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: