INFLUENCE OF MIXED TRAFFIC FLOWS ON CAPACITY OF A ROUNDABOUT

Abstract

Intersections are an unavoidable part of a road network and commands a lot of attention in its design. There are various ways in which traffic can be controlled at an intersection. One of the commonly used methods is a roundabout. Also called as rotary or traffic circle, roundabout consists of a central island around which vehicles move clockwise for left-side driving and anti-clockwise for right-side driving. Merging and diverging of vehicles takes place simultaneously which is termed as weaving. Roundabouts are capable of reducing accidents and increasing the capacity of the intersection. The vehicles entering the roundabout slowdown in order to find a suitable gap in the circulating stream of vehicles. Therefore, the capacity of a roundabout depends, besides the geometric elements of the roundabout, on the availability of gaps between the circulating vehicles. This dissertation attempts to determine the entry capacity of a roundabout and the effect of mixed traffic on entry capacity. The data was collected at three roundabouts in Chandigarh city having suitable site conditions on a typical weekday. The collected data consisted of geometric details and traffic flow at the roundabout. For determining the entry capacity of an approach, the entry flow and circulating flow were plotted when there was queue formation in the approach. The plot showed that as the circulating flow increased, the entry flow decreases. These plots were compared with the equation given in HCM 2010. The comparison showed that the entry capacity values from the field data are much higher than that obtained from the HCM 2010 equation. Further, the critical gap and follow-up times for the approaching traffic was calculated and plotted against the circulating traffic. The critical gap values are found to be extremely low as compared to the HCM 2010 values.