Geometric Design

Dr. Hana Naghawi

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 Geometric Design
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

The highway design process requires knowledge of most

subdisciplines of civil eng., including:
Traffic & transportation Eng.
Geotechnical Eng.
Material Eng.
Structural Eng.
Hydraulic Eng.
Surveying

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 Geometric Design
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

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 Highway Classification
Classification of roads is necessary for
communication among engineers, administrators, and
the general public
Classification schemes applies for different
purposes:
 By design type/function
Grouping of roads by character of service that they
provide
•Used for transportation planning and design
•The most widely used (Arterial, Collector, Local)

Different function - different characteristics

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 Highway Classification
By route numbering – traffic operation
state – primary
- secondary

A – primary state road, B- secondary state road

A45 is more important than A456
A456 is more important than B1563
A45

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B1563
 Highway Classification
Administrative classification – to identify the level of
government responsibility – method of financing
state/ federal aid – primary
- secondary

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 Functional Classification
Classify the road by function (LOS)

Two important consideration to be taken into account where

functional classification:
 access - provide land access to serve each end of the trip
 mobility - provide travel mobility at varying levels

There is a basic relationship between functionally classified highway

systems in serving traffic mobility and land access, as illustrated in the
next slide

 Arterials provide a high level of mobility and a greater degree of

access control, while local facilities provide a high level of access to
adjacent properties but a low level of mobility. Collector roadways
provide a balance between mobility and land access
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 Functional Classification
Arterials
•higher mobility
•low degree of access
Collectors
•balance between
mobility and access

Locals
•lower mobility
•high degree of access

Source: Safety Effectiveness of Highway Design

Features, Volume I, Access Control, FHWA, 1992

Access mobility
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 Functional Classification
 Conclusion:
The extent and degree of access control is a significant
factor in defining the functional class/category of the road

Freeway – accessibility lowest

mobility highest

What factors do we need to define LOS & class of road

1. Trip time – basic function
2. Operating speed
3. Absences of sudden changes of speed
4. Riding comfort

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 Functional Classification
Definition and characteristics of roads are based on
functional classification

Urban and rural functional systems are classified

separately since these areas differ in characteristics
with regard to:
1. Density and type of land use (L.U)
2. Density of population
3. Density of road network
4. Nature of travel pattern
5. The way in which these elements interact
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 Functional Classification
Urban area: in the USA urban areas are those places within
the boundaries set by responsible authority (officials) having
population > 5,000
Urbanized area > 50,000
Small urban area (urban clusters) 5,000 – 50,000

It is characterized by high population density

Forecast over population of the design year

Rural areas are places that fall outside the boundaries of

the urban area

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 Functional Classification
In addition to operational consideration, a road’s functional
classification influences several key design parameters
including:

Traffic volume that the road will carry

Traffic composition
Design & Operating speed
Design Standards
Type of trip

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 Functional Classification

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 Functional Classification
Criteria to classify rural roads:
1. Trip length
2. Population size
3. Amount of traffic
4. Route spacing

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 Functional Classification
Functional system classification for rural roads:
1. Arterial
 principle
• freeway
• other principle
 minor
2. Collector
 Major
 Minor
3. Local

Ex: Rural principle arterial road

Rural minor arterial road
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 Functional Classification
 Rural principle arterial road:
Serves corridor movements of substantial statewide
or interstate travel

Serves all urban areas of 50,000 and over population

and a majority of those over 25,000

The trip length and travel density are suitable for

substantial statewide interstate travel

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 Functional Classification
 Rural minor arterial road:
Link cities and large towns (and other generators, such as
major resorts)

Spaced at such intervals so that all developed areas of the

state are within a reasonable distance of an arterial highway

Provide service to corridors with trip lengths and travel

density greater than those served by rural collectors or local
systems

Design should be expected to provide for relatively high

overall speeds, with minimum interference to through
movement
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 Functional Classification
 Rural major collector

Provide service to any county seat not on an arterial system,

to larger towns not directly served by higher systems

Link the above to nearby larger towns or routes of higher

classification

Serve the more important intra-county travel corridors

Travel shorter, speed moderate, penetrates neighborhoods

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 Functional Classification
 Rural minor collector

Spaced at intervals, consistent with population density

Collect traffic from local roads and bring all developed areas
within a reasonable distance of a collector road

Provide service to the remaining smaller communities

Link local traffic generators with their rural neighborhoods

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 Functional Classification
 Rural local

Serves primarily to provide direct access to adjacent

land

Provide service to travel over relatively short

distances as compared to collectors or other higher
systems

All facilities not on one of the higher systems

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 Functional Classification
Functional system classification for urban roads:
1. Arterial
 principle
• Interstate freeway
• Other freeway and expressway
• Other principle arterial
 minor
2. Collector
3. Local

Ex: urban interstate freeway

urban minor arterial road
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 Functional Classification
Criteria to classify urban roads:
1. Trip length
2. Population size
3. Amount of traffic
4. Route spacing
5. Access control

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 Functional Classification
Urban principal arterial
Accessibility lowest, mobility highest, high operating speed
and high LOS

Serves the major centers of activity of a metropolitan area

Highest traffic volume corridors

Roads serving the longest trip desires

Carry a high proportion of the total urban area travel on a

minimum distance

Carry
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significant amounts of intra-area travel
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 Functional Classification
Urban minor arterial
Interconnect the urban principal arterial system and
provide service to trips of moderate length at a lower
level of travel mobility than principal arterials

Include all arterials not classified as a principal and

contains facilities that place more emphasis on land
access, and offer a lower level of traffic mobility, less
spacing in CBD 1/8 – ½ ml, suburban 2 -3 ml, and in
fully developed areas 2 -3 ml

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 Functional Classification
Urban collector
Provides land access and traffic circulation within
residential neighborhoods, commercial, and industrial
areas (penetrates neighborhoods)

Distributes trips from the arterials through these

areas to their ultimate/final destination

Collects traffic from local streets and channels it to

the arterial system

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 Functional Classification
Urban local
All facilities not on one of the higher systems

Serves primarily as direct access to adjacent land

Serves as access to the higher order systems

Through traffic movement is discouraged

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 Hierarchies of Movement (HOM)
A road hierarchy has, for some time, been accepted as one
of the important tools used for road network and land use
planning

It is a means of defining each roadway in terms of its

function such that appropriate objectives for that roadway
can be set and appropriate design criteria can be
implemented

These objectives and design criteria are aimed at achieving

an efficient & safe road system whereby conflicts between
the roadway and the adjacent land use are minimized and the
appropriate level of interaction between the roadway and
land2/8/2018
use is permitted Dr. Hana Naghawi 27
 Hierarchies of Movement (HOM)
Transition from high speeds to lower speeds in a trip

Stages included in a trip

1. Main movement – on freeway
2. Transition – on freeway ramp to reduce speed
3. Distribution – on a moderate speed road
(Arterial/distributor)
4. Collection – on collector which passes through the
neighborhood
5. Access – on local road (access) that leads to individual
residences
6. Termination – as the vehicle is parked at the terminal
facility
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 Hierarchies of Movement (HOM)

Each element of the Functional Hierarchy can serve as a

collecting facility for the next higher element. However the
space and volume requirements for each element should be
defined in order to determine the cases where intermediate
element
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can be bypassed Dr. Hana Naghawi 29
 Hierarchies of Movement (HOM)
Avoid sudden changes
Comments on HOM
1. HOM is based on the total amount of traffic
freeway
dist
collector
local
terminal
2. Intermediate stages are not always necessary
3. Complete stages of HOM relates specially to conditions of
low density suburban development where traffic flows are
cumulative on successive elements of the system

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 Hierarchies of Movement (HOM)
4. Conflict and congestion occur at the interface between
public roads and traffic generators where functional
transition is inadequate Shopping center

collector
Distributor
terminal Local

Freeway

5. Each functional category is related to a range of vehicle

speeds
6. The principles of HOM are related to the intensity of
traffic generator
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 Access Control
Access Control refers to the control or limitations on the
number of points of access to roads adjacent to property

Spacing & # of access points depends on speed/functional

classification of the road- the higher speed the longer spacing

Types of access control

Fully access control (when selected access points are
permitted)
Partially access control (when no direct access is
permitted)
No access control

