Developing passenger car equivalents for heavy vehicles on freeways during queue discharge flow

This paper investigates the hypothesis that the effect of heavy vehicles on traffic is greater during congestion than during undersaturated conditions. A new approach was developed to quantify this effect by deriving passenger car equivalents (PCEs) using queue discharge flow (QDF) capacity as the equivalency criterion. This approach is based on the premise that QDF capacity observations can be expected to show minimal variation if traffic stream was uniform and consisted of passenger cars only. Two sites in Ontario, Canada were used for this research. The first is located at an entrance ramp merge area and the second at a long-term freeway reconstruction zone. Nonlinear programming was utilized to perform optimizations on a number of data sets at each site. Results strongly suggest that the research hypothesis is true and that the approach developed by this research is both plausible and feasible. The mean PCE factor at the first site was 2.36 versus 1.5 in the Highway Capacity Manual (HCM) 2000. At the second site, the mean PCE factors in the two directions of travel were 3.21 and 2.7 versus 2.0 in the HCM 2000. Results also showed that the PCE factor developed from the optimization runs behaves as a random variable that generally follows the normal distribution. Furthermore, the PCE factor was neither a function of weather conditions nor of roadside maintenance work.