AADT

AADT, short for Average Annual Daily Traffic, is a standard traffic engineering metric describing the average number of vehicles passing a point or road segment on a typical day over a year. Expressed as vehicles per day (vpd), it provides a single, comparable measure of traffic volume for planning and design purposes.

Calculation and data: AADT is usually derived from continuous traffic counts at permanent or temporary counters over a full year, with missing data imputed and seasonal adjustments applied. When long-term counts are not available, estimates are made from shorter counts or models using factors such as road type, location, and nearby demand. Agencies may also report separate AADT values for different vehicle types, for example Truck AADT (T-AADT) or adjust by the percentage of trucks.

Applications: AADT supports highway planning and capacity analysis, corridor evaluation, safety assessments, and funding decisions. It is used to compare traffic volumes across routes and to project future conditions for design standards and maintenance planning.

Limitations: As an annual average, AADT smooths daily and hourly variability and can obscure peak-period stresses or seasonal effects. It does not convey the timing of traffic or vehicle mix unless supplemented by additional metrics such as peak hour volumes, the peak-hour factor, or vehicle-type breakdowns.