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 Access Control
Accessibility

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Dr. Hana Naghawi 33
 Access Control
Notes:-
The functional conflicts lead to traffic congestion & safety
problems as the traffic mix changes to combine both through
traffic seeking to travel at higher speeds and low speed local
traffic turning in and out of local roads

Access should not be allowed at locations where entering

and leaving vehicles will create a hazard such as locations
where sight distance is limited or at point too close to an
intersection

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 Access Control
Types of access:
Roadside access

Median access/opening

Higher access lower capacity lower speed

lower LOS higher accident rates
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 Traffic Stream Performance (TSP)
Any factor that affects traffic stream performance
affects capacity as well

Factors affecting TSP:

1- geometric design features (H & V alignment,
grades, climbing lanes, sight distance, number of
lanes, width,…
2- traffic composition (HGV, LGV, PC, BUS)
3- road furniture (signs, signals, street lighting, traffic
control devices …)

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 Traffic Stream Performance (TSP)

4- type and condition of road pavement (road surface)

5- number and form of intersections
6- attractiveness of the road (sight seeing, terrain,..)
7- general environment (weather)
8- parking (bus stop)

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 Traffic Facility Classification

Traffic stream = drivers + vehicles + roadway

Uninterrupted flow facility( rural area):
In which no fixed element external to traffic stream
such as traffic signal that cause interruption to traffic
flow. Traffic conditions are result vehicles interaction
among each other in the same traffic stream and
between vehicles and the environment (Freeways)

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 Traffic Facility Classification
Interrupted flow facility (urban areas):
In which fixed elements such as signal, yield or stop signs
causes interrupts the traffic stream flow. Flow on such
facilities depends not only on the interaction between vehicles,
but is affected by control devices (City street & Arterials)

External factors:
•Roadway crossing
•Ped. Crossing
•Parking, bus stop
•Traffic signals and signs

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 Traffic Stream Parameters
Levels of Analysis
Macroscopic (uninterrupted flow) : describe the
traffic stream as a whole
•Volume or flow rate
•Speed and travel time
•Density

Microscopic (interrupted flow): describe the

behavior of individual vehicles or pairs of vehicles
within the traffic stream
•Spacing
•headway
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 Geometric Design Controls & Criteria
It relates to those characteristics of road user,
vehicle, and traffic that will affect the
improvement or optimization of the design of
various highway and street functional classes

Design parameters:
•Vehicle
•Road user
•Traffic
•Speed
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 Vehicle Characteristics
The criteria of highway G.D is partly based on
the vehicle ch. (superelevation, widening)
We use the design vehicle

Power and Speed

Lighting
Performance (acceleration, breaking/deceleration)
Dimensions (Size & weight)
Wheels conditions
Instruments (seat belt)
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 Vehicle Characteristics
The physical characteristics of the vehicle in use and
the anticipated for future are essential in highway G.D

Dimensions (Size & weight)

G.D (lane & shoulder width, grads,
pavement design

Performance
Speed (G.D, S.D)
Power (resistance)
Acceleration
Deceleration
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 Vehicle Characteristics

Safety Standards
Instruments (air bags, seat belt)
Lighting (this affects night operation)

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 Design Vehicle
1- Design vehicle – the vehicle within a class that
represents all vehicles in that class – the worst in the
class
Design vehicles are selected motor vehicles with the
weight, dimensions, and operating characteristics
used to establish highway design controls for
accommodating all vehicles of that class and is
expected to use the road with considerable frequency
For the purposes of geometric design, each design
vehicle has larger physical dimensions and a larger
minimum turning radius than most vehicles in its
class.
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 Design Vehicle Dimensions

AASHTO,
Exhibit 2-1

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 Design Vehicle
The choice of design vehicle is influenced by:
Functional classification of a roadway
Proportions of the various types and sizes of
vehicles expected to use the facility
On rural facilities, to accommodate truck traffic,
one of the semitrailer combination trucks should be
considered in design.
In urban areas that are highly built-up, intersections
may be designed to provide fully for passenger
vehicles but require the larger vehicles to swing wide
upon turning.
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 Design Vehicle
The boundaries of the turning paths of the several design
vehicles when making the sharpest turns are established by
the outer trace of the front overhang and the path of the
inner rear wheel. (front left & rear right)

