EFFECT OF PHYSICAL FACTORS ON GAP ACCEPTANCE AT A ROUNDABOUT

Abstract

Roundabouts perform better than other types of uncontrolled intersections when traffic flow in low to medium. One such approach is based on gap acceptance behavior of the entering vehicles. This requires estimation of critical gap and measurement of follow-up time. The models based on gap acceptance approach are from developed countries and cannot be used under heterogeneous traffic condition prevailing in developing countries like India. Further, the physical geometry of roundabout controls and slows down the speed of vehicles entering the roundabout in a range of 30-35 km/h and thus increases their safety. Lowering down of these speeds also signify the lowering of level of service at that roundabout. This thesis will discuss the gap acceptance and vehicle speed aspects on roundabouts under heterogeneous traffic condition. This thesis addresses the most important element of operational performance of roundabout traffic intersections in Chandigarh. The data collected at two roundabouts with central-island diameter 25 m and 37 m were selected in Chandigarh city, India. The data collected can be broadly classified into two categories: inventory data and traffic data. Gaps accepted and rejected by entering vehicle have been extracted by vehicle type. These are used to estimate critical gap by using maximum likelihood method. The circulating flow has been counted for every 10 minutes interval and is correlated with critical gap. Similarly, follow-up time is also estimated and correlated. Average speed has been estimated by entry approach section and within circulating section It was found that, the critical gap was decreasing with the increase or decrease in circulating traffic flow while follow-up time was decreasing with an increase in circulating traffic flow. The critical gap ad follow-up time are found decreasing for small car and motorized two-wheeler with are increase in the diameter of central-island. Speed of the vehicles at entry and within circulating area of roundabouts was extracted. Its relation with the entry traffic flow and circulating traffic flow was studied. It was found that, speed of entry of approaches and within circulating area was decreasing with an increase in circulating traffic flow. It was also found that the circulating speed increased with an increase in entry speed on an approach section.Hence both roundabouts are significantly different on performance of roundabouts in terms of gap acceptance parameters and speed.