SATURATION HEADWAYS AT STOP-CONTROLLED INTERSECTIONS

Capacity analysis procedures for stop-controlled intersections require saturation headways or related parameters as inputs. Unfortunately, no data base currently exists for these parameters, including critical gaps and follow-up times for two-way stop-controlled (TWSC) intersections and saturation headways for all-way stop-controlled (AWSC) intersections, for conditions found in the United States. The results of a set of recent measurements of these parameters are reported, and several important issues are explored: (a) how are the critical gap and follow-up gap measured at a TWSC intersection and what is the relationship between them? (b) what is the saturation headway (i.e., follow-up gap) for a TWSC intersection? (c) what is the effect of turning movements on the saturation headway at an AWSC intersection? and (d) should other cases be considered, in addition to the standard four cases, when measuring the saturation headway at an AWSC intersection? For TWSC intersections, a relationship was found between the critical gap and the follow-up time. In addition, the importance of the directional movement of the major street