Estimation of Passenger Car Equivalents on Basic Freeway Segments from Field-Observed Traffic Data

Abstract

Passenger car equivalents (PCEs) represent the effects of heavy vehicles on traffic operations. PCE estimation is based on equating the passenger-car-only flow rate to the mix-fleet flow rate such that it results in the same performance for the selected measure. Most existing methods rely on simulation to generate PCE values. However, this approach generates PCEs using the truck characteristics of the simulation rather than local field conditions and heavy vehicle types. Most existing methods estimate the marginal effect of adding one specific truck in the traffic stream assuming relatively high volumes, which results in higher PCE values. This study proposes a new method for estimating PCEs using field-observed traffic data considering the impact of each truck type for a broad set of flows, not just their marginal effect at high flows. Based on density equivalency, the maximum likelihood estimation is used to estimate the means and the variances of the PCE values from traffic data collected at 10 sites on the national motorways of Thailand. When aggregated across all observed percentages of heavy vehicles and flow rates, the PCE values are 1.46 (trucks with length between 5.2 and 13.0 m) and 2.08 (trucks with length of 13.1 m or longer). For low to medium percentage of trucks, the PCEs become lower after the onset of oversaturation. However, for high percentages of trucks the PCEs tend to be higher after the onset of oversaturation. The proposed methodology can be applied using data from other locations to estimate the corresponding PCEs from field-observed data.