RESEARCH ARTICLES CURRENT SCIENCE, VOL. 114, NO. 7, 10 APRIL 2018 1461 *For correspondence. (e-mail: ankit.civ@iitbhu.ac.in) Modelling U-turning behaviour of vehicles at mid-block median openings in multilane urban roads Ankit Gupta*, Satyajit Mondal and Vinay Kumar Sharma Department of Civil Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221 005, India Gap acceptance and lateral placing of vehicles are considered as prime parameters for analysing the behaviour of U-turning in vehicles at un-signalized median opening in urban areas. Critical gap is the sole parameter in gap acceptance phenomenon where crit- ical position gives the vehicle’s location during U-turn operation. Data for this study were collected using video photography at six median openings on a six- lane road and three median openings on a four-lane road from two different cities of India. Data as ex- tracted by categorizing the traffic stream into five dif- ferent classes such as two-wheelers, three-wheelers, car/SUVs, LCVs (light commercial vehicles) and HCVs (heavy commercial vehicles). Three different method- ologies (namely the traditional method, INAFOGA and modified Raff’s method) were utilized to estimate the critical gap. Critical position of vehicles is esti- mated using the markings on the pavement surface. Regression technique was used for modelling critical gap and position using different variables for each motorized mode. All the proposed models have a high coefficient of determination value which indicates its high significance level. Modelling of critical position for 3-wheelers was not framed as the P-value for each variable was higher than 0.05. The reason could be the randomness in data because of the undisciplined lateral movement of three-wheelers. A design recom- mendation for right turn pocket lanes near the median opening area is proposed especially for U-turning traf- fic based on the percentage of the U-turning traffic and maximum critical position of vehicles from the median edge. Keywords: Critical position, critical gap, gap accep- tance, median opening, U-turning vehicles. RAPID civilization has increased the number of vehicles significantly in the last two decades. In most of the sig- nalized intersections, U-turning movements are not permitted so as to improve the operational condition of intersection by optimizing the conflict points. Hence, openings are provided in the median in divided urban roads at an adequate interval particularly for U-turn operations. Median openings are quite effective to access opposing traffic without creating any conflicts between the vehicles travelling through the stretch and vehicles taking U-turn operation. While the turning operation through median openings is complex due to the opposing high speed traffic stream. Most of the median openings in India are uncontrolled. Thus a U-turning vehicle needs to accept a time or gap between the arrivals of successive vehicles through traffic of the opposite lanes to perform a safe turn. This is the gap acceptance phenomenon of U- turning vehicles. Critical gap is the time gap between two successive vehicles during which a U-turning vehicle may decide to cross or merge with the traffic stream. It is the minimum time interval that allows a vehicle to merge or cross the opposite traffic flow safely. The duration of critical gap affects the waiting time and merging time of the U-turning vehicles and also the behaviour of vehicles in the opposite traffic stream. Values of critical gap that are accepted vary for different vehicle classes and are dependent on various parameters such as types of U-turning vehicles, several stream parameters of oppos- ing lane traffic, geometrical elements of the carriageway including the median, etc. The U-turn operation sometime creates congestion resulting in reduction of roadway capacity in both the vehicle approach lanes. In most cas- es, it is observed that a larger radius is required for huge vehicles to turn 180 degrees which increases the delay, fuel consumption and are prone to accidents 1 . U-turning region is defined as a hypothetical road space where a lar- ger number of automobiles try to share the available space at the same time, thus creating more conflict as shown in Figure 1. Critical position is the distance of Figure 1. U-turning operation at median opening.