Auxiliary turning lanes at urban intersections

TLDR: Guidelines for determining the number and length of left-turn lanes needed at signalized intersections have been developed and equivalence factors for converting right-turn traffic volumes to equivalent straight-through volumes at stop-sign controlled intersections are presented.

Abstract: The capacity of an intersection is affected significantly by the relative proportion of straight-through and turning vehicles in each approaching lane. In many situations - particularly at signalized intersections - traffic throughput on an approach can be increased by adding auxiliary lanes to accommodate only turning vehicles. Techniques for designing left-turn and right-turn lanes at intersections and evaluating the related traffic performance are needed. Guidelines for determining the number and length of left-turn lanes needed at signalized intersections have been developed. The guidelines are implemented primarily through a microcomputer program called "Left-Turning Movement Analysis Program" (LTMAP). This program provides the engineer/user with an interactive means for entering descriptive data concerning intersection turn- lane configuration, traffic volumes, vehicle classes, traffic behavior parameters, and signal timing. A range of descriptive quantitative information about expected queue lengths, likely signal-cycle failures, volume-to-capacity statistics, and various delay estimates is produced immediately by the program. Different intersection operational situations can be compared quickly and easily in this way. Equivalence factors for converting right-turn traffic volumes to equivalent straight-through volumes at stop-sign controlled intersections are presented, and guides for determining the length of right-turn bays at a signalized intersection are shown graphically. These tools aid the engineer in designing and analyzing auxiliary-lane treatments required for various intersection conditions.