Impact of Assumed Versus Measured Vehicle Distributions on County Road Flexible Pavement Design

Abstract

Accurate characterization of traffic is critical to the design of pavement structures. At the low-volume road level, a common practice is to use assumed vehicle type distributions based on statewide or regional studies instead of measuring the traffic for a particular location. The impact of this practice was investigated, and equivalent single-axle loads determined from measured vehicle distributions on 29 county roads in Minnesota were compared with those calculated from assumed distributions. It was found that significant differences exist between the assumed distribution and the measured distribution. Additionally, the impact of using assumed distributions on the structural design of flexible pavements was investigated. With data collected from 21 different county roads in Minnesota, it was found that using assumed distributions can result in over- or under-designed pavements, with asphalt concrete thicknesses often deviating by more than 50 mm (2 in.). Based on the results, it was recommended that Minnesota counties begin to evaluate when more detailed traffic studies are warranted to improve their designs and use of resources.