This document discusses traffic characteristics that influence highway design, including volume, composition, speed, and projections. It notes that traffic volume indicates the need for improvements and influences design features like lanes and widths. Average daily traffic and peak hour volumes are important metrics. Directionality of traffic must also be considered. Traffic composition, including the number of trucks, affects design due to size and speed differences versus passenger vehicles. Design speed, a selected speed used for geometric design, should fit drivers' expectations and be consistent along a highway. Projecting future traffic over a 20-year period is common for design.

1. Design Control and
Criteria-2
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2. Traffic characteristics - Introduction
• The design of a highway depends upon:
– traffic volumes
– traffic characteristics
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3. Traffic characteristics - Introduction
• Other factors - ?
• Traffic volumes indicates the need for the improvement and directly
influence the selection of geometric design features, such as
number of lanes, widths, alignments, and grades
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4. Traffic characteristics - Volume
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• Traffic Volume.
Number of vehicles passing a point on a highway during a
specified time interval. Units: Simply vehicles or vehicles per unit
time (veh/day; veh/hr)
• Average annual daily traffic (AADT).
The average 24-hour volume at a given location over a full 365-
day year.
• Average daily traffic (ADT).
The average 24-hour volume at a given location over a defined
time period less than one year; a common application is to
measure an ADT for each month of the year

5. Traffic characteristics - Volume
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Peak-Hour Traffic
• Peak Hour Volume - The single hour of the day that has the highest
hourly volume is referred to as “Peak Hour volume”.
• Design traffic volume
– a value that will not be exceeded very often or by very much
– Sufficiently utilized
• It is recommended that the hourly traffic volume that should
generally be used in design is the 30th highest hourly volume of the
year, abbreviated as 30 HV

6. Traffic characteristics - Volume
• Basic measure of the traffic
demand - average daily traffic
• ADT measurement - periodic
traffic counts
• Season, month, or day of week
adjustment to period counts
• ADT - does not indicate traffic
volume variations
• A highway designed for the traffic
on an average day would need to
carry a greater volume on some
days
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7. Traffic characteristics - Volume
Directional Distribution
• Highways must be designed to
adequately serve the peak-hour traffic
volume in the peak direction of flow.
• Total hourly traffic in both directions is
used to design two-lane roads
• Typically, one direction contributes by
55-70% in total traffic, although
occasionally 80% is observed.
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8. Traffic characteristics - Volume
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Directional Distribution - Example
• For example, consider a rural road with a design volume of 4,000
vehicles per hour (vph) for both directions of travel combined
• If during the design hour, the directional distribution is equally split,
or 2,000 vph is one direction, two lanes in each direction may be
adequate
• If 80 percent of the DHV is in one direction, at least three lanes in
each direction would be needed for the 3,200 vph

9. Traffic characteristics - Volume
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Directional Distribution -Directional Design Hourly Volumes
 DDHV- ADTs are converted to a peak-hour volume in the peak
direction of flow
DDHV  AADT * K(Peakhr)* D(Peakdir  flow)
 K = proportion of daily traffic occurring during peak hour
 D = proportion of peak hour traffic travelling in peak direction
of flow
 For design, the K factor often represents the proportion of ADT
occurring during the 30th peak hour of the year

10. • Example Consider the case of a rural highway that has a
20-year forecast of AADT of 30,000 veh/day, for given
highway, the K factor ranges from 0.15 to 0.25, and the D
factor ranges from 0.65-0.80.
D D H V  A D T * K ( P ea kh r ) * D ( P ea kd ir  flo w )
DDHV (low)  30,000*.15*.65  2,925veh / h
DDHV(high)  30,000*.25*.80  6,000veh/ h
Traffic characteristics - Volume
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11. Traffic characteristics - Volume
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Composition of Traffic –Traffic Mix
• Vehicles operating characteristics:
– Sizes
– Weight
– Speed
– Space consumed on highway

12. Traffic characteristics - Volume
Composition of Traffic –Traffic Mix
• Consequently, trucks have a greater individual effect on highway
traffic operation than do passenger vehicles
• The effect on traffic operation of one truck is often equivalent to
several passenger cars
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13. Traffic characteristics - Volume
Composition of Traffic –Traffic Mix
• Trucks are equivalent to several passenger cars depending on the
gradient and passing sight distance available
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14. Traffic characteristics - Volume
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Projection of Future Traffic Demands
• Geometric design based on future traffic volumes
• Defining life of highway / Life-cycle
• Physical life expectancies:
– right-of-way and grading
– bridges
– pavement
- 100 years,
- 25-100 years,
- 10-30 years.
• Period of 20 years is widely used as a basis for design

15. Traffic characteristics - Speed
Speed - distance per unit time
• Speed is one of the most important factors considered by
travelers in selecting alternative routes or transportation
modes
• Travelers assess the value of a transportation facility by its
convenience and economy, which are directly related to its
speed
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16. Traffic characteristics - Speed
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Design Speed:
• Design speed is a selected speed used to determine the various
geometric design features of the roadway
• The selected design speed should be a logical one with respect to
the anticipated operating speed, topography, the adjacent land use,
and the functional classification of the highway
• In selection of design speed, every effort should be made to attain a
desired combination of safety, mobility, and efficiency within the
constraints of environmental quality, economics, aesthetics, and
social or political impacts
• Some design features, such as curvature, superelevation, and sight
distance, are directly related to, and vary appreciably with, design
speed

17. Traffic characteristics - Speed
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Design Speed Important Considerations
• Should be consistent with drivers' expectations
• Should fit the traffic habits of nearly all drivers
• Should not change over a substantial length of highway
km/h with 10 km/h
• Depends on the functional class
• Ranges from 30 km/h to 110
increment