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

UTC: Safety Research Using Simulation (SAFER-SIM)
Speaker: Boris Claros from the University of Wisconsin Madison

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 recordings of left turn maneuvers, opposing traffic, and traffic signal indication at urban signalized intersections with permissive or protected/permissive left turns were used. The research team obtained and collected about 500 hours of video data between 2016 and 2019, from three different geographical regions in the United States— East (Florida, North Carolina, Virginia), Midwest (Wisconsin), and West/Southwest (Arizona). Using the Maximum Likelihood method proposed by Troutbeck, the aggregated mean critical headway was 4.87 seconds, and the aggregated mean follow-up headway was 2.73 seconds. Headway estimates of this study were higher than the HCM values. The mean critical headway for large vehicles was 6.03 seconds which is different than the aggregated estimate of 4.87 seconds which only included passenger vehicles. With a decreasing value of the posted speed limit and width of opposing traffic lanes, the smaller the critical and follow-up headways result in higher saturation flow estimates.