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 s and the mean follow-up headway was 2.73 s. 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 decreased, resulting in greater saturation flows. When site-specific saturation flow estimates were compared with HCM saturation flow estimates, the differences ranged from −30% to +23%. This paper quantifies and illustrates the impact of site-specific characteristics on gap acceptance parameters and saturation flow.

关键和后续进展是估算信号交叉口允许左转弯饱和流量的基础。《 2016年公路通行能力手册》中建议的当前关键和后续进展（HCM）是基于1970年代从德克萨斯州五个十字路口收集的有限数据得出的。这项研究分析了45个交叉路口处的2,500多辆左转弯车辆，提供了对间隙接受参数的见解，并评估了不同地点特定因素的影响。视频数据是从美国的不同地理区域（亚利桑那州，佛罗里达州，北卡罗来纳州，弗吉尼亚州和威斯康星州）收集和处理的。使用最大似然法估计间隙接受参数时，平均临界进展为4.87 s，平均随访进展为2.73 s。为了说明特定地点的特征，研究了几个几何和操作变量对关键和后续进展的影响。通过元回归分析，发现张贴的速度限制和相对行车道的宽度对间隙接受参数有显着影响。结果表明，随着张贴速度限制和相对车道宽度的减小，临界和后续车道也减小，从而导致更大的饱和流量。将特定地点的饱和流量估算值与HCM饱和流量估算值进行比较时，差异范围为-30％至+ 23％。本文量化并说明了现场特定特征对间隙接受参数和饱和流的影响。将特定地点的饱和流量估算值与HCM饱和流量估算值进行比较时，差异范围为-30％至+ 23％。本文量化并说明了现场特定特征对间隙接受参数和饱和流的影响。将特定地点的饱和流量估算值与HCM饱和流量估算值进行比较时，差异范围为-30％至+ 23％。本文量化并说明了现场特定特征对间隙接受参数和饱和流的影响。