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

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

Capacity Analysis of Single-Lane Roundabouts in The State of Qatar

This master thesis presents capacity analysis of Single-Lane roundabouts in the State of Qatar. The aim of this paper is to update Qatar highway manual, by developing a new capacity model using empirical data collected from directly the field for a single lane roundabout, also to compare the obtained capacity model with the one provided by the Highway Capacity Manual (HCM, 2016). The operations of single-lane roundabouts should be studied to ensure a safe and smooth flow for road users. This study aims to analyze drivers’ behavior and estimate the capacity of two single-lane roundabouts using critical gap, entry headway gap and circulating headway gap. Video data were collected at two roundabouts located in Doha, Qatar. The analysis estimated the critical gap tc, follow up headway gap tf and circulating headway gap for each approach alone. Then capacity was calculated using the approach of the most critical conditions. The capacity was estimated using the Highway Capacity Manual (HCM, 2016) approach. The critical gap was different among the two sites with an average of 3.39 sec for Site 2 and 2.61 secs for Site 1. The average circulating headway gap was 2. 51 and 2.24 seconds for Site 2 and Site 1 respectively. The average follow-up headway gap was between 2.61 and 2.67 seconds for Site 2 and Site 1 respectively. The results signify that the HCM 2016 fails at higher circulating flow rates in capacity estimation in Qatar. Further, the importance of roundabouts geometry and drivers’ behavior on impacting the capacity estimation of single-lane roundabouts was reflected on the critical gap parameters