Probabilistic Capacity Modeling for Single-Lane Roundabout

HCM 2000 model estimates entry capacity of roundabouts under an assumption of headway (time gaps) following an exponential distribution. Entry capacity is thus calculated by circulating flow, critical gap and follow-up time. This paper aims to analyse: (1) the actual distribution type of headway; and (2) the relative error resulting from the assumption. To this end, a simulation that simulates the real traffic operation is conducted based on survey data from an existing roundabout in Queensland, Australia. A comparative study is then carried out to compare the results from the HCM 2000 model and the simulation. It indicated that the HCM 2000 model is not appropriate to be established under an exponential distribution. Although HCM 2000 model over-estimated capacity in our case study, it is still an acceptable approximation. Results suggest that the relative error is 6% under the circulating flow of 468 vehicles/hour.