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 AASHTO Turning Templates

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 Characteristics of Drivers
1- Psychological Factors
• Motivation: business, recreational, shopping,
work ….
• Emotional factors (anger, aggressiveness ,… )
• Maturity
2- Physical Factors
• Visual characteristics of drivers: the most
important
• Hearing: warning sounds
• Perception- Reaction Process
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 Visual Factors in Driving

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 Visual Factors in Driving
a) Visual Acuity: the ability to see fine details
clearness of vision
- varies among people
- depends on the light level
- the best vision occurs within a cone of 3⁰, clear
vision occurs within 10⁰ & satisfactory vision
occurs within 20⁰
- traffic signs and marking should fall within the
cone of clear vision
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 Visual Factors in Driving

b) Depth vision: important in judging distance &

speed

c) Peripheral vision: the ability to understand objects

outside the cone of clear vision 120⁰ -180⁰.
The peripheral field narrows as speed increases to as
little as 100⁰ at 20mph and 40⁰ at 60mph

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 Visual Factors in Driving
d) Color vision: the ability to differentiate one color
from another
- deficiency- color blindness – not of great
significance- shapes,…
- about 8% of male & 4% of females suffer from some
degree of color blindness

e) Glare vision and recovery:

time needed by a person to recover when moving
from dark to light is about 3 sec and can be 6 sec or
more when moving from light to dark
sunglasses can reduce this time
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 Driver Performance
The way how drivers interact with the design elements
& its information system and how they make errors
accidents and congestion
It is important to know the capabilities & limitations of
drivers which affects efficiency and safety of the roads
Errors occur when the design of the road is
incompatible with the drivers capabilities
Special consideration should be given to drivers > 65
1900 – 4%
1986 – (12 -16)%
2030 – 22%
Design driver !!!!
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 Driver Performance
Driving task:
Driving task can be complex under conditions such as
driving at high speed, poor environmental conditions,
where several individual activities need to be
performed simultaneously. Those requiring an
efficient & smooth integration and handling
information
The key for efficient and safe driving is free
information handling (without errors)

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 Driver Performance
 Causes of errors:
1. Inconsistent design
2. Deficiency of design
3. Unclear information
4. Complex situations
5. Pressure of time

 Levels of performance of activities

1. Control steering
2. Guidance road following Accidents
3. Navigation trip planning Inefficiency

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Accidents57
 Driver Performance/Guidance
Drivers use there senses to gather information
There are 2 types/source of information
1. Formal information (TCD)
2. Informal information (traffic, design elements, pav,..)

The needed information must:

1. Be available when needed
2. In usable form
3. In the field of clear view
4. Capable of getting the drivers attention

 Drivers shift their attention from one source to another

1. Making some decisions immediately
2. Delaying others
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 Perception Reaction Time (PRT)

The process though which a driver

evaluates and reacts to a situation
can be divided into 4 sub processes

PRT

Perception/ Emotion/ Reaction/

Identification
Detection Decision Response
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 Perception Reaction Time (PRT)
Perception: the driver sees a control device,
warning sign, or an object on the road

Identification: understanding the situation

Emotion: the driver decides what action to

take in response to the situation

Reaction: executing/implementing the

decision
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 Perception Reaction Time (PRT)
PRT is an important factor in the determination
of the breaking distance, so it is an important
factor in determining the min sight distance and
the yellow phase at a signalized intersection

Varies among individuals and may vary for the

same person as the occasion changes

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 Perception Reaction Time (PRT)
 PRT depends:
1. Feature expectancy
2. Complexity of the situation
3. Content of information bits
4. Age
5. DUI

 Bits quantifies the amount of information needed

to make a decision

 The longer PRT the greater the chance for an

error which leads directly to accidents
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 Perception Reaction Time (PRT)
Expectancy relates to the driver readiness to
response to common situation in a predictable
and successful way. This aids performance by
enabling rapid correct response (on freeway
ramps)

Errors may result when drivers don’t get what

they expect or get what they don’t expect
Avoid unusual design elements
Apply consistent design elements
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 Perception Reaction Time (PRT)
PRT varies bet. 3-10 sec.

