Estimation of Passenger Car Equivalents for single-lane roundabouts using a microsimulation-based procedure

This paper addresses the question of how to estimate the passenger car equivalents for heavy vehicles on single-lane roundabouts. First, a comparison was performed between the empirical capacity functions based on a meta-analytic estimation of the critical and the follow up headways and the simulation outputs manually obtained for a single-lane roundabout built in Aimsun microscopic simulator. A genetic algorithm-based calibration procedure, therefore, was used to reach a better convergence between the simulation outputs and the empirical capacities. Based on the calibrated model, the passenger car equivalents were determined by comparing the capacity functions built for a fleet of passenger cars with the capacity functions calculated for different percentages of heavy vehicles. Differently from HCM 2010 which assumes a heavy vehicle to be equivalent to two passenger cars and sets as 2.0 the passenger car equivalents for heavy vehicles for roundabouts, a higher PCE effect would be expected on the quality of traffic conditions when the traffic stream contains a high number of heavy vehicles; this effect should be accounted for when calculating capacity and level-of-services.