Page 94 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 77

what traffic demands will be 20 years from the current year. Shorter time periods, such
as 10 years, may apply to resurfacing projects or other minor repair projects. It is impor-
tant that within the same jurisdiction traffic data be forecast using the same methods
and techniques, in order to ensure similar designs for similar type roadways. This is
especially true for roadways in a given state jurisdiction.
The following types of traffic numbers are used most frequently in design:
Average daily traffic (ADT). The average number of vehicles using a roadway in
a 24-hour period.
Design hourly volume (DHV). The estimated number of vehicles using the roadway
in the 30th highest hour of the year. This number is generally 8 to 12 percent of the
ADT and is used extensively in determining lane widths and shoulder characteristics
of the roadway cross section.
Directional design hourly volume (DDHV). The estimated number of vehicles
traveling in one direction of a two-way roadway in the 30th highest hour of the
year. This number must be at least 50 percent of the DHV and is usually in the range
of 50 to 60 percent. A higher value would indicate that the roadway is a major link
in the commuter network, carrying a heavy inbound load in the morning and
reversing that flow in the evening.
Truck percentage (T). The portion of the ADT which consists of B and C trucks.
Traffic counts are usually separated according to vehicle type:

P passenger cars (%)
A commercial (%), consisting of light delivery trucks, panel trucks, and pickup
trucks
B commercial (%), consisting of semitrailer and truck-trailer combinations
C commercial (%), consisting of buses or dual-tired trucks having single or tandem
rear axles

Traffic counts sometimes group the P and A vehicles together and the B and C together.
Terrain is a factor that can significantly influence design features, especially in
rural areas. Various categories of terrain are level, rolling, and hilly. They are further
described as follows:
Level terrain. Any combination of grades and horizontal and vertical alignment
permitting heavy vehicles to maintain approximately the same speed as passenger
cars. Grades are generally limited to 1 or 2 percent.
Rolling terrain. Any combination of grades and horizontal and vertical alignment
causing heavy vehicles to reduce their speeds substantially below those of passenger
cars, but not to operate at crawl speeds.
Hilly terrain. Any combination of grades and horizontal and vertical alignment
causing heavy vehicles to operate at crawl speed.

Heavy vehicles are defined as any vehicle having a weight (pounds) to horsepower
ratio of 200 or greater (Ref. 1). Crawl speed is defined as the maximum sustained
speed heavy vehicles can maintain on an extended upgrade. See Ref. 1 for graphs
showing the effect of grades on acceleration and deceleration of heavy vehicles.
Locale describes the character and extent of development in the vicinity. It can be
considered commercial, industrial, or residential, as well as rural or urban.