Operational and Safety Effects of Using Left-Turn Lanes with Substandard Lengths at Unsignalized Median Openings: Case Study in Houston, Texas

For median-divided roadways, the AASHTO Greenbook specifically encourages the use of left-turn lanes at median openings to eliminate stopping in through-traffic lanes. However, in urban areas, the Greenbook requirements for the lengths of median turn lanes often exceed the available length between two adjacent openings. Thus, left-turn lanes with substandard lengths have been in wide use at median openings on urban arterial roads. The objective of this study was to investigate whether it is operationally effective and safe to use median left-turn lanes of substandard lengths at unsignalized median openings. For this purpose, an analytical model was developed to estimate the delay incurred by left-turn vehicles as they decelerate in through-traffic lanes in preparation for the left-turn maneuvers. A case study was conducted on a 3,000-ft corridor on Jones Rd. in Houston, Texas, where eight median left-turn lanes have been installed, all with substandard lengths. Using calibrated micro-simulation models as benchmarks, the proposed model showed reasonable performance. Actual rear-end crashes and simulated rear-end conflicts generally revealed that such lanes did not result in compromised safety performance.