Effect on Utilization of Auxiliary through Lanes of Downstream Right-Turn Volume

To increase the capacity of through traffic at signalized intersections, additional lanes with limited lengths—called auxiliary lanes—are usually added to the roadway at the intersection. Because of their limited length, as well as other factors, these lanes are not fully utilized as other continuous through lanes. The objectives of this research were (1) to identify the level of utilization of auxiliary through lanes added at intersections of four-lane roadways with right-turn driveways and/or side streets present within the downstream length of the auxiliary lane; and (2) to study the effects of auxiliary lane length, right-turn volume exiting downstream into driveways/side streets, and through/right-turn lane group delay on the level of their utilization. Lane-use and right-turning volume data collected during 866 saturated cycles at eight signalized intersections with different auxiliary lane lengths and right-turn volumes were used to accomplish the research objectives. Auxiliary lane length and right-turn volume downstream were found to significantly contribute to the use of auxiliary lanes at the 0.01 level of significance. Lane-use of nearly 0.4–1.9 straight-through vehicles per cycle, depending on lane length and downstream right-turn volume, was observed at the study locations. Though not directly comparable to this study, previous studies had indicated lane-use values within this range. Longer auxiliary lanes and lower downstream right-turn volumes encouraged the use of auxiliary lanes by straight-through vehicles. The range of lane utilization adjustment factors $(f_{\mathrm{LU}}$-factors) calculated from field data was 0.72–0.80, which is lower than the 1997 U.S. Highway Capacity Manual default value of 0.91 for a three-lane through/right-turn group.