracks are expected in the slab, usually at intervals of 3 to 5 ft (0.91 to 1.52 m).
These pavements are designed with enough steel, 0.60.7% by cross-sectional area, so that
cracks are held together tightly. Determining an appropriate spacing between the cracks is part
of the design process for this type of pavement.
A concrete road in Ewing, New Jersey. The original pavement was laid in the 1950s and has not been
significantly altered since.

Continuously reinforced designs may cost slightly more than jointed reinforced or jointed plain
designs due to increased quantities of steel. Often the cost of the steel is offset by the reduced cost
of concrete because a continuously reinforced design is nearly always significantly thinner than a
jointed design for the same traffic loads. Properly designed, the two methods should demonstrate
similar long-term performance and cost-effectiveness. A number of agencies have made policy
decisions to use continuously reinforced designs in their heavy urban traffic corridors.
One of the major advantages of concrete pavements is they are typically stronger and more durable
than asphalt roadways. They also can be grooved to provide a durable skid-resistant surface. A
notable disadvantage is that they typically can have a higher initial cost, and can be more time-
consuming to construct. This cost can typically be offset through the long life cycle of the pavement.
Concrete pavement can be maintained over time utilizing a series of methods known as concrete
pavement restoration which include diamond grinding, dowel bar retrofits, joint and crack sealing,
cross-stitching, etc. Diamond grinding is also useful in reducing noise and restoring skid resistance
in older concrete pavement.[7][8]
The first street in the United States to be paved with concrete was Court Avenue in Bellefontaine,
Ohio in 1893.[9][10] The first mile of concrete pavement in the United States was on Woodward
Avenue in Detroit, Michigan in 1909.[11]

Composite pavement[edit]

An example of composite pavement: hot-mix asphalt overlaid onto Portland cement concrete pavement

Composite pavements combine a Portland cement concrete sublayer with an asphalt. They are
usually used to rehabilitate existing roadways rather than in new construction.
Asphalt overlays are sometimes laid over distressed concrete to restore a smooth wearing
surface.[12] A disadvantage of this method is that movement in the joints between the underlying
concrete slabs, whether from thermal expansion and contraction, or from deflection of the concrete
slabs from truck axle loads, usually causes reflective cracks in the asphalt. To decrease reflective
cracking, concrete pavement is broken apart through a break and seat, crack and seat,
or rubblization process. Geosynthetics can be used for reflective crack control.[13] With break and
seat and crack and seat processes, a heavy weight is dropped on the concrete to induce cracking,
then a heavy roller is used to seat the resultant pieces into the subbase. The main difference
between the two processes is the equipment used to break the concrete pavement and the size of
the resulting pieces. The theory is frequent small cracks will spread thermal stress over a wider area
than infrequent large joints, reducing the stress on the overlying asphalt pavement. Rubblization is a
more complete fracturing of the old, worn-out concrete, effectively converting the old pavement into
an aggregate base for a new asphalt road.[14]
Whitetopping uses Portland cement concrete to resurface a distressed asphalt road.

Recycling[edit]
An asphalt milling machine in Boise, Idaho.

Distressed road materials can be reused when rehabilitating a roadway. The existing pavement is
ground or broken up into small pieces, through a process called milling. It can then be transported to
an asphalt or concrete plant and incorporated into new pavement, or recycled in place to form the
base or subbase for new pavement. Some methods used include:
In-place recycling[edit]

Rubblizing of concrete pavement. Existing concrete pavement is broken into gravel-sized

particles. Any steel reinforcing is removed, then the remaining gravel-sized particles are
compacted and overlaid with asphalt pavement.[15]
Cold in-place recycling. Bituminous pavement is ground or milled into small particles. The
asphalt millings are blended with a small amount of asphalt emulsion or foamed bitumen, paved
and compacted, allowed to cure for seven to ten days, then overlaid with asphalt.[16]
Hot in-place recycling. Bituminous pavement is heated to 250 to 300 F (120 to 150 C), milled,
combined with a rejuvenating agent or virgin asphalt binder, and compacted. It may then be
overlaid with a new asphalt overlay. This process only recycles the top two inches (50 mm) or
less, so it can be used to correct rutting, polishing or other surface defects. It is not a good
procedure for roads with structural failures. It also generates high heat and vapor emissions, and
may not be a good candidate for built-up areas.[16]
Full depth reclamation is a process which pulverizes the full thickness of the asphalt pavement
and some of the underlying material to provide a uniform blend of material. A binding agent may
be mixed in to form a base course for the new pavement, or it may be left unbound to form a
subbase course. Common binding agents include asphalt emulsion, fly ash, Portland cement or
calcium chloride. It can also be mixed with aggregate, recycled asphalt millings, or crushed
Portland cement to improve the gradation of the material, and can provide a design life cycle of
30 years with proper lab testing and field verification.[16][17]

Bituminous surface[edit]
Main article: Chipseal
Bituminous surface treatment (BST) or chipseal is used mainly on low-traffic roads, but also as a
sealing coat to rejuvenate an asphalt concrete pavement. It generally consists of aggregate spread
over a sprayed-on asphalt emulsion or cut-back asphalt cement. The aggregate is then embedded
into the asphalt by rolling it, typically with a rubber-tired roller. This type of surface is described by a
wide variety of regional terms including "chip seal", "tar and chip", "oil and stone", "seal coat",
"sprayed seal"[18] or "surface dressing"[19] or as simply "bitumin."
BST is used on hundreds of miles of the Alaska Highway and other similar roadways in Alaska,
the Yukon Territory, and northern British Columbia. The ease of application of BST is one reason for
its popularity, but another is its flexibility, which is important when roadways are laid down over
unstable terrain that thaws and softens in the spring.
Other types of BSTs include micropaving, slurry seals and Novachip. These are laid down using
specialized and proprietary equipment. They are most often used in urban areas where the
roughness and loose stone associated with chip seals is considered undesirable.
Thin membrane surface[edit]
A thin membrane surface (TMS) is an oil-treated aggregate which is laid down upon a gravel
road bed, producing a dust-free road.[20] A TMS road reduces mud problems and provides stone-free
roads for local residents where loaded truck traffic is negligible. The TMS layer adds no significant
structural strength, and so is used on secondary highways with low traffic volume and minimal
weight loading. Construction involves minimal subgrade preparation, following by covering with a 50-
to-100-millimetre (2.03.9 in) cold mixasphalt aggregate.[5] The Operation Division of the Ministry of
Highways and Infrastructure in Saskatchewan has the responsibility of maintaining 6,102 kilometres
(3,792 mi) of thin membrane surface (TMS) highways.[21]
Otta seal[edit]
Otta seal is a low-cost road surface using a 1630-millimetre (0.631.18 in) thick mixture
of bitumen and crushed rock.[22]

Gravel surface[edit]
Main article: Gravel road
Gravel is known to have been used extensively in the construction of roads by soldiers of the Roman
Empire (see Roman road) but in 1998 a limestone-surfaced road, thought to date back to the Bronze
Age, was found at Yarnton in Oxfordshire, Britain.[23] Applying gravel, or "metalling," has had two
distinct usages in road surfacing. The term road metalrefers to the broken stone or cinders used in
the construction or repair of roads or railways,[24] and is derived from the Latin metallum, which
means both "mine" and "quarry".[25]The term originally referred to the process of creating a gravel
roadway. The route of the roadway would first be dug down several feet and, depending on local
conditions, French drains may or may not have been added. Next, large stones were placed and
compacted, followed by successive layers of smaller stones, until the road surface was composed of
small stones compacted into a hard, durable surface. "Road metal" later became the name
of stone chippings mixed with tar to form the road surfacing material tarmac. A road of such material
is called a "metalled road" in Britain, a "paved road" in Canada and the US, or a "sealed road" in
parts of Canada, Australia and New Zealand.[26]
A granular surface can be used with a traffic volume where the annual average daily traffic is 1,200
vehicles per day or less.[citation needed] There is some structural strength as the road surface combines a
sub base and base and is topped with a double graded seal aggregate with emulsion.[5][27] Besides
the 4,929 kilometres (3,063 mi) of granular pavements maintained in Saskatchewan, around 40%
of New Zealand roads are unbound granular pavement structures.[21][28]
The decision whether to pave a gravel road or not often hinges on traffic volume. It has been found
that maintenance costs for gravel roads often exceed the maintenance costs for paved or surface-
treated roads when traffic volumes exceed 200 vehicles per day.[29]
Some communities are finding it makes sense to convert their low-volume paved roads to aggregate
surfaces.[30]