American Association of State Highway &

Transportation Officials recommends: 2.5 sec.

It was found that this value (2.5 sec) is insufficient in

complicated situations and might be exceeded by 35%

Reaction distance: dᵣ = ST
where S: speed T: time
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 Volume

Distribution
Direction
Urban – consistent
Rural - variation
Composition
Passenger car (include light delivery veh)
Trucks

Trend to estimate future traffic

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 Volume
Volume or flow rate
•Traffic volume is defined as the number of vehicles passing a
point on a highway or given lane or direction of a highway,
during a specified period of time.
•Unite …. Vehicles but it is often expressed as vehicle per
unite time (veh/day, veh/hr)
•Daily volumes are used for general planning (trends)
•Hourly volumes for (peak hours of the day) for detailed
design
•Rates of flow are generally stated in unites of veh/hr but
represent flows that exist for periods less than an hour

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 Volume
Volume or flow rate
A volume of 200 vehicle observed over 15 min period
=
200 * 4 = 800 veh/hr flow rate

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 Volume
Daily Volumes
There are 4 daily volume parameters that are used in
traffic engineering

•Average annual daily traffic (AADT). The number of

vehicles passing a site in a year divided by 365 days
(veh/day)

•Average annual weekday traffic (AAWT).The number

of vehicles passing a site on weekdays in a year
divided by the number of weekdays (260) (veh/day)
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 Volume

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 Volume
Daily Volumes

•Average daily traffic (ADT). The average 24 hr volume

at a given location over a defined time period less
than a year; common application is to measure an
ADT for each month of the year (veh/day)

•Average weekday traffic (AWT). The average 24 hr

volume weekday volume at a given location over a
defined time period less than a year; common
application is to measure an AWT for each month of
the year (veh/day)
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 Volume

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 Volume
Daily Volumes are generally not differentiated by direction or
lane but are totals for the entire facility at the designated
location

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 Volume
Hourly volumes
Max volumes occurs during the morning &
evening peak (rush hour)
The single hour of the day that has the highest
hourly volume is referred to as the peak hour
The peak hour volume is stated as a
directional volume (each direction of flow is
counted separately)
Use peak hour volume in the peak direction
for design
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 Volume
In design, peak hourly volumes are estimated
from projections of AADT
Directional design hourly volumes (DDHV)
DDHV = AADT* K* D
Where:
K: proportion of daily traffic occurring
during peak hour (30th peak hour of
the year)
D: proportion of peak hour traffic
traveling in the peak direction of flow
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 Volume

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 Volume

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 Volume
Rural highway
AADT (after 20 yr) = 30,000 veh/day

What range of directional design hour volume

might be expected?
DDHV(low) = 30,000* 0.15* 0.65 = 2,925
DDHV(high) = 30,000* 0.25* 0.80 = 6,000

Large range difficulty in projecting future

traffic demand accurately
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 Volume
Sub-hourly Volumes & Rate of Flow
PHF = hourly volume = V____
max rate of flow 4*v15

PHF: peak hour factor

v15: max 15 min volume within the hour, vehs

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 Volume

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 Volume

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 Volume
Example:-
Time interval VOLUME FOR TIME INTERVAL (vehs)
5:00 – 5:15 PM 1,000
5:15 – 5:30 PM 1,100
5:30 – 5:45 PM 1,200
5:45 – 6:00 PM 900
5:00 -6:00 PM 4,200

PHF = 4,200/(4*1,200) = 0.875

Max PHF = ????, min PHF = ????
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 Traffic Composition
For design of a particular hwy, data on traffic
composition should be determined by traffic
studies

Truck traffic should be expressed as a % of

total traffic during the design hour
 For a 2-lane hwy, as a % of total 2-way
traffic
 For multilane hwy, as a % of total traffic in
peak direction of travel
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 Traffic Composition
Criteria for determining traffic composition
under urban interrupted flow conditions
At important intersections, the % of trucks
during morning and evening peaks should be
determined separately
Variation in truck traffic bet. Int. movements
may be substantial

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 Volume / Future trends
Future trends/ projection of traffic demand:

Highways are designed to accommodate

traffic that is expected to occur within the life
expectancy of the highway- assuming adequate
maintenance

Life expectancy is difficult to find since each

segment is affected by different factors
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 Design Period
Life/design period:
R.O.W 100 yr
BASE COURSE 50 yr
DRIANAGE STRUCTURE 50 yr
BRIDGE (25 – 100) yr
SURFACING (10 -30) yr

What design period do we use?

