Determining Roundabout Capacity by Modifying HCM Model Under Mixed Traffic Conditions

Abstract

Roundabouts are commonly implemented as transportation infrastructure facilities due to their improved performance. Capacity is a key performance measure of a roundabout which is considered at its design stage. Highway Capacity Manual (HCM) 2010 provides an exponential regression model for determining capacity at entry for a roundabout. However its applicability in heterogeneous traffic conditions has not yet been thoroughly researched. In Sri Lanka the island nation in the Indian Ocean, the percentage of contribution to the vehicle mix by motorcycles and three-wheelers are 45–55%. Thus, in this study HCM 2010 roundabout capacity model was modified to determine the capacity of a selected four legged roundabout in Kandy, Sri Lanka. Data acquisition was carried out by a turning movement count survey and video recording. Circulatory traffic volume, composition of each vehicle class, critical gap and follow-up headway were determined. A method incorporating the mixed traffic conditions was adopted to determine critical gap value. Stream equivalent follow up headway of 2.97 s and critical gap value of 5.07 s were used to determine the entry capacity. The graph plotted for variation between circulatory flow and entry flow at the roundabout was in negative exponential behaviour. The graph showed that entry capacity reduced exponentially when the circulatory flow increased. The study was further extended to determine adjustment factors for capacity estimation from the HCM 2010 roundabout capacity model directly. The results of this study could be useful in sustainable design of roundabouts in countries that experience mixed traffic conditions like Sri Lanka.