Other surfaces[edit]
Pavers (or paviours), generally in the form of pre-cast concrete blocks, are often used for aesthetic
purposes, or sometimes at port facilities that see long-duration pavement loading. Pavers are rarely
used in areas that see high-speed vehicle traffic.
Brick, cobblestone, sett, wood plank, and wood block pavements such as Nicolson pavement, were
once common in urban areas throughout the world, but fell out of fashion in most countries, due to
the high cost of labor required to lay and maintain them, and are typically only kept for historical or
aesthetic reasons.[citation needed] In some countries, however, they are still common in local streets. In
the Netherlands, brick paving has made somewhat of a comeback since the adoption of a major
nationwide traffic safety program in 1997. From 1998 through 2007, more than 41,000 km of city
streets were converted to local access roads with a speed limit of 30 km/h, for the purpose of traffic
calming.[31]One popular measure is to use brick paving - the noise and vibration slows motorists
down. At the same time, it is not uncommon for cycle paths alongside a road to have a smoother
surface than the road itself.[32][33]
Likewise, macadam and tarmac pavements can still sometimes[when?] be found buried underneath
asphalt concrete or Portland cement concrete pavements, but are rarely[clarification needed] constructed
today[when?].
There are also other methods and materials to create pavements that have appearance of brick
pavements. The first method to create brick texture is to heat an asphalt pavement and use metal
wires to imprint a brick pattern using a compactor to create stamped asphalt. A similar method is to
use rubber imprinting tools to press over a thin layer of cement to create decorative concrete.
Another method is to use a brick pattern stencil and apply a surfacing material over the stencil.
Materials that can be applied to give the color of the brick and skid resistance can be in many forms.
An example is to use colored polymer-modified concrete slurry which can be applied by screeding or
spraying.[34]Another material is aggregate-reinforced thermoplastic which can be heat applied to the
top layer of the brick-pattern surface.[35] Other coating materials over stamped asphalt are paints and
two-part epoxy coating.[36]

Concrete pavers

Replacing the old road with concrete blocks in Bo'ao Road area, HaikouCity, Hainan, China


Polymer cement overlaying to change asphalt pavement to brick texture and color to create decorative
crosswalk

Acoustical implications[edit]
Roadway surfacing choices are known to affect the intensity and spectrum of sound emanating from
the tire/surface interaction.[37] Initial applications of noise studies occurred in the early 1970s. Noise
phenomena are highly influenced by vehicle speed.
Roadway surface types contribute differential noise effects of up to 4 dB, with chip seal type and
grooved roads being the loudest, and concrete surfaces without spacers being the
quietest. Asphaltic surfaces perform intermediately relative to concrete and chip seal. Rubberized
asphalt has been shown to give a marginal 35 dB reduction in tire-pavement noise emissions, and
a marginally discernible 13 dB reduction in total road noise emissions when compared to
conventional asphalt applications.

Cobbles

Rectangles


Decorative wavy pattern onLa Rambla.

Decorative mock-brick pattern.

More decorative brickwork patterns.

Surface deterioration[edit]
See also: Pothole, Crocodile cracking, Rut (roads), and Bleeding (roads)

Deteriorating asphalt
As pavement systems primarily fail due to fatigue (in a manner similar to metals), the damage done
to pavement increases with the fourth power of the axle load of the vehicles traveling on it.
According to the AASHO Road Test, heavily loaded trucks can do more than 10,000 times the
damage done by a normal passenger car. Tax rates for trucks are higher than those for cars in most
countries for this reason, though they are not levied in proportion to the damage done.[38] Passenger
cars are considered to have little practical effect on a pavement's service life, from a materials
fatigue perspective.
Other failure modes include aging and surface abrasion. As years go by, the binder in a
bituminous wearing course gets stiffer and less flexible. When it gets "old" enough, the surface will
start losing aggregates, and macrotexture depth increases dramatically. If no maintenance action is
done quickly on the wearing course, potholes will form. The freeze-thaw cycle in cold climates will
dramatically accelerate pavement deterioration, once water can penetrate the surface.
If the road is still structurally sound, a bituminous surface treatment, such as a chipseal or surface
dressing can prolong the life of the road at low cost. In areas with cold climate, studded tires may be
allowed on passenger cars. In Sweden and Finland, studded passenger car tires account for a very
large share of pavement rutting.
The physical properties of a stretch of pavement can be tested using a falling weight deflectometer.
Several design methods have been developed to determine the thickness and composition of road
surfaces required to carry predicted traffic loads for a given period of time. Pavement design
methods are continuously evolving. Among these are the Shell Pavement design method, and
the American Association of State Highway and Transportation Officials (AASHTO) 1993 "Guide for
Design of Pavement Structures". A new mechanistic-empirical design guide has been under
development by NCHRP (Called Superpave Technology) since 1998. A new design guide called
Mechanistic Empirical Pavement Design Guide (MEPDG) was developed and is about to be adopted
by AASHTO.
Further research by University College London into pavements has led to the development of an
indoor, 80-sq-metre artificial pavement at a research centre called Pedestrian Accessibility and
Movement Environment Laboratory (PAMELA). It is used to simulate everyday scenarios, from
different pavement users to varying pavement conditions.[39] There also exists a research facility
near Auburn University, the NCAT Pavement Test Track, that is used to test experimental asphalt
pavements for durability.
In addition to repair costs, the condition of a road surface has economic effects for road
users. Rolling resistance increases on rough pavement, as does wear and tear of vehicle
components. It has been estimated that poor road surfaces cost the average US driver $324 per
year in vehicle repairs, or a total of $67 billion. Also, it has been estimated that small improvements
in road surface conditions can decrease fuel consumption between 1.8 and 4.7%.[40]

Markings[edit]
Main article: Road surface marking
Road surface markings are used on paved roadways to provide guidance and information to drivers
and pedestrians. It can be in the form of mechanical markers such as cat's eyes, botts'
dots and rumble strips, or non-mechanical markers such as paints, thermoplastic, plastic and epoxy.

https://en.wikipedia.org/wiki/Road_surface
 Road Alignment Consideration

2.5.1 Alignment

The position or the layout of the central line of the highway on the ground is called the
alignment. Horizontal alignment includes straight and curved paths. Vertical alignment includes
level and gradients. Alignment decision is important because a bad alignment will enhance the
construction, maintenance and vehicle operating cost. Once an alignment is fixed and
constructed, it is not easy to change it due to increase in cost of adjoining land and construction
of costly structures by the roadside.

Requirements

The requirements of an ideal alignment are

The alignment between two terminal stations should be short and as far as possible be straight,
but due to some practical considerations deviations may be needed.

The alignment should be easy to construct and maintain. It should be easy for the operation of
vehicles. So to the maximum extend easy gradients and curves should be provided.

It should be safe both from the construction and operating point of view especially at slopes,
embankments, and cutting. It should have safe geometric features.

The alignment should be economical and it can be considered so only when the initial cost,
maintenance cost, and operating cost is minimum.
Factors controlling alignment

We have seen the requirements of an alignment. But it is not always possible to satisfy all these
requirements.

Hence we have to make a judicial choice considering all the factors.

The various factors that control the alignment are as follows:

obligatory points These are the control points governing the highway alignment. These
points are classified into two categories. Points through which it should pass and points through
which it should not pass.
Some of the examples are:
 bridge site: The bridge can be located only where the river has straight and permanent
path and also where the abutment and pier can be strongly founded. The road approach
to the bridge should not be curved and skew crossing should be avoided as possible.
Thus to locate a bridge the highway alignment may be changed.

mountain: While the alignment passes through a mountain, the various alternatives are
to either construct a tunnel or to go round the hills. The suitability of the alternative
depends on factors like topography, site conditions and construction and operation
cost.

intermediate town: The alignment may be slightly deviated to connect an intermediate

town or village nearby.