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 Design Period
Design period (year) should be no further ahead than that
for which traffic can be estimated with reasonable degree of
accuracy
Use (15 – 25) yr
20 years design period is the most commonly used
We can't justify the prediction of volumes beyond this
period due to changes in regional economy, pop., land use,…
along the road which can’t be predicted with any degree of
assurance
For rehabilitation projects, 5 -10 years design period is used
due to uncertainty in traffic prediction and funding
constraints
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 Speed
Speed and Travel Time

Speed is the rate of motion in distance per

unite time
Travel time is the time taken to traverse a
defined section of the roadway
Speed and travel time are inversely related
S=d
t
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 Speed
Limitations introduced by speed
1. Reduce visual field…..Restricts peripheral vision
2. Limits time to receive & process information

How does the designer compensate for this

limitations
1. Aiding the driver by providing adequate
information
2. Placing the information in the cone of clear vision
3. Simplification of activities

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 Speed
1. Operating speed
2. Desired speed
3. Design speed
4. Running speed
1. Operating speed (OS): the speed at which drivers are
observed operating their vehicles during free-flow
conditions. The 85th percentile of the distribution of
observed speeds is most frequently used to measure the OS
associated with a particular location or geometric feature

 The 85th speed is generally determined from speed studies

made
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on a roadway afterDr.itHanahas been opened to traffic 89
Naghawi
 Speed
2. Posted speed: 2 popular methods
 10 mph under the design speed
 The use of 85th percentile speed/operating
speed

3. Desired speed: the operating speed that

drivers will adopt on less constrained
elements (large horizontal curves of
reasonably uniform section of road when not
constrained by other vehicles)
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 Speed
4. Design speed: the speed that is adopted for the calculation
of various G.D parameters (SD, superelevation, H curve radii)

Except for local streets where speed controls are frequently

included, every effort should be made to use as high design
speed as practical to attain a desired degree of safety,
mobility, and efficiency within the constraints of
environmental quality, economics and social or political
impacts

The design safe speed is usually equal to or greater than the

85th percentile speed/operating speed
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 Speed

Higher design speed – cost more, higher standard,

higher construction cost

What are the G.D features affected by the design

speed?
S.D
Curvatures
Superelevation
Friction

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 Speed
Guidelines for the selection of the design speed
1. Logical with regard to topography
2. Functional class of the road (driver’s expectation)
3. Adjacent land use (satisfy the travel desires &
habits of nearly all drivers)
4. Use as high speed as practicable to obtain a desire
degree of Safety, Efficiency & Mobility while under
Economics, Environmental consideration &
Social/political impact
5. Don’t use extremity high or low speed at which
small percentage use
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 Speed
6. Consider the average trip length
7. Use speed increments of 10 mph for low speed and of 5
mph at high speed. Each segment of the road is subjected to
different conditions leading to different speed

Design speed impacts most safety-related features of design

as well as those associated with rideability and comfort, it can
also impact efficiency & capacity of the roadway
(R, e, S.D, g are directly related to DS)

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 Design Speed (DS)

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 Highway Design Standards
Design speed
Depends on
Functional class of hwy
Topography
Highway location
Adjacent LU

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 Highway Design Standards
5. Running speed: speed of veh over
section of the road = distance traveled/
time veh in motion

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 Density

 Defined as the number of vehs occupying a

given length of highway or lane

 Expressed as (veh/mi/ln) or (veh/mi)

 It is difficult to measure directly, it can be

estimated from speed & flow rate

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 Relationships among v, S & D
flow, speed and density, are related as follow:
v = S*D
Where:
v: rate of flow (veh/hr) or (veh/hr/ln)
S: SMS (ml/hr)
D: density (veh/mi) or (veh/mi/ln)

