Proactive safety evaluation of a multilane unsignalized intersection using surrogate measures

Abstract

Road traffic safety is emerging as an area of increased attention and concern in many countries including India. Safety evaluation by conflict study using surrogate measures is a proactive method which does not require any accident data. Conflict study may be carried out using post encroachment time (PET) as a surrogate measure, where critical conflicts are determined based on certain threshold value of PET. Identifying critical conflicts using a threshold value of PET is good and correct for highways and major roads where traffic follows posted speed, but on highways with mixed traffic for varied speeds this is not correct. It can be seen that conflicts with PET less than the threshold value may not be critical if the speed of conflicting vehicle is less. Similarly, conflicts with PET more than threshold value may also be critical if the speed of conflicting vehicle is high. The speed of conflicting vehicle is also important in evaluating severity of conflict. Hence, the use of PET alone for conflict study is not correct for evaluating safety of intersections on highways where traffic follows varied speeds. Therefore, in the present study, safety evaluation of an unsignalized intersection is carried out using two surrogate measures, PET and the speed of corresponding conflicting through vehicle. A term critical speed is proposed to identify critical conflicts. Critical speed for a particular PET value is determined using stopping/braking distance concept. Results show that there are significant percent of conflicts which are critical at the intersection. This shows that drivers of right-turning vehicles do take risks in accepting small gaps in through traffic at the intersection, which is dangerous. As crossing maneuver depends on length and speed of turning vehicle, the effect of type of turning vehicle has also been studied. Results show that right-turning light motor vehicles are at higher risk compared to two wheelers and heavy vehicles at the intersection. Safety evaluation of such unsignalized intersections may be carried out using this concept.

Keywords:

• Conflict
• post encroachment time
• speed of conflicting vehicle
• threshold value
• critical speed
• braking distance

Acknowledgments

The authors are grateful to the Commandant, College of Military Engineering, Pune, for extending cooperation and encouragement for this research work.