These were some of the obligatory points through which the alignment should pass. Coming to
the second category, that is the points through which the alignment should not pass are:

religious places: These have been protected by the law from being acquired for any
purpose. Therefore, these points should be avoided while aligning.

very costly structures: Acquiring such structures means heavy compensation which
would result in an increase in initial cost. So the alignment may be deviated not to pass
through that point.

lakes/ponds etc: The presence of a lake or pond on the alignment path would also
necessitate deviation of the alignment.

Traffic: The alignment should suit the traffic requirements. Based on the origin-destination
data of the area, the desire lines should be drawn. The new alignment should be drawn keeping
in view the desire lines, traffic flow pattern etc.
Geometric design: Geometric design factors such as gradient, radius of curve, sight distance
etc. also governs the alignment of the highway. To keep the radius of curve minimum, it may be
required to change the alignment of the highway. The alignments should be finalized such that
the obstructions to visibility do not restrict the minimum requirements of sight distance. The
design standards vary with the class of road and the terrain and accordingly the highway should
be aligned.

Economy: The alignment finalized should be economical. All the three costs i.e. construction,
maintenance, and operating cost should be minimum. The construction cost can be decreased
much if it is possible to maintain a balance between cutting and filling. Also try to avoid very
high embankments and very deep cuttings as the construction cost will be very higher in these
cases.
Other considerations : various other factors that govern the alignment are drainage
considerations, political factors and monotony.

Drainage :Add

Political: If a foreign territory comes across a straight alignment, we will have to deviate
the alignment around the foreign land.

Monotony: For a at terrain it is possible to provide a straight alignment, but it will be

monotonous for driving. Hence a slight bend may be provided after a few kilometers of
straight road to keep the driver alert by breaking the monotony.

Hydrological (rainfall/water table):add

Special consideration for hilly areas

Stability of the slopes

Hill side drainage
Special geometric standards
Ineffective rise and fall

Geometric design of roads

From Wikipedia, the free encyclopedia

The examples and perspective in this article deal primarily with the United
States and do not represent a worldwide view of the subject. You
may improve this article, discuss the issue on the talk page, or create a new
article, as appropriate.(April 2009) (Learn how and when to remove this template
message)

The Autova del Olivar which unitesbeda with Estepa in Andalucia in southern Spain. A geometric design
saved on construction costs and improved visibility with the intention to reduce the likelihood of traffic incidents

The geometric design of roads is the branch of highway engineering concerned with the
positioning of the physical elements of the roadway according to standards and constraints. The
 basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and
environmental damage. Geometric design also affects an emerging fifth objective called "livability,"
which is defined as designing roads to foster broader community goals, including providing access to
employment, schools, businesses and residences, accommodate a range of travel modes such as
walking, bicycling, transit, and automobiles, and minimizing fuel use, emissions and environmental
damage.[1]
Geometric roadway design can be broken into three main parts: alignment, profile, and cross-
section. Combined, they provide a three-dimensional layout for a roadway.
The alignment is the route of the road, defined as a series of horizontal tangents and curves.
The profile is the vertical aspect of the road, including crest and sag curves, and the straight grade
lines connecting them.
The cross section shows the position and number of vehicle and bicycle lanes and sidewalks,
along with their cross slope or banking. Cross sections also show drainage features, pavement
structure and other items outside the category of geometric design.

Contents
[hide]

1Design standards
2Profile
o 2.1Terminology
o 2.2Sag Curves
o 2.3Crest Curves
3Alignment
o 3.1Terminology
o 3.2Geometry
o 3.3Curve sight Distance
4Cross section
o 4.1Lane width
o 4.2Cross slope
5Safety effects of road geometry
o 5.1Design consistency
o 5.2Safety effects of alignment
o 5.3Safety effects of cross section
6Sight distance
o 6.1Stopping sight distance
o 6.2Decision sight distance
o 6.3Intersection sight distance
6.3.1Corner sight distance
6.3.1.1Uncontrolled and yield controlled intersections
6.3.1.2Two-way stop control
6.3.1.3All-way stop control and signalized intersections
o 6.4Effects of insufficient sight distance
7See also
o 7.1Road standards setting bodies
8Notes
9References
o 9.1Law Reviews
 Design standards[edit]
Roads are designed in conjunction with design guidelines and standards. These are adopted by
national and sub-national authorities (e.g., states, provinces, territories and municipalities). Design
guidelines take into account speed, vehicle type, road grade (slope), view obstructions, and stopping
distance. With proper application of guidelines, along with good engineering judgement, an engineer
can design a roadway that is comfortable, safe, and appealing to the eye.[citation needed]
The primary US guidance is found in A Policy on Geometric Design of Highways and
Streets published by the American Association of State Highway and Transportation
Officials(AASHTO).[2] Other standards include the Australian Guide to Road Design, and the
British Design Manual for Roads. An open source version of the green book is published online
by The Council for Scientific and Industrial Research (CSIR) office in Zimbabwe.[3]

Profile[edit]
The profile of a road consists of road slopes, called grades, connected by parabolic vertical curves.
Vertical curves are used to provide a gradual change from one road slope to another, so that
vehicles may smoothly navigate grade changes as they travel.
Sag vertical curves are those that have a tangent slope at the end of the curve that is higher than
that of the beginning of the curve. When driving on a road, a sag curve would appear as a valley,
with the vehicle first going downhill before reaching the bottom of the curve and continuing uphill or
level.
Crest vertical curves are those that have a tangent slope at the end of the curve that is lower than
that of the beginning of the curve. When driving on a crest curve, the road appears as a hill, with the
vehicle first going uphill before reaching the top of the curve and continuing downhill.
The profile also affects road drainage. Very flat roads and sag curves may have poor drainage, and
steep roads have high velocity flows.
Terminology[edit]

BVC = Beginning of Vertical Curve

EVC = End of Vertical Curve

= initial roadway grade, expressed in percent

= final roadway grade, expressed in percent

A = absolute value of the difference in grades (initial minus final), expressed in percent

= Height of eye above roadway, measured in meters or feet

= Height of object above roadway, measured in meters or feet

L = curve length (along the x-axis)
PVI = point of vertical interception (intersection of initial and final grades)
tangent elevation = elevation of a point along the initial tangent
x = horizontal distance from BVC
Y (offset) = vertical distance from the initial tangent to a point on the curve
Y = curve elevation = tangent elevation - offset [2]
Sag Curves[edit]
Sag vertical curves are curves which, when viewed from the side, are concave upwards. This
includes vertical curves at valley bottoms, but it also includes locations where an uphill grade
becomes steeper, or a downhill grade becomes less steep.
The most important design criterion for these curves is headlight sight distance.[2] When a driver is
driving on a sag curve at night, the sight distance is limited by the higher grade in front of the vehicle.
This distance must be long enough that the driver can see any obstruction on the road and stop the
vehicle within the headlight sight distance. The headlight sight distance (S) is determined by the
angle of the headlight and angle of the tangent slope at the end of the curve. By first finding the
headlight sight distance (S) and then solving for the curve length (L) in each of the equations below,
the correct curve length can be determined. If the S<L curve length is greater than the headlight
sight distance, then this number can be used. If it is smaller, this value cannot be used. Similarly, if
the S>L curve length is smaller than the headlight sight distance, then this number can be used. If it
is larger, this value cannot be used.[4]

Units Sight Distance < Curve Length (S<L) Sight Distance > Curve Length (S>L)

Metric

US Customary

These equations assume that the headlights are 600 millimetres (2.0 ft) above the ground, and the
headlight beam diverges 1 degree above the longitudinal axis of the vehicle.[5]
Crest Curves[edit]
Crest vertical curves are curves which, when viewed from the side, are convex upwards. This
includes vertical curves at hill crests, but it also includes locations where an uphill grade becomes
less steep, or a downhill grade becomes steeper.
The most important design criterion for these curves is stopping sight distance.[2] This is the distance
a driver can see over the crest of the curve. If the driver cannot see an obstruction in the roadway,
such as a stalled vehicle or an animal, the driver may not be able to stop the vehicle in time to avoid
a crash. The desired stopping sight distance (S) is determined by the speed of traffic on a road. By
first finding the stopping sight distance (S) and then solving for the curve length (L) in each of the
equations below, the correct curve length can be determined. The proper equation depends on
whether the vertical curve is shorter or longer than the available sight distance. Normally, both
equations are solved, then the results are compared to the curve length.[4][5]
Sight Distance > Curve Length (S>L)

