Safety performance evaluation of driveway left turning alternatives using traffic conflicts

Safety of left turning vehicles from driveways onto major arterials carrying high traffic volumes is of particular importance. Left turn movement at such locations could be accommodated either through direct left turns (DT) or right turns followed by U-turns (RUT). The study summarized in this paper evaluated the effects of these two alternatives where the safety comparison using traffic conflicts is described here. This was achieved through field data collection of traffic and traffic conflict data at locations where one or both of the two alternative treatments were utilized. Two types of conflict rates, conflicts per hour and conflicts per thousand involved vehicles were used in comparing the safety of the two left turning alternatives. Average number of conflicts per hour for DT and RUT were 6.35 and 4.20, respectively. When data were separated into peak and non-peak periods, average peak hourly conflicts were 7.01 and 3.98 for DT and RUT respectively. When average number of conflicts per thousand involved vehicles was considered, the rates for DT and RUT were 26.43 and 16.08 respectively. RUT conflict rate was 39% lower as compared to DLT conflict rate. Severity of conflicts for DT and RUT was also analysed by using two approaches based on Risk of Collision and Time to Collision scores. In both cases, average DT conflict severity was significantly larger than that of RUT. Both conflict rates and severity indicated more favourable safety effects due to RUT, where a case study of converting a DT location into a RUT location confirmed the findings.