Analysis of gap parameters for the estimation of single lane roundabouts' capacity in the State of Qatar

Abstract

Entry capacity is an important parameter for evaluating the roundabout performance. In general, entry capacity is influenced by three gap parameters: critical gap, follow-up time and minimum headway of circulating vehicles. These gap parameters certainly are correlated with the driving behavior as well as the roundabout characteristics. In this paper, video records of two single roundabouts in the state of Qatar were used to estimate the gap parameters. In addition, the entry capacity of both roundabouts was estimated and compared through different methodologies. Results showed that the entry capacity is directly related with the inscribed circle diameter (ICD) until 400 vph of circulating flow. However, it is inversely related with ICD if the circulating flow exceeds 400 vph. Moreover, we found that the entry capacity has a negative relation with the gap parameters. Estimated entry capacities with the calibrated Highway Capacity Manual 6th Edition model (HCM, 2016) were significantly higher than estimated ones through the proposed approach by Qatar Highway Design Manual (QHDM, 2015). Interestingly, the entry capacity was underestimated in both approaches if default gap parameter values are used compared to the calibrated models using the observed gap parameters (36.7% less for HCM, and 19.4% less for QHDM). Thus, the utilization of realistic gap parameter values representing local traffic condition is essential for traffic planners to accurately estimate the entry capacity and accordingly ensure feasible design of different types of roundabouts.