Approach saturation headways at all-way stop control intersections: empirical investigation

This paper presents an empirical investigation into approach saturation headways at All-Way Stop-Controlled (AWSC) intersections. The mean approach saturation headway is a surrogate measure for approach capacity, which is largely a function of traffic demand on other intersection approaches. Video data was collected over four days at an AWSC intersection site in Bozeman, Montana. The site is characterized by single-lane approaches and a high level of vehicular and pedestrian traffic. Using strict protocols, video records were processed at the individual vehicle level and several information metrics were extracted for each vehicle in the data set on all approaches. Study results indicate that the mean approach saturation headways varied in a wide range due to variations in pedestrian activity, level of conflict, and the intended movement of the subject vehicle. The results suggest that, when no pedestrians were present, the maximum approach capacity in the absence of vehicular conflicts was in the order of 570 vph versus only 360 vph when conflicts were present on all other approaches. Statistical tests confirmed that the three study variables: pedestrian crossing activity, level of conflict, and subject vehicle movement all had significant effects on approach saturation headways. The pedestrian crossing activity was found to have the most significant impact on saturation headways, followed by the level of conflict and the subject vehicle movement, respectively. The results presented in this paper offer valuable information on approach saturation headways and capacity, a major input to operational analyses for this important type of intersection.