The physical design of a new highway is controlled by many factors. This module addresses factors common to most of the functional classifications.

Design Speed

"Design speed is the maximum safe speed that can be maintained over a specified section of a highway when conditions are so favorable that the design features of the highway govern." (AASHTO, 1990). The selection of a suitable design speed will depend on the terrain and functional class of the highway. Typical design speeds for freeways range from 50 mph to 70 mph depending on the terrain type (level, rolling or mountainous).

Traffic Volume

The traffic engineer’s measure or indicator of traffic volume is the average daily traffic (ADT). The ADT is the volume that results from dividing a traffic count obtained during a given time period by the number of days in that time period. For example, given a traffic count of 52,800 vehicles that was taken over a continuous period of 30 days, the ADT for this count equals 1,760 vehicles (52,800 divided by 30). Another commonly used measure of traffic volume is the annual average daily traffic (AADT), which is determined by dividing a count of the total yearly traffic volume by 365. The ADT and the AADT are not the same and it’s important to be aware of the time period when calculating the ADT.

Design Hour Volume

The DHV is a two-way traffic volume that is determined by multiplying the ADT by a percentage called the K-factor. Values for K typically range from 8 to 12% for urban facilities and 12 to 18% for rural facilities. Neither the AADT nor the ADT indicate the variations in traffic volumes that occur on an hourly basis during the day, specifically high traffic volumes that occur during the peak hour of travel. The traffic engineer has to balance the desire to provide an adequate level of service (LOS) for the peak hour traffic volume with proposing a design in which the highway capacity would only be utilized for a few hours of the year. This is where the design hour volume (DHV) comes in.

Directional Design Hour Volume

The directional design hour volume (DDHV) is the one-way volume in the predominant direction of travel in the design hour, expressed as a percentage of the two-way DHV. For rural and suburban roads, the directional distribution factor (D) ranges from 55 to 80 percent. A factor of approximately 50 percent is used for urban highways. Keep in mind that the directional distribution can change during the day. For example, traffic volume heading into the central business district is usually higher than outbound traffic in the morning, but the reverse is true during the afternoon peak hour. In summary, DDHV = ADT (or AADT)*K*D.

Vehicle Characteristics

Traffic engineers design highways that will accommodate all classes of vehicles. Width and height, overhangs and minimum turning paths at intersections are important parameters to have at hand during the design process. AASHTO states that the vehicle which should be used in designing for normal operations is the largest one that represents a significant percentage of the traffic for the design year.

Geometric Design Elements