This paper examines the 1985 highway capacity manual (hcm) definition of critical gap for two-way stop-controlled (twsc) intersections. Minor street left-turning movements at unsignalized three-leg intersections are examined. On the basis of this examination, a revision is recommended for the hcm definition, which better reflects actual driver behavior and provides a better estimate of the capacity oftwsc intersections. The effects of front-of-queue delay on the length of the critical gap were investigated. On the basis of a limited amount of field data, the critical gap was found to be significantlyaffected by the amount of front-of-queue delay incurred by individual drivers. It is therefore recommended that any delay-based level of service (los) criterion for twsc intersections should incorporate lower delay thresholds than are used for signalized intersections atleast in the los d, e, and f regions. Finally, the collected field data demonstrate that the critical gap for minor street left-turningvehicles is affected by the type of major street conflict (same direction versus opposite direction) experienced. From this finding, itis concluded that the distribution of major street traffic can havea substantial effect on the capacity of the minor street left-turn movement. This paper appears in transportation research record no. 1320, Freeway operations, highway capacity, and traffic flow 1991.
Abstract