Influence of restricted sight distances on permitted left-turn operation at signalized intersections

Adequate sight distance is an important factor needed to provide safe and efficient operation of left-turn movements at signalized intersections. This factor becomes more crucial for left turners attempting this maneuver during the permitted phase of the traffic signal. The large size median of the major road is commonly designed to accommodate an exclusive left-turn lane. However, studies have proved that if the median of that major road is relatively wide, the simultaneously turning vehicles in the opposite left-turn lanes often restrict driver's sight distances for each other. In this study, a field observation was conducted to analyze the changes of left-turn gap acceptance parameters associated with drivers' restricted sight distances. Theoretical models were developed and used to quantify the negative effect of the restricted sight problem on the left-turn capacity. The results show that the sight-distance problem may contribute to significant increments of the critical gap and follow-up time. This influence on driver's gap-acceptance behavior may result in a 70% reduction in the left-turn capacity when the opposing-through traffic volume increases up to 1,800 vehicles/h. The capacity reduction rate increases with the increase in both the opposing-through traffic volume and the volume-to-capacity ratio of the opposing left-turn traffic.