Effect of Large Vehicles on Left Turn Gap Acceptance at Signalized Intersections

Critical and follow-up headways are the foundation for estimating the saturation flow of permissive left turns at signalized intersections. Current critical and follow-up headways recommended in the 2016 Highway Capacity Manual (HCM) are based on limited data collected from five intersections in Texas in the 1970s. This study analyzed over 2,500 left-turning vehicles at 45 intersection approaches, provides insights into gap acceptance parameters, and evaluates the effect of different site-specific factors. Video data were collected and processed from different geographical regions in the United States—Arizona, Florida, North Carolina, Virginia, and Wisconsin. Using the Maximum Likelihood method to estimate gap acceptance parameters, the mean critical headway was 4.87 seconds and the mean follow-up headway was 2.73 seconds. The mean critical headway for large vehicles was 6.03 seconds. To account for site-specific characteristics, the effect of several geometric and operational variables on critical and follow-up headway were explored. Through a meta-regression analysis, the posted speed limit and width of opposing travel lanes were found to have a significant effect on gap acceptance parameters. Results showed that with decreasing posted speed limit and width of opposing lanes, critical and follow-up headways also decrease resulting in greater saturation flows. When site-specific saturation flow estimates are compared with HCM saturation flow estimates, the differences ranged from -30% to +23%. This research quantifies and illustrates the impact of site-specific characteristics on gap acceptance parameters and saturation flow.