highway geometric design control and criteria-2

Design Controland Criteria-2

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Traffic characteristics - Introduction• The design of a highway depends upon:

– traffic volumes

– traffic characteristics

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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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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


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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

Traffic characteristics - Volume• Basic measure of the traffic

demand - average daily traffic

• ADT measurement - periodic traffic counts

• Season, month, or day ofweekadjustment 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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Traffic characteristics - VolumeDirectional Distribution

• Highways must be designed toadequately serve the peak-hour trafficvolume in the peak direction of flow.

• Total hourly traffic in both directions isused to design two-lane roads

• Typically, one direction contributes by 55-70% in total traffic, although occasionally 80% is observed.

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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


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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

• 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 i r

f l o w ) DDHV (low) 30,000*.15*.65

2,925veh / hDDHV(high) 30,000*.25*.80 6,000veh/ h


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Composition of Traffic –Traffic Mix• Vehicles operating characteristics:– Sizes– Weight– Speed– Space consumed on highway

Traffic characteristics - VolumeComposition 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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Traffic characteristics - VolumeComposition of Traffic –Traffic Mix• Trucks are equivalent to several passenger cars depending on the

gradient and passing sight distance available

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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

Traffic characteristics - SpeedSpeed - 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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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

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 to110increment

highway geometric design control and criteria-2

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