Capacity-based calculation of passenger car equivalents using traffic simulation at double-lane roundabouts

Calculation of passenger car equivalents for heavy vehicles represents the starting point for the operational analysis of road facilities and other traffic management applications. This paper introduces a criterion to find the passenger car equivalents that reflect traffic conditions at double-lane roundabouts, where the capacity is typically estimated for each entry lane. Based on the equivalence defined by the proportion of capacity used by vehicles of different classes, the criterion implies a comparison between the capacity that would occur with a traffic demand of passenger cars only and the capacity reached beginning from a demand with a certain percentage of heavy vehicles. A preliminary activity consisted of the comparison of the empirical capacity functions based on a meta-analytical estimation of critical and follow up headways, and simulation output data derived for a double-lane roundabout built in AIMSUN. The formulation of the calibration process as an optimisation problem enabled to minimize an objective function using the genetic algorithm tool in MATLAB®. A subroutine in Python implemented the automatic interaction with AIMSUN. Differently from methods that propose constant values for the passenger car equivalents, the results highlighted that the passenger car equivalents at double-lane roundabouts increased when the circulating flow increased, while a higher effect was expected when the traffic streams included a higher number of heavy vehicles.