Signalised Intersection Capacity Reduction of Trucks

Abstract

The lower acceleration capability of a heavy vehicle (HV) compared to that of a passenger car results in greater headways for the HV and vehicles behind it departing a queue at an intersection approach. The Highway Capacity Manual would associate this with increased start loss if the HV were located in the first four vehicles in a queue of vehicles, or with reduced saturation flow if located in the fifth or greater queue position. However, in determining saturation flow rate without conducting a survey, HVs are accounted for through the use of a passenger-car equivalency factor to reduce saturation flow – no provision is available to adjust start loss or end gain with HVs. A simple microsimulation model was developed to demonstrate the effects of HVs at different queue positions at a signalised intersection approach. Control delay was found to be greatest with the HV at the front of the queue, decreasing to that of the non-HV case as the HV was moved rearwards. Knowing the probability distribution of HVs amongst queue positions, an average control delay could be determined. Additional traffic-related effects of HVs are discussed, such as disproportionate lane utilisation behind and beside HVs.