Proactive Safety Evaluation of Unsignalized Intersection Using Surrogate Measures

Road traffic safety is emerging as an area of increased attention and concern in many countries including India. Several countermeasures are being practiced across the world to improve traffic safety. To evaluate the impact of these measures on safety many predictive methods are suggested by several researchers. Safety evaluation by conflict technique using surrogate safety measures is a faster and resource effective method. This does not require any accident data, hence is a proactive method of safety evaluation. Conflict study may be carried out using surrogate measure like Post Encroachment Time (PET). Post Encroachment Time is defined as the time lag between the passages of offending vehicle and conflicting vehicle on a conflict zone. The critical conflicts are determined based on certain threshold value of PET. Many researchers have used different threshold values for PET ranging from 1s to 5s for their specific studies. There is no specific value defined which can be used to determine critical conflicts. Therefore in the present study safety evaluation of an unsignalized intersection is carried out by observing conflicts using Post Encroachment Time (PET) as safety indicator and the speeds of corresponding conflicting vehicles. To identify the critical conflicts a term critical speed is proposed in this study. The critical speed for a particular PET value is worked out using stopping /braking distance concept. The concept and logic of evaluation of safety using PET as surrogate safety indicator, speed of conflicting vehicles and the critical speed are described in the paper. Using the above concept the safety evaluation of an unsignalized intersection in Thane city has been carried out. The results show that there are significant percent of conflicts which are critical at the intersection. This shows that the drivers of right turning vehicles do take risk in accepting small gaps in through traffic at the intersection which is dangerous. As the crossing maneuver depends on the length and speed of the turning vehicle, the effect of type of vehicle has also been studied. The results show that the right turning Light Motor Vehicles are at higher risk as compared to two wheelers and heavy vehicles at the intersection. Safety evaluation of such unsignalized intersections may be carried out using the above concept.