Sight Distance < Curve Length (S<L)

 US standards specify the height of the drivers eye is defined as 1080 mm (3.5 ft) above the
pavement, and the height of the object the driver needs to see as 600 mm (2.0 ft), which is
equivalent to the taillight height of most passenger cars.[6]
For bicycle facilities, the cyclist's eye height is assumed to be at 1.4 m (4.5 ft), and the
object height is 0 inches, since a pavement defect can cause a cyclist to fall or lose control.[7]

Alignment[edit]
Horizontal alignment in road design consists of straight sections of road, known as tangents,
connected by circular horizontal curves.[2] Circular curves are defined by radius (tightness)
and deflection angle (extent). The design of a horizontal curve entails the determination of a
minimum radius (based on speed limit), curve length, and objects obstructing the view of the
driver.[4]
Using AASHTO standards, an engineer works to design a road that is safe and comfortable.
If a horizontal curve has a high speed and a small radius, an increased superelevation
(bank) is needed in order to assure safety. If there is an object obstructing the view around a
corner or curve, the engineer must work to ensure that drivers can see far enough to stop to
avoid an accident or accelerate to join traffic.
Terminology[edit]

R = Radius
PC = Point of Curvature (point at which the curve begins)
PT = Point of Tangent (point at which the curve ends)
PI = Point of Intersection (point at which the two tangents intersect)
T = Tangent Length
C = Long Chord Length (straight line between PC and PT)
L = Curve Length
M = Middle Ordinate, now known as HSO - Horizontal Sightline Offset (distance from sight-
obstructing object to the middle of the outside lane)
E = External Distance

= Coefficient of Side Friction

u = Vehicle Speed

= Deflection Angle [2]

Geometry[edit]
[2]

Curve sight Distance[edit]

Cross section[edit]
The cross section of a roadway can be considered a representation
of what one would see if an excavator dug a trench across a
roadway, showing the number of lanes, their widths and cross
slopes, as well as the presence or absence of shoulders, curbs,
sidewalks, drains, ditches, and other roadway features.
Lane width[edit]
The selection of lane width affects the cost and performance of a
highway. Typical lane widths range from 3 metres (9.8 ft) to 3.6
metres (12 ft). Wider lanes and shoulders are usually used on
roads with higher speed and higher volume traffic, and significant
numbers of trucks and other large vehicles. Narrower lanes may be
used on roads with lower speed or lower volume traffic.
Narrow lanes cost less to build and maintain, but also reduce the
capacity of a road to convey traffic.[8] 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 stormwater runoff.
Cross slope[edit]
See also: Cant (road/rail)

Cross slope 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 withhydroplaning, and ice
accumulation in cold weather.
In tangent (straight) sections, the road surface cross slope is
commonly 1-2% to enable water to drain from the roadway. Cross
slopes of this size, especially when applied in both directions of
travel with a crown point along the centerline of a roadway are
commonly referred to as "normal crown" and are generally
unnoticeable to traveling motorists.
In curved sections, the outside edge of the road
is superelevated above the centerline. Since the road is sloped
down toward the inside of the curve, gravity draws the vehicle
toward the inside of the curve. This causes a greater proportion
of centripetal force to supplant the tyre friction that would otherwise
be needed to negotiate the curve.
Superelevation slopes of 4 to 10% are applied in order to aid
motorists in safely traversing these sections, while maintaining
vehicle speed throughout the length of the curve. An upper bound
of 12% was chosen to meet the demands of construction and
maintenance practices, as well as to limit the difficulty of driving a
steeply cross-sloped curve at low speeds. In areas that receive
significant snow and ice, most agencies use a maximum cross
slope of 6 to 8%. While steeper cross slope makes it difficult to
traverse the slope at low speed when the surface is icy, and when
accelerating from zero with warm tyres on the ice, lower cross
slope increases the risk of loss-of-control at high speeds, especially
when the surface is icy. Since the consequence of high speed
skidding is much worse than that of sliding inward at a low speed,
sharp curves have the benefit of greater net safety when designers
select up to 8% superelevation, instead of 4%.[citation needed] A lower
slope of 4% is commonly used on urban roadways where speeds
are lower, and where a steeper slope would raise the outside road
edge above adjacent terrain.[5]
The equation for the desired radius of a curve, shown below, takes
into account the factors of speed and superelevation (e). This
equation can be algebraically rearranged to obtain desired rate of
superelevation, using input of the roadway's designated speed and
curve radius.

The American Association of State Highway and

Transportation officials (AASHTO) provides a table from which
desired superelevation rates can be interpolated, based on the
designated speed and radius of a curved section of roadway.
This table can also be found in many state roadway design
guides and manuals in the U.S.
Recent research has shown that, considering rollover risk for
heavy vehicles (semitrailers & buses), which have a relatively
high centre-of-gravity, the above equation yields cross slope
values which are too low.[9]

Safety effects of road geometry[edit]

The geometry of a road influences its safety performance.
While studies of contributing factors to road accidents show
that human factors predominate, roadway factors are the
second most common category, with vehicle factors last.
Design consistency[edit]
Collisions tend to be more frequent in locations where a
sudden change in road character violates the driver's
expectations. A common example is a sharp curve at the end
 of a long tangent section of road. The concept of design
consistency addresses this by comparing adjacent road
segments and identifying sites with changes the driver might
find sudden or unexpected. Locations with large changes in the
predicted operating speed are likely to benefit from additional
design effort. A horizontal curve with a significantly smaller
radius than those before it may need enhanced curve
signs.[10] This is an improvement on the concept of design
speed, which only sets a lower limit for geometric design. In the
example given above, a long tangent followed by a sharp curve
would be acceptable if a 30 mph design speed was chosen.
Design consistency analysis would flag the decrease in
operating speed at the curve.
Safety effects of alignment[edit]
The safety of a horizontal curve is affected by the length of the
curve, the curve radius, whether spiral transition curves are
used, and the superelevation of the roadway. For a given curve
deflection, crashes are more likely on curves with a smaller
radius. Spiral transitions decrease crashes, and insufficient
superelevation increases crashes.
A safety performance function to model curve performance on
two-lane roads is:bharat[11]

where
AMF = Accident modification factor, a multiplier that describes how many more crashes are
likely to occur on the curve compared to a straight road
Lc = Length of the horizontal curve in miles.
R = Radius of the curve in feet.
S = 1 if spiral transition curves are present
= 0 if spiral transition curves are absent
Safety effects of cross section[edit]
Cross slope and lane width affect the safety
performance of a road.
Certain types of crashes, termed "lane
departure crashes", are more likely on roads
with narrow lanes. These include run-off-road
collisions, sideswipes, and head-on collisions.
For two-lane rural roads carrying over 2000
vehicles per day, the expected increase in
crashes is:

Lane width Expected increase in crashes

 12 feet (3.7 m) 0%

11 feet (3.4 m) 5%

10 feet (3.0 m) 30%

9 feet (2.7 m) 50%

The effect of lane width is reduced on urban

and suburban roads[12] and low volume
roads.[11]
Insufficient superelevation will also result in an
increase in crash rate. The expected increase
is shown below:[11]

Expected
Expected
increase in
Superelevation increase in
crashes for
deficiency crashes for
heavy
cars
trucks[13]

