Utilization of Auxiliary Through Lanes at Intersections of Four-Lane, Two-Way Roadways

Abstract

To increase the capacity of through traffic at signalized intersections, additional lanes with limited length—called auxiliary lanes—are added to the roadway at the intersection. Because of their limited length, as well as other factors, these lanes are not as fully utilized as other continuous through lanes. Research was undertaken with two objectives: ( a) to observe and identify the level of use of auxiliary through lanes added at intersections of four-lane, two-way roadways; and ( b) to study the effects of auxiliary lane length, right-turn volume, and through/right-turn lane group delay on the level of their use. Lane-use data collected during 1,050 saturated cycles at eight signalized intersections with different auxiliary lane lengths were used to accomplish research objectives. All factors investigated—auxiliary lane length, right-turn volume, and stopped-delay—were found to contribute significantly to the use of auxiliary lanes at 0.01 level. The level of each factor’s contribution, however, was dependent on the level of the other two. Lane use of nearly one to seven straight-through vehicles per cycle, depending on levels of factors investigated, was observed at the study locations. Longer auxiliary lanes, lower right-turn volumes, and excessive approach delays encouraged the use of auxiliary lanes by straight-through vehicles. The range of lane utilization adjustment factors ( fLU-factors) calculated from field data was 0.73 to 0.82, which is lower than the 1997 Highway Capacity Manual default value of 0.91 for a three-lane through/right-turn group.