Application of Seasonal Adjustment Factors to Subsequent Year Data

Seasonal adjustment factors are used by state highway agencies to convert short-term traffic counts into estimates of annual average daily traffic (AADT). These factors are typically calculated from continuous traffic counter data. The protocol for the process is described in FHWA's Traffic Monitoring Guide. This protocol specifies that temporary counts collected during a year should be adjusted by seasonal factors computed from continuous data collected during the same calendar year. The authors had suggested that, if the prior year seasonal factors could be used, AADT estimates would be available soon after temporary counts were collected. This study investigated whether the seasonal factors from sequential years could be applied without loss of statistical accuracy. No significant loss of accuracy was found in estimating AADT when prior-year seasonal adjustment factors were applied to current-year short-term counts of traffic volume. Likewise, no significant loss of accuracy was found when vehicle miles of travel were computed by using prior-year seasonal adjustment factors.