At an unsignalized t-intersection, where a major two-lane roadway intersects a minor roadway, criteria that justify a left-turn laneon the major roadway are analyzed. Three criteria are considered: (a) probability that one or more waiting through vehicles are presenton the approach; (b) delay (average delay to the "caught" through vehicles, average delay to all through vehicles, and delay savings due to the left-turn lane); and (c) degradation of the level of service. The volume combinations (through, left-turn, and opposing flow) that would justify a left-turn lane under each of the criteria are presented. The current aashto guidelines are based on the probability that one or more through vehicles are in the queue behind a waiting left-turn vehicle. The original mathematical formulation of the aashto guidelines is examined and corrected, and a new set of volume warrants is developed. A simulaiton model of the movements of the vehicles on the approach is simulated, and delays to through vehicles with and without a left-turn lane for different traffic volumes are computed. Finally, a set of traffic volumes at which the level of service of the approach changes from a to b is developed. The warrant volumes based on the three criteria are different. Delay and the level of service are more easily understandable measures of traffic performance than probability, so the volume combinations based on these two criteria should also be considered. The result provides a range ofvolume combinations within which an engineering judgment should be made. Discussions of other considerations for justification of a left-turn lane are also provided. This paper appears in transportation research record no. 1327, Visibility, rail-highway grade crossings, and highway improvement evaluation 1991
Samenvatting