Evaluation of Roundabout Capacity Models: An Empirical Case Study

Based on field data collected at nine roundabouts in Gold Coast, Australia, this paper evaluates the performance of the capacity estimation for single-lane roundabouts using analytical models [including the highway capacity manual (HCM) 2000 model, the German Highway Capacity Manual (GHCM) model, the signalized and unsignalized intersection design and research aid (SIDRA) model and a new roundabout capacity (NRC) model] and an empirical model (the HCM 2010 model). First, this study calibrates critical gaps, follow-up times, and conflicting flows. Compared with the capacities measured in the field, the authors carry out a study to analyze the accuracy of the HCM, GHCM, SIDRA, and NRC models. The results show that the five models underestimate capacity but the NRC model produces a smaller range of relative error (-1.07 to -5.74%) and root-mean-square deviation (47.68) than the HCM 2010 (-4.62 to -16.14% and 105.00, respectively), HCM 2000 (-5.76 to -17.21% and 115.29, respectively), GHCM (-8.95 to -21.26% and 146.82, respectively), and SIDRA (-5.15 to -17.51% and 113.48, respectively) models. Accordingly, the NRC outperforms the other four models for the surveyed roundabouts. The five models estimate a similar capacity in low and medium traffic volumes, and exiting vehicles play an important role in high traffic volumes, and at high traffic condition exiting vehicles warrant more entry opportunities that exceeds the impact of high circulating volumes on entry capacity.