Evaluating the safety and operational impacts of left-turn bay extension at signalized intersections using automated video analysis

Left-turn lanes are commonly introduced to provide space to accommodate comfortable deceleration and adequate storage of turning vehicles. Operational shortcomings may arise due to inadequate length, including overflow and blockage of left-turn entrance by queues on an adjacent through lane. This study investigates the potential safety and operational benefits of treating left-turn lanes by extending the length further upstream a signalized intersection. Video data was collected at three treated left-turn lanes as well as three matched control lanes; all in both before and after treatment conditions. Safety parameters consisted of the counts and severities of traffic conflicts occurring on the left-turn lanes and inside the intersection. There was a marked reduction in traffic conflict counts in all treated sites. The overall treatment effect, which accounts for the simultaneous change in control sites, was 63.2% (p < 0.05). There was a marked reduction in frequency of traffic conflicts at different severity levels. The mobility benefit of the treatment was demonstrated in terms of the reduction in average travel time for left-turn as well as through vehicles. The count of traffic signal cycles with blocked left-turn entrance was considerably reduced after the treatment. The use of collision data gathered from more sites is suggested as potential future work to further evaluate this treatment.