Traffic Factoring

The Planning Data Team is able to calculate an Annual Average Daily Traffic (AADT) from a short-term count using factoring procedures.

Axle Factoring compensates for any overcounting that results from using road tubes. Road tubes count the number of times the tubes are compressed and divides the total by two. This would cause overcounting in the raw count because multi-axle vehicles would be counted as multiple passenger vehicles. TDOT currently applies a single axle-correction factor to each raw count depending on which of the 76 classifications groups matches the station information. The factor is derived from the distribution of class data from the stations within the same classification group that collected class data.

Seasonal Factoring adjusts the raw short-term so that it is not skewed by the seasonality of collection. Each raw count is assigned to a seasonal group of continuous count stations. Seasonal adjustment factors are calculated by taking the percentage of each weekday of each month compared to the average. For example, a count collected on Wednesday, March 6 would be multiplied by the ratio of the AADT divided by the count for an average Wednesday in March for the same seasonal group. If AADT/(Wednesday in March) = .989, then the raw count would be multiplied by .989, reducing the total since it was collected on a higher than average traffic day. The hours counted on distinct days are assigned a different seasonal factor, so if the count started mid-day on a Wednesday and ended mid-day on the following Thursday, two different factors would be applied.

Class Distribution Factoring calculates class data for stations without raw classification counts. Each station is assigned the average distribution into all 13 FHWA vehicle classes based on which of the 76 classifications groups matches the station information. The percent distribtions are applied to the AADT to create 13 Class AADTs.