Existing Warrants for Left-turn Lanes

• Authored by:  Ida Van Schalkwyk
• Co-authored by:  Dr. Vergil G. Stover

Van Schalkwyk, I. and V.G. Stover. “Revisiting Existing Warrants for Left-turn Lanes at Unsignalized Intersections on Two-Way Roadways,” Paper 07-0784, Presented at the 86th Annual Transportation Research Board meeting, Washington, D.C. (2007)
This paper reviews the Harmelink methodology for left-turn lane warrants at unsignalized intersections and draws on results from subsequent research efforts, specifically focusing on older drivers, to make recommendations for improved turn lane warrants and storage length requirements. In addition, the paper presents the results of a comparison analysis in which the Harmelink values were compared to five different sets of values, ranging from minimum accepted values to conservative values for older driving populations. Based on this analysis as well as findings from a thorough literature review, the authors suggest that the Harmelink methodology underestimates the traffic volumes that should warrant left-turn lane installations at unsignalized intersections on two-lane roadways. To better account for the aging population of drivers, the authors recommend warrants based on the 85th percentile values instead of the 50th percentile values currently used. More specifically, the authors suggest adjusting Harmelink’s calibration factors and using the following values when larger populations of older drivers are present: an eight second critical gap, five seconds to perform a left-turn movement, and 6.6 seconds to clear the left-turn lane.