Guidelines for left-turn bays at unsignalized access locations on arterial roadways

Abstract

It has long been recognized that effective access management along arterial streets can alleviate traffic congestion. A major goal within access management is to limit the speed differential between turning and through vehicles. Left-turn bays reduce the speed differential due to left-turning vehicles on major roadways. The development of the left-turn lane guidelines and warrants in existence today are primarily focused on how the left-turn maneuver affects the left-turning vehicle. Harmelink investigated the probability of a left-turning vehicle in the through lane, however, he did not couple that with the advancing volume (by itself, the probability of a left-turning vehicle in the through lane does not show how the through traffic is impacted). The purpose of this thesis was to develop left-turn lane guidelines based on the delay to through vehicles. The TEXAS Model for Intersection Traffic simulated the impact of left turning vehicles on the through traffic at various speeds, and advancing, opposing and left-turn volumes. The minimum delay to through vehicles was used as the criteria in developing the turn lane guidelines. With the curves developed, a conflict analysis assessed the safety aspects of the guidelines. A probability of 0.01 was selected as the maximum likelihood of a conflict. The conflict analysis showed that guidelines would result in a probability of 0.01 or less for all but the highest through volumes.