<0.01 0% <5%

0.02 6% 10%

0.03 9% 15%

0.04 12% 20%

0.05 15% 25%

Sight distance[edit]
Road geometry affects the sight distance
available to the driver. Sight distance, in the
context of road design, is defined as "the
length of roadway ahead visible to the
driver."[1] Sight distance is how far a road user
(usually a vehicle driver) can see before the
line of sight is blocked by a hill crest, or an
obstacle on the inside of a horizontal curve or
intersection. Insufficient sight distance can
adversely affect the safety or operations of a
roadway or intersection.
The sight distance needed for a given situation
is the distance travelled during the two phases
of a driving maneuver: perception-reaction
time (PRT), and maneuver time (MT).
Perception-reaction time is the time it takes for
a road user to realize that a reaction is needed
to a road condition, decided what maneuver is
appropriate, and start the maneuver.
Maneuver time is the time it takes to complete
the maneuver. The distance driven during
perception-reaction time and maneuver time is
the sight distance needed.
During highway design and traffic safety
investigations, highway engineers compare the
available sight distance to how much sight
distance is needed for the situation.
Depending on the situation, one of three types
of sight distances will be used:
Stopping sight distance[edit]
Main article: Stopping sight distance

Stopping sight distance is the distance

traveled during perception-reaction time (while
the vehicle driver perceives a situation
requiring a stop, realizes that stopping is
necessary, and applies the brake), and
maneuver time (while the driver decelerates
and comes to a stop). Actual stopping
distances are also affected by road conditions,
the mass of the car, the incline of the road,
and numerous other factors. For design, a
conservative distance is needed to allow a
vehicle traveling at design speed to stop
before reaching a stationary object in its path.
Typically the design sight distance allows a
below-average driver to stop in time to avoid a
collision.[14] [15]
Decision sight distance[edit]
Decision sight distance is used when drivers
must make decisions more complex than stop
or don't stop. It is longer than stopping sight
distance to allow for the distance traveled
while making a more complex decision. The
decision sight distance is "distance required
for a driver to detect an unexpected or
otherwise difficult-to-perceive information
source or hazard in a roadway environment
that may be visually cluttered, recognize the
hazard or its threat potential, select an
appropriate speed and path, and initiate and
complete the required maneuver safely and
efficiently".[16] Ideally, roads are designed for
the decision sight distance, using 6 to 10
seconds for perception-reaction time and 4 to
5 seconds to perform the right maneuver.
Intersection sight distance[edit]
Intersection sight distance is the sight distance
needed to safely proceed through an
intersection. The distance needed depends on
the type of traffic control at the intersection
(uncontrolled, yield sign, stop sign or signal),
and the maneuver (left turn, right turn, or
proceeding straight). All-way stop intersections
need the least, and uncontrolled intersections
require the most. Intersection sight distance is
a key factor in whether no control or yield
control can be safely used, or more restrictive
control in needed.[17]
Corner sight distance[edit]
Corner sight distance (CSD) is the road
alignment specification which provides a
substantially clear line of sight so that the
driver of a vehicle, bicyclist or pedestrian
waiting at the crossroad may safely anticipate
the driver of an approaching vehicle. Corner
sight provides an adequate time for the waiting
user to either cross all lanes of through traffic,
cross the near lanes and turn left, or turn right,
without requiring through traffic to radically
alter their speed.
Uncontrolled and yield controlled
intersections[edit]
Uncontrolled and yield (give way) controlled
intersections require large sight triangles clear
of obstructions in order to operate safely. At
uncontrolled intersections, the basic right-of-
way rules apply (either yield to the vehicle on
the right, or the boulevard rule, depending on
the location). Vehicle drivers must be able to
see traffic approaching on the intersecting
road at a point where they can adjust their
speed, or stop if need be, to yield to the other
traffic before reaching the intersection. It isn't
the only criterion for allowing these types of
intersection control. Changing an intersection
to stop control is a common response to poor
safety performance.
Two-way stop control[edit]
When determining corner sight distance, a set
back distance for the vehicle waiting at the
crossroad must be assumed. Set back for the
driver of the vehicle on the crossroad has been
standardized by some state MUTCDs and
design manuals to be up to a minimum of 10
feet plus the shoulder width of the major road
but not less than 15 feet.[18] However, the
Federal MUTCD requires that a stop line, if
used, shall be at least 4 feet from the nearest
travel lane.[19] Line of sight for corner sight
distance is to be determined from a 3 and 1/2-
foot eye height at the vehicle driver's location
on the minor road to a 4 and 1/4-foot object
height in the center of the approaching lane of
the major road.[20][21] Corner sight

distance, , is equivalent to a specified

time gap, , at the design speed, ,

required for a stopped vehicle to turn right or
left:

For passenger vehicles at two lane

intersections, this time gap equivalence is
commonly a distance 7.5 seconds away at
the design speed. Longer gaps are
required for trucks and buses, and for
multilane roads.[22] Generally, the public
right-of-way should include and maintain
this line-of-sight.
All-way stop control and signalized
intersections[edit]
Drivers at intersections with all-way stop
control or traffic signals need the least
sight distance. At all-way stops, drivers
need to be able to see vehicles stopped at
other approaches. At signals, drivers
approaching the intersections need to see
the signal heads. In jurisdictions that
allow right turn on red, drivers in the right
lane stop control need the same sight
distance as two-way stop control. Although
not needed during normal operations,
additional sight distance should be
provided for signal malfunctions and
power outages.[citation needed]
Effects of insufficient sight
distance[edit]
Many roads were created long before the
current sight distance standards were
adopted, and the financial burden on many
jurisdictions would be formidable
to: acquire and maintain additional right-of-
way; redesign roadbeds on all of them; or
implement future projects on rough terrain,
or environmentally sensitive areas. In such
cases, the bare minimum corner sight
distance should be equal to the stopping
sight distance.[23] While a corner sight
distance which far exceed the braking
distance at the design speed should be
afforded to the driver, he or she is still
generally required to maintain such control
and operating speed as to be able to stop
within the Assured Clear Distance Ahead
(ACDA),[24] and the basic speed
rule always applies. Jurisdictions often
provide some level of design immunity
against government claims actions, in
such cases.[Note 1]
Warning signs are often used where sight
distance is insufficient. The US MUTCD
requires Stop Ahead, Yield Ahead or
Signal Ahead signs at intersections where
the traffic control device is not visible from
a distance equal to the stopping sight
distance at speed of approaching traffic.
Hill Blocks View signs can be used where
crest vertical curves restrict sight
distance.[25] However, many jurisdictions
still expect drivers to use ordinary
care regarding conditions readily apparent
to a driver, without the prompting of a
sign.[Note 2] The care and focus ordinarily
required of a driver against certain types of
hazards may be somewhat amplified on
roads with lower functional
classification.[26][27] The probability of
spontaneous traffic increases
proportionally to the density of access
points, and this density should be readily
apparent to a driver even when a specific
access point is not.[28] For this reason, full
 corner sight distance is almost never
required for individual driveways in urban
high-density residential areas, and street
parking is commonly permitted within
theright-of-way.

See also[edit]
Cognitive ergonomics
Degree of curvature
Design speed
Human factors
Road traffic safety
Stopping sight distance
Traffic psychology
Transition curve
Structural road design
Road standards setting
bodies[edit]

American Association of State

Highway and Transportation Officials
National Cooperative Highway
Research Program
Transportation Research Board

https://en.wikipedia.org/wiki/Geometric_design_of_roads

ection 6: Cross Sectional Elements

Overview
This section includes information on the following cross sectional design elements:

Pavement Cross Slope

Median Design

Lane Widths

Shoulder Widths

Sidewalks and Pedestrian Elements

Curb and Curb and Gutters

 Roadside Design

Slopes and Ditches

Lateral Offset to Obstructions

Clear Zone

Pavement design is covered in TxDOTs Pavement Design Guide.

Pavement Cross Slope

The operating characteristics of vehicles on crowned pavements are such that on
cross slopes up to 2 percent, the effect on steering is barely perceptible. A reasonably
steep lateral slope is desirable to minimize water ponding on flat sections of uncurbed
pavements due to imperfections or unequal settlement. With curbed pavements, a
steep cross slope is desirable to contain the flow of water adjacent to the curb. The
recommended pavement cross slope for usual conditions is 2 percent. In areas of high
rainfall, steeper cross slopes may be used (see AASHTOs A Policy on Geometric
Design of Highways and Streets).

On multilane divided highways, pavements with three or more lanes inclined in the
same direction desirably should have greater slope across the outside lane(s) than
across the two interior lanes. The increase in slope in the outer lane(s) should be at
least 0.5 percent greater than the inside lanes (i.e., slope of 2.5 percent). In these
cases, the inside lanes may be sloped flatter than normal, typically at 1.5 percent but
not less than 1.0 percent.

