Transportation planners use a variety of traffic volume statistics to monitor and plan for traffic on roadways. Two of the most important statistics used are ADT, or average daily traffic, and AADT, or average annual daily traffic.
A roadway’s average daily traffic is the volume of vehicles counted over a given time period -- greater than one day but less than one year -- divided by the number of days in that time period. Average annual daily traffic is a similar measure. To compute the AADT of a roadway, the daily traffic counts collected over one year are added and then divided by 365 days.
Automated traffic counters on major roadways, installed by state departments of transportation, provide continuous counting of vehicle traffic. Temporary automated counters -- including road tubes or video cameras -- collect traffic data over a shorter duration, usually less than a week.
Shorter-term traffic counts -- weekly or monthly -- estimate annual traffic demand when complete annual counts are not available. Adjustments to actual traffic counts to reflect daily, weekly and seasonal variations of traffic on the roadway under study provide an estimate of annual traffic.
Uses for Data
Analysts use average daily traffic estimates to monitor the growth in traffic on a roadway from year to year and for programming and funding of major improvements, such as roadway widening. Average daily traffic is also useful for analyzing the rate of traffic accidents on a roadway.
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