Estimation of Passenger Car Equivalents for Two-Lane and Turbo Roundabouts Using AIMSUN

Granà, Anna; Giuffrè, Tullio; Macioszek, Elżbieta; Acuto, Francesco · 2020 · OpenAlex-citations

DOI: 10.3389/fbuil.2020.00086

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Summary

This study addresses the estimation of Passenger Car Equivalents (PCEs) for heavy vehicles at two-lane and turbo roundabouts, challenging the constant PCE value of 2.0 recommended by the Highway Capacity Manual (HCM). The authors argue that curvilinear geometric designs significantly impact heavy vehicle trajectories and swept envelopes, suggesting that PCEs should vary based on traffic composition and roundabout type. The research aims to provide a methodological approach using traffic microsimulation to determine more accurate PCEs, specifically comparing a standard two-lane roundabout with a comparable-sized basic turbo roundabout. The methodology employed the AIMSUN microscopic traffic simulation software to model a two-lane roundabout in Palermo, Italy, and a designed basic turbo roundabout. Empirical capacity functions for both facilities were derived using the Hagring model, calibrated with behavioral headway data from field studies and literature. These empirical capacities served as target values for model calibration. A genetic algorithm optimized AIMSUN parameters to minimize the difference between simulated and empirical entry capacities. The study simulated traffic scenarios with varying percentages of single-unit trucks (10% and 20%) to isolate their impact on capacity. PCEs were estimated by comparing the capacity reduction caused by heavy vehicles against a baseline of passenger cars, focusing on entry lanes with similar maneuver mechanisms. The results indicate that PCEs are not constant but depend on operational conditions, particularly the level of circulating flow and the percentage of heavy vehicles. The study found that higher circulating flows necessitate higher PCE values than those proposed by the HCM. Furthermore, the analysis revealed distinct impacts of heavy vehicles on the operational performance of two-lane versus turbo roundabouts. The physical lane separation in turbo roundabouts influences gap acceptance and capacity differently than in standard multi-lane roundabouts, necessitating differentiated PCE estimations for each entry lane configuration. The microsimulation approach successfully captured these variations, demonstrating that constant PCE values can misinterpret the effect of trucks on traffic quality and capacity. The significance of this work lies in its contribution to more accurate capacity and level-of-service determinations for roundabouts, particularly when evaluating design alternatives such as converting standard roundabouts to turbo configurations. By providing a simulation-based method for estimating PCEs, the study offers a tool for engineers to better assess the life-cycle costs and operational performance of intersection designs. The findings suggest that future capacity models should account for the dynamic nature of PCEs in mixed traffic conditions, improving the reliability of traffic analysis for both standard and alternative roundabout geometries.

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• intersection design