For tangent sections on divided highways, each pavement should have a uniform
cross slope with the high point at the edge nearest the median. Although a uniform
cross slope is preferable, on rural sections with a wide median, the high point of the
crown is sometimes placed at the centerline of the pavement with cross slopes from
1.5 to 2 percent. At intersections, interchange ramps or in unusual situations, the high
point of the crown position may vary depending upon drainage or other controls.

For two lane roadways, cross slope should also be adequate to provide proper
drainage. The cross slope for two lane roadways for usual conditions is 2 percent and
should not be less than 1.0 percent.

Shoulders should be sloped sufficiently to drain surface water but not to the extent that
safety concerns are created for vehicular use. The algebraic difference of cross slope
between the traveled way and shoulder grades should not exceed 6 to 7 percent.
Maximum shoulder slope should not exceed 10 percent. Following are recommended
cross slopes for various types of shoulders:

Bituminous and concrete-surface shoulders should be sloped from 2 to 6 percent

(often the slope rate is identical to that used on the travel lanes).

Gravel or crushed rock shoulders should be sloped from 4 to 6 percent.

Turf shoulders should be sloped at about 8 percent.

Pavement cross slopes on all roadways, exclusive of superelevation transition

sections, should not be less than 1 percent.

Median Design
A median (i.e., the area between opposing travel lane edges) is provided primarily to
separate opposing traffic streams. The general range of median width is from 4 ft to 76
ft [1.2 m to 22.8 m], with design width dependent on the type and location of the
highway or street facility.

In rural areas, median sections are normally wider than in urban areas. For multi-lane
rural highways without access control, a median width of 76 ft [22.8 m] is desirable to
provide complete shelter for trucks at median openings (crossovers). These wide,
depressed medians are also effective in reducing headlight glare and providing a
horizontal clearance for run-off-the-road vehicle encroachments.

Where economically feasible, freeways in rural areas should also desirably include a
76 ft [22.8 m] median. Since freeways by design do not allow at-grade crossings,
median widths need not be sufficient to shelter crossing trucks. In this regard, where
right-of-way costs are prohibitive, reduced median widths (less than 76 ft [22.8 m])
may be appropriate for certain rural freeways. Statistical studies have shown that over
90 percent of median encroachments involve lateral distances traveled of 48 ft [14.4
m] or less. In this regard, depressed medians on rural freeways sections should be 48
ft [14.4 m] or more in width.

Urban freeways generally include narrower, flush medians with continuous longitudinal
barriers. For urban freeways with flush median and six or more travel lanes, full (10 ft
[3.0 m]) inside shoulders should be provided to provide space for emergency parking.
Median widths vary up to 30 ft [9.0 m], with 24 ft [7.2 m] commonly used. For projects
involving the rehabilitation and expansion of existing urban freeways, the provision of
wide inside shoulders may not be feasible.
For low-speed urban arterial streets, flush or curbed medians are used. A width of 16
ft [4.8 m] will effectively accommodate left-turning traffic for either raised or flush
medians. Where the need for dual left turns are anticipated at cross streets, the
median width should be 28 ft [8.4 m]. The two-way (continuous) left-turn lane design is
appropriate where there exists (or is expected to exist) a high frequency of mid-block
left turns. Median types for urban arterials without access control are further discussed
in Chapter 3, Section 2, Urban Streets.

When flush median designs are selected, it should be expected that some crossing
and turning movements can occur in and around these medians. Full pavement
structure designs will usually be carried across flush medians to allow for traffic
movements.

Lane Widths
For high-speed facilities such as all freeways and most rural arterials, lane widths
should be 12 ft [3.6 m] minimum. For low-speed urban streets, 11 ft or 12 ft [3.3 m or
3.6 m] lanes are generally used. Subsequent sections of this manual identify
appropriate lane widths for the various classes of highway and street facilities.

Bicycle accomodations should be considered when a project is scoped. Bicycle

consideration is required on urban facilities. To accommodate bicycles, the outside
curb lane should be 14 ft [4.2m] from the lane stripe to the gutter joint or gutter lip on a
monolithic curb. For a striped bicycle lane, the clear width is 5 ft [1.5m] minimum. For
additional guidance, refer to the AASHTO Guide for the Development of Bicycle
Facilities.

Shoulder Widths
Wide, surfaced shoulders provide a suitable, all-weather area for stopped vehicles to
be clear of the travel lanes. Shoulders are of considerable value on high-speed
facilities such as freeways and rural highways. Shoulders, in addition to serving as
emergency parking areas, lend lateral support to travel lane pavement structure,
provide a maneuvering area, increase sight distance of horizontal curves, and give
drivers a sense of safe, open roadway. Design shoulder widths for the various classes
of highways are shown in the appropriate subsequent portions of this manual.

Shoulder widths should accomodate bicycle facilities and provide a 1 ft offset to

barriers across bridges being replaced or rehabilitated.
On urban collector and local streets, parking lanes may be provided instead of
shoulders. On arterial streets, parking lanes decrease capacity and generally are
discouraged.

Sidewalks and Pedestrian Elements

Walking is an important transportation mode that needs to be incorporated in
transportation projects. Planning for pedestrian facilities should occur early and
continuously throughout project development. Sidewalks provide distinct separation of
pedestrians and vehicles, serving to increase pedestrian safety as well as to enhance
vehicular capacity. When any of the following factors are present, sidewalks should be
included on a project located in an urban setting where:

Construction is within existing right-of-way, and the scope of work involves

pavement widening;

Full reconstruction or new construction that requires new right-of-way.

In typical suburban development, there are initially few pedestrian trips because there
are few closely located pedestrian destinations. However, when pedestrian demand
increases with additional development, it may be more difficult and more costly to go
back and install pedestrian facilities if they were not considered in the initial design.
Early consideration of pedestrian facility design during the project development
process may also greatly simplify compliance with accessibility requirements
established by theAmericans with Disabilities Act Public Accessibility Guidelines for
Pedestrian Facilities in the Public Right of Way (PROWAG) and the Texas
Accessibility Standards (TAS).

Sidewalk Location. For pedestrian comfort, especially adjacent to high speed traffic,
it is desirable to provide a buffer space between the traveled way and the sidewalk as
shown in Figure 2-7(A). For curb and gutter sections, a buffer space of 4 ft to 6 ft
[1.2m to 1.8m] between the back of the curb and the sidewalk is desirable. Roadways
in urban and suburban areas without curb and gutter require sidewalks , which should
be placed between the ditch and the right of way line if practical. Note that pedestrian
street crossings must be ADA compliant. For roadways functionally classified as rural,
the shoulder may be used to accommodate pedestrian and bicycle traffic. Where a
shoulder serves as part of the pedestrian access route, it must meet ADA/TAS
requirements.

Sidewalk Width. Sidewalks should be wide enough to accommodate the volume and
type of pedestrian traffic expected in the area. The minimum clear sidewalk width is 5
ft [1525 mm]. Where a sidewalk is placed immediately adjacent to the curb as shown
in Figure 2-7(B), a sidewalk width of 6 ft [1830 mm] is recommended to allow
additional space for street and highway hardware and allow for the proximity of moving
traffic. Sidewalk widths of 8 ft [2440 mm] or more may be appropriate in commercial
areas, along school routes, and other areas with concentrated pedestrian traffic.

Where necessary to cross a driveway while maintaining the maximum 2 percent cross
slope, the sidewalk width may be reduced to 4 ft [1220 mm] (Figure 2-8). Also,if
insufficient space is available to locate street fixtures (elements such as sign supports,
signal poles, fire hydrants, manhole covers, and controller cabinets that are not
intended for public use) outside the 5 ft [1525 mm] minimum clear width, the sidewalk
width may be reduced to 4 ft [1220 mm] for short distances.

Street Crossings. Intersections can present formidable barriers to pedestrian travel.

Intersection designs which incorporate properly placed curb ramps, sidewalks,
crosswalks, pedestrian signal heads and pedestrian refuge islands can make the
environment more accommodating for pedestrians. Desirably, drainage inlets should
be located on the upstream side of crosswalks and sidewalk ramps.