This equation applies only for uninterrupted

flow conditions
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 Relationships among v, S & D
The exact shape
and calibration of
these relationships
depends upon
prevailing conditions
which vary from
location to location
and even over time
at the same location

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 Relationships among v, S & D
Note that:
A flow rate of 0 veh/hr occurs under 2 very different
conditions:
When there are no veh on the hwy – density is 0
veh/mi – no vehs can be observed passing a point,
under this condition speed is free flow speed
When there are so many vehs on the road that all
motion stops – very high density – jam density
Between these 2 extreme points, the peaking
flow-speed and flow density curves is the max v or
capacity of the roadway
Operation under cap is so unstable
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 Relationships among v, S & D
The solid portion of the curves represents stable
flow
The dashed portion of the curves represents
unstable or forced flow (breakdown, queuing,.. v >
cap.)
Except for cap. flow any flow rate may exist under 2
conditions:
A condition of relatively high speed and low
density (on stable portion of flow relationship)
A condition of relatively low speed and high
density (on the unstable portion of flow
relationship)
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 Capacity & LOS
Capacity is the max hourly rate at which vehs/persons can
reasonably be expected to traverse a point or uniform section
during a given time period under prevailing roadway, traffic
and control conditions

It is essential for:

1. Determine type of hwy facility
2. Estimate the max amount of traffic that can be
accommodated by a facility
3. Provide basis for planning, design & operation of roads
4. For a given # of lanes, it provide a means for determining
the traffic carrying performance (LOS)
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 Capacity & LOS
Prevailing roadway conditions - refer to the
geometric characteristics of the road
1. Type of facility
2. # of lanes
3. Lane & shoulder width
4. Lateral clearance
5. Design speed
6. H & V alignment

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 Capacity & LOS
Prevailing traffic conditions - refer to the characteristics of
TS using the facility
1. Traffic composition
2. Distribution of veh by lane and by direction
3. Speed, vol, density,….

Prevailing control conditions – refer to type & specific

design of control device & traffic regulations
1. Location & timing of traffic signals
2. Turn restrictions

 Under ideal conditions

Freeway cap = 2,000 pcphpl
2- lane
2/8/2018
2-way hwy = 2,800 pcph
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 Capacity & LOS
Level of Service (LOS)
A qualitative measure describing the traffic
operational conditions and perception by motorists/
passengers within TS

We can say that the LOS is a qualitative measure

representative of the operation of a road for a given
traffic flow, that takes into account a group of factors
(speed, manoeuvre, safety, comfort and cost) that
take place in it

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 Capacity & LOS

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 Capacity & LOS
It is from A to F, A – best & F – worst

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LOS Flow conditions Freedom to select Level of comfort &
speed & maneuver convenience
A Free flow Extremely high Excellent
B Stable Reasonable Less than A
C Stable Begins to be Decline noticeably
restricted
D High density- limit of Severely restricted Poor
stable approaching
unstable
E Unstable/ near Low uniform Extremely poor
capacity- any increase speed extremely
of flow will cause difficult to
operational problem maneuver
F Forced flow none Breakdown occurs

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 Service Flow Rate (SFR)
SFR is the max hourly rate at which vehs/persons
can reasonably be expected to traverse a point or
uniform section during a given time period under
prevailing roadway, traffic and control conditions
while maintaining a designated LOS (cap of each LOS)

Every LOS has SFR except LOS F – no flow

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 Safety of Design
Safety of design
1. Full access control- the most significant measure to
improve safety
2. Speed
3. Use either control device or channelization at
intersections
4. Consider driver characteristics
5. Environmental impact of design
Measures to improve safety & efficiency of traffic
operations:-
•Reduce # of conflict points
•Signal phase for pedestrians
•Convert 2-way to one way
•Reduce turning & unnecessary
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cross walks 111
 Basic Design Considerations
The geometric design of roads covers the design
features of roadway associated with safe, efficient and
comfortable travel

Basic design consideration includes:

1. Design speed
2. Sight distance
3. Horizontal alignment
4. Vertical alignment
5. Co-ordination of H & V alignments

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