Effects of opposing flow on left-turn reduction factors at two-phase signalized intersections

This paper reports results of a field study to evaluate the effects of opposing flow on left-turn reduction factors in computing capacity of two-lane approaches to two-phase signalized intersections with no separate left-turn lanes. Time-lapse photography was used to gather data on the approaching and opposing volumes and the delaying effects of left turners. Reduction factors were computed for different numbers of left turns per cycle by using regression analysis. The two variables that explained most of the variance were the ratio of the opposing flow in the curb lane to its capacity and the ratio of the arrival time of the first left-turning car to the green-plus-yellow time. Results indicate that, when the opposing curb lane is loaded, left turns have a much greater reducing effect than that given in the Highway Capacity Manual. The capacity-reducing effect of a single left-turning vehicle is greater than the incremental reduction effect of the second, third, and fourth left turners in a signal phase.