Refuge islands enhance pedestrian comfort by reducing effective walking distances

and pedestrian exposure to traffic. Islands should be a minimum of 6 ft [1.8m] wide to
afford refuge to people in wheelchairs. A minimum 5 ft [1.5m] wide by 6 ft [1.8m] long
curb ramp should be cut through the island for pedestrian passage. Install curb ramps
with a minimum 5 ft x 5 ft [1525 mm x 1525 mm] landing in the island if room allows,
see Figure 2-9. Curb ramps and crosswalks must be aligned behind the nose of the
median island to provide adequate refuge.
Figure 2-7. Curb Ramps and Landings
Figure 2-8. Sidewalks at Driveway Aprons.
Figure 2-9. Curb Ramps at Median Islands.

NOTE: Online users can view the metric version of this figure.

Curb Ramps and Landings. Curb ramps must be provided in conjunction with each
project where the following types of work will be performed:

reconstruction, rehabilitation and resurfacing projects, including overlays, where a

barrier exists to a sidewalk or a prepared surface for pedestrian use

construction of curbs, curb and gutter, and/or sidewalks

installation of traffic signals which include pedestrian signals

installation of pavement markings for pedestrian crosswalks

A sidewalk curb ramp and level landing will be provided wherever a public sidewalk
crosses a curb or other change in level. The maximum grade for curb ramps is 8.3
percent. The maximum cross slope for curb ramps is 2 percent. Flatter grades and
slopes should be used where possible and to allow for construction tolerances and to
improve accessibility. The preferred width of curb ramps is 5 ft [1.5m] and the
minimum width is 4 ft [1.2m], exclusive of flared sides. Where a side of a curb ramp is
contiguous with a public sidewalk or walking surface, it will be flared with a slope of 10
percent maximum, measured parallel to the curb.

Where a perpendicular or directional curb ramp is provided, a landing must be

provided at the top of the ramp run. The slope of the landing will not exceed 2 percent
in any direction. The landing should have a minimum clear dimension of 5 ft x 5 ft
[1.5m x 1.5m] square or accomodate a 5 ft [1.5m] diameter circle and will connect to
the continuous passage in each direction of travel as shown in Figure 2-7. Landings
may overlap with other landings.

Where a parallel curb ramp is provided (i.e., the sidewalk ramps down to a landing at
street level) a minimum 5 ft x 5 ft [1.5m x 1.5m] landing should be provided at the
entrance to the street.

The bottom of a curb ramp run should be wholly contained within the markings of the
crosswalk. There should be a minimum 4 ft x 4 ft [1.2m x 1.2m] maneuvering space
wholly contained within the crosswalk, whether marked or unmarked and outside the
path of parallel vehicular traffic.

Manhole covers, grates, and obstructions should not be located within the curb ramp,
maneuvering area, or landing.
The standard sheet PED may be referenced for additional information on the
configuration of curb ramps.

Cross Slope. Sidewalk cross slope will not exceed 1:50 (2 percent). Due to
construction tolerances, it is recommended that sidewalk cross slopes be shown in the
plans at 1.5 percent to avoid exceeding the 2 percent limit when complete. Cross
slope requirements also apply to the continuation of the pedestrian route through the
cross walk. Sidewalks immediately adjacent to the curb or roadway may be offset to
avoid a non-conforming cross slope at driveway aprons by diverting the sidewalk
around the apron as shown in Figure 2-8. Where the ramp sidewalk must be sloped to
cross a driveway, the designer is encouraged to use a running slope of 5 percent or
less on the sloping portions of the sidewalk to avoid the need for handrails.

Street Furniture. Special consideration should be given to the location of street

furniture (items intended for use by the public such as benches, public telephones,
bike racks, and parking meters). A clear ground space at least 2.5 ft x 4 ft [760 mm x
1.2m] with a maximum slope of 2 percent must be provided and positioned to allow for
either forward or parallel approach to the element in compliance with PROWAG/TAS.
The clear ground space must have an accessible connection to the sidewalk and must
not encroach into the 5 ft [1.5m] minimum sidewalk width by more than 2 ft [610 mm].
Pedestrian push buttons must also be within specified reach ranges of a ground
space.

PROWAG/TAS. Specific design requirements to accommodate the needs of persons

with disabilities are established by the PROWAG/TAS and related rulemaking. A
request for a design variance for any deviations from TAS requirements must be
submitted to the Texas Department of Licensing and Regulation (TDLR) for approval.

Curb and Curb and Gutters

Curb designs are classified as vertical or sloping. Vertical curbs are defined as those
having a vertical or nearly vertical traffic face 6 inches [150 mm] or higher. Vertical
curbs are intended to discourage motorists from deliberately leaving the roadway.
Sloping curbs are defined as those having a sloping traffic face 6 inches [150 mm] or
less in height. Sloping curbs can be readily traversed by a motorist when necessary. A
preferable height for sloping curbs at some locations may be 4 inches [100 mm] or
less because higher curbs may drag the underside of some vehicles.

Curbs are used primarily on frontage roads, crossroads, and low-speed streets in
urban areas. They should not be used in connection with the through, high-speed
traffic lanes or ramp areas except at the outer edge of the shoulder where needed for
drainage, in which case they should be of the sloping type.

Roadside Design
Of particular concern to the design engineer is the number of single-vehicle, run-off-
the-road accidents which occur even on the safest facilities. About one-third of all
highway fatalities are associated with accidents of this nature. The configuration and
condition of the roadside greatly affect the extent of damages and injuries for these
accidents.

Increased safety may be realized through application of the following principles,

particularly on high-speed facilities:

A forgiving roadside should be provided, free of unyielding obstacles including

landscaping, drainage facilities that create obstacles, steep slopes, utility poles, etc.
For adequate safety, it is desirable to provide an unencumbered roadside recovery
area that is as wide as practicable for the specific highway and traffic conditions.

For existing highways, treatment of obstacles should be considered in the following

order:

o Eliminate the obstacle.

o Redesign the obstacle so that it can by safely traversed.

o Relocate the obstacle to a point where it is less likely to be struck.

o Make the obstacle breakaway.

o Apply a cost-effective device to provide for redirection (longitudinal barrier) or

severity reduction (impact attenuators). Barrier should only be used if the
barrier is less of an obstacle than the obstacle it would protect, or if the cost of
otherwise safety treating the obstacle is prohibitive.

o Delineate the obstacle.

Use of higher than minimum design standards result in a driver environment which
is fundamentally safer because it is more likely to compensate for driver errors.
Frequently, a design, including sight distances greater than minimum, flattened
slopes, etc., costs little more over the life of a project and increases safety and
usefulness substantially.

For improved safety performance, highway geometry and traffic control devices
should merely confirm drivers' expectations. Unexpected situations, such as left side
 ramps on freeways, sharp horizontal curvature introduced within a series of flat
curves, etc., have demonstrated adverse effects on traffic operations.

These principles have been incorporated as appropriate into the design guidelines
included herein. These principles should be examined for their applicability at an
individual site based on its particular circumstances, including the aspects of social
impact, environmental impact, economy, and safety.

Slopes and Ditches

Sideslopes. Sideslopes refer to the slopes of areas adjacent to the shoulder and
located between the shoulder and the right-of-way line. For safety reasons, it is
desirable to design relatively flat areas adjacent to the travelway so that out-of-control
vehicles are less likely to turn over, vault, or impact the side of a drainage channel.

Slope Rates. The path that an out-of-control vehicle follows after it leaves the traveled
portion of the roadway is related to a number of factors such as driver capabilities,
slope rates, and vehicular speed. Accident data indicates that approximately 75
percent of reported encroachments do not exceed a lateral distance of 30 ft [9 m] from
the travel lane edge where roadside slopes are 1V:6H or flatter - slope rates that
afford drivers significant opportunity for recovery. Crash test data further indicates that
steeper slopes (up to 1V:3H) are negotiable by drivers; however, recovery of vehicular
control on these steeper slopes is less likely. Recommended clear zone width
associated with these slopes are further discussed in Clear Zone.

Design Values. Particularly difficult terrain or restricted right-of-way width may require
deviation from these general guide values. Where conditions are favorable, it is
desirable to use flatter slopes to enhance roadside safety.

Front Slope. The slope adjacent to the shoulder is called the front slope. Ideally,
the front slope should be 1V:6H or flatter, although steeper slopes are acceptable in
some locations. Rates of 1V:4H (or flatter) facilitate efficient operation of
construction and maintenance equipment. Slope rates of 1V:3H may be used in
constrained conditions. Slope rates of 1V:2H are normally only used on bridge
header banks or ditch side slopes, both of which would likely require rip-rap.

When the front slope is steeper than 1V:3H, a longitudinal barrier may be
considered to keep vehicles from traversing the slope. A longitudinal barrier
should not be used solely for slope protection for rates of 1V:3H or flatter since
the barrier may be more of an obstacle than the slope. Also, since recovery is
less likely on 1V:3H and 1V:4H slopes, fixed objects should not be present in the
 vicinity of the toe of these slopes. Particular care should be taken in the treatment
of man-made appurtenances such as culvert ends.

Back Slope. The back slope is typically at a slope of 1V:4H or flatter for mowing
purposes. Generally, if steep front slopes are provided, the back slopes are
relatively flat. Conversely, if flat front slopes are provided, the back slopes may be
steeper. The slope ratio of the back slope may vary depending upon the geologic
formation encountered. For example, where the roadway alignment traverses
through a rock formation area, back slopes are typically much steeper and may be
close to vertical. Steep back slope designs should be examined for slope stability.

Design. The intersections of slope planes in the highway cross section should be well
rounded for added safety, increased stability, and improved aesthetics. Front slopes,
back slopes, and ditches should be sodded and/or seeded where feasible to promote
stability and reduce erosion. In arid regions, concrete or rock retards may be
necessary to prevent ditch erosion.

Where guardrail is placed on side slopes, the area between the roadway and barrier
should be sloped at 1V:10H or flatter.

Roadside drainage ditches should be of sufficient width and depth to handle the
design run-off and should be at least 6 inches [150 mm] below the subgrade crown to
insure stability of the base course. For additional information, see Drainage Facility
Placement.

Lateral Offset to Obstructions

It is generally desirable that there be uniform clearance between traffic and roadside
features such as bridge railings, parapets, retaining walls, and roadside barriers. In an
urban environment, right of way is often limited and is characterized by sidewalks,
enclosed drainage, numerous fixed objects (e.g., signs, utility poles, luminaire
supports, fire hydrants, sidewalk furniture, etc.), and traffic making frequent stops.
Uniform alignment enhances highway safety by providing the driver with a certain level
of expectation, thus reducing driver concern for and reaction to those objects. The
distance from the edge of the traveled way, beyond which a roadside object will not be
perceived as an obstacle and result in a motorists reducing speed or changing vehicle
position on the roadway, is called the lateral offset. This lateral offset to obstructions
helps to:
 Avoid impacts on vehicle lane position and encroachments into opposing or
adjacent lanes

Improve driveway and horizontal sight distances

Reduce the travel lane encroachments from occasional parked and disabled
vehicles

Improve travel lane capacity

Minimize contact from vehicle mounted intrusions (e.g., large mirrors, car doors, and
the overhang of turning trucks.

As a minimum, as long as the obstruction is located beyond the recommended paved

shoulder of a roadway, it will have minimum impact on driver speed or lane position
and meet the lateral offset requirement. Where a curb is present, the lateral offset is
measured from the face of curb and shall be a minimum of 1.5 ft [0.5 m]. A minimum
of 1 ft [0.3 m] lateral offset should be provided from the toe of barrier to the edge of
traveled way.

Clear Zone
A clear recovery area, or clear zone, should be provided along high-speed rural
highways. A clear zone is the unobstructed, traversable area provided beyond the
edge of the through traveled way for the recovery of errant vehicles. The clear zone
includes shoulders, bike lanes, and auxiliary lanes, except those auxiliary lanes that
function like through lanes. Such a recovery area should be clear of unyielding objects
where practical or shielded by crash cushions or barrier. Table 2-12 shows criteria for
clear zones.
Table 2-12: Clear Zones

Location Functional Classification Design Speed (mph) Avg. Daily Traffic Clear Z

- - - - Minimu

Rural Freeways All All 30 (16

0 - 750 10
Rural Arterial All 750 - 1500 16
>1500 30

Rural Collector 50 All Use ab

Rural Collector 45 All 10

Rural Local All All 10

Suburban All All <8,000 106

 Suburban All All 8,000 - 12,000 106

Suburban All All 12,000 - 16,000 106

Suburban All All >16,000 206

Urban Freeways All All 30 (16

Use ab
Urban All (Curbed) 50 All
permits

Urban All (Curbed) 45 All 4 from

Urban All (Uncurbed) 50 All Use ab

Urban All (Uncurbed) 45 All 10

1 Because of the need for specific placement to assist traffic operations, devices such as traffic signal supports, railroad signal/warn
excluded from clear zone requirements. However, these devices should be located as far from the travel lanes as practical. Other n
prescribed clear zone or these devices should be protected with barrier.
2Average ADT over project life, i.e., 0.5 (present ADT plus future ADT). Use total ADT on two-way roadways and directional ADT on
3 Without barrier or other safety treatment of appurtenances.
4 Measured from edge of travel lane for all cut sections and for all fill sections where side slopes are 1V:4H or flatter. Where fill slop
10 ft area free of obstacles beyond the toe of slope.
5 Desirable, rather than minimum, values should be used where feasible.
6 Purchase of 5 ft or less of additional right-of-way strictly for satisfying clear zone provisions is not required.

NOTE: Online users can view the metric version of this table in PDF format.

The clear zone values shown in Table 2-12 are measured from the edge of travel lane.
These are appropriate design values for all cut sections (see Drainage Facility
Placement), for cross sectional design of ditches within the clear zone area) and for all
fill sections with side slopes 1V:4H or flatter. It should be noted that, while a 1V:4H
slope is acceptable, that a 1V:6H or flatter slope is preferred for both errant vehicle
performance and slope maintainability. For fill slopes steeper than 1V:4H, errant
vehicles have a reduced chance of recovery and the lateral extent of each roadside
encroachment increases. It is therefore preferable to provide an obstacle-free area of
10 ft[3.0m] beyond the toe of steep side slopes even when this area is outside the
clear zone.

http://onlinemanuals.txdot.gov/txdotmanuals/rdw/cross_sectional_elements.htm
 Clear delineation is required at intersections to inform road users that
there is an intersection present and to provide information about the
types of manoeuvres that may occur.
In the worst case situation, road users may not realise that an intersection is present, and collide with
other vehicles or road users, often at high speeds. Poor delineation may also result in late braking
behaviour by road users who are required to stop, or wish to make turns. Improvements to intersection
delineation can be made by making adjustments to, or installing new traffic islands, street lighting,
linemarking and signs.
Linemarking deficiencies (such as unclear approach lane lines, and faded or missing Stop or Give Way
markings) are easily and cheaply treated. Warning signs can be used to give drivers advance notice of an
upcoming intersection. They are also cheap to install and particularly useful where the intersection is sub-
standard. Median islands (or splitter islands) can be used on the approaches to intersections to improve
the prominence of intersections (including by the provision of additional signs on median islands), and
provide an additional benefit as they channelise traffic and may provide pedestrian protection if designed
well. Improvingstreet lighting at such locations should be also considered.

Benefits

Reduction in intersection crashes.

Reductions in speed.
Awareness of the intersection is increased.
Vehicles are directed to a clearer path through the intersection.
Median islands (if used) can create a refuge for pedestrians crossing the road, thus reducing the
likelihood of pedestrian/vehicle crashes

Implementation issues

Old linemarking should be properly removed (eg. by grinding) or it may remain visible and confuse
drivers.
Warning signs should be placed at sufficient distance from the intersection to ensure drivers have
enough time to take necessary action (e.g. to slow down).
Warning signs and median islands should not be located or designed in such a way as to be hazards.

http://toolkit.irap.org/default.asp?page=treatment&id=6