Abstract
Due to its geometric design, turbo-roundabouts impose greatest constraints to the vehicular trajectories; by consequence, one can expect a more unfavourable impact of heavy vehicles on the traffic conditions than on other types of roundabouts. The present paper addresses the question of how to estimate Passenger Car Equivalents (PCEs) for heavy vehicles driving turbo-roundabouts. The microsimulation approach used revealed as a useful tool for evaluating the variation of quality of traffic in presence of mixed fleets (different percentages of heavy vehicles). Based on the output of multiple runs of several scenarios simulation, capacity functions for each entry lane of the turbo-roundabout were developed and variability of the PCEs for heavy vehicles were calculated by comparing results for a fleet of passenger cars only with those of the mixed fleet scenarios. Results show a dependence of PCEs for heavy vehicles on operational conditions, which characterise the turbo-roundabout. Assuming the values of PCEs for roundabouts provided by the 2010 Highway Capacity Manual (HCM), depending on entering manoeuvring underestimation and overestimation of the effect of heavy vehicles on the quality of traffic conditions have been found.
Highlights
Turbo-roundabouts represent a recent scheme of roundabouts, which provide a spiralling traffic flow and require drivers to choose their direction before entering the intersection, since physical barriers mark the lanes on the ring (Fortuijn 2009)
Starting from the initial belief that the constraints to the vehicular trajectories imposed by the turbo-roundabout design imply that the impact of heavy vehicles on the quality of traffic flow is higher if compared to other roundabouts, a microsimulation-based method was applied to evaluate the variation of the quality of traffic in presence of mixed fleets, each having different percentages of heavy vehicles
Based on the output of multiple runs of the different mixed fleet scenarios simulated in Aimsun, capacity functions for each lane from the turbo-roundabout entries were developed and the Passenger Car Equivalents (PCEs) for heavy vehicles driving turbo-roundabouts were calculated by comparing a fleet of passenger cars only with a mixed fleet characterised each time by different percentages of heavy vehicles
Summary
Turbo-roundabouts represent a recent scheme of roundabouts, which provide a spiralling traffic flow and require drivers to choose their direction before entering the intersection, since physical barriers mark the lanes on the ring (Fortuijn 2009). This paper addresses the question of how to estimate Passenger Car Equivalents (PCEs) for heavy vehicles driving turbo-roundabouts. This study starts from the belief that constraints to the vehicular trajectories imposed by the turbo-roundabout design imply a more unfavourable impact of heavy vehicles on the quality of traffic flow than on other roundabouts; higher PCEs than the established values (Highway Capacity Manual 2010) are expected. As Huber (1982) proposed, PCEs were calculated by comparing entry-lane capacities both for a fleet of passenger cars only and for a mixed fleet. Estimations of capacity for each entry lane of the turbo-roundabout were obtained by using microsimulation, varying the percentage of heavy vehicles in traffic demand. Some researchers have used Aimsun for traditional roundabouts in the recent past (Gingrich, Dion 2012; Zhang, Excell 2013; Zeņina, Merkuryev 2009)
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