Empirical Estimation of Capacity for Roundabouts Using Adjusted Gap-Acceptance Parameters for Trucks

Transportation Research Record: Journal of the Transportation Research Board, No. 2312, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 34–45. DOI: 10.3141/2312-04 Department of Civil and Environmental Engineering, University of Windsor, Windsor, Ontario N9B 3P4, Canada. Current affiliation for J. Dahl: Lea Consulting Ltd., 625 Cochrane Drive, Suite 900, Markham, Ontario L3R 9R9, Canada. Corresponding author: C. Lee, [email protected]. higher than passenger cars, it is expected that the capacity will decrease more rapidly as the number of trucks in the circulating flow increases. However, this adjustment method did not capture the effect of trucks on driver gap-acceptance behavior, which is essential to understand the roundabout operation affected by trucks (2). It was expected that a truck driver’s gap-acceptance behavior would be different from a car driver’s as the result of slower truck entry and longer gaps required by trucks. Current gap-acceptance capacity models describe the capacity as a function of the circulating flow, the critical headway, and the follow-up time. The “critical headway” is defined as the minimum time gap between the circulating vehicles accepted by the entering vehicles. The “follow-up time” is defined as the time gap between two queued vehicles that enter the roundabout by using the same gap between the circulating vehicles. However, most past studies assume only single values of the critical gap and the follow-up time for cars and trucks. Thus, the objectives of this study are to (a) identify the limitations of the existing roundabout capacity estimation methods with respect to truck traffic, (b) develop a method to take into account the effect of trucks on the capacity, and (c) evaluate the accuracy of capacity estimation with the proposed method by comparing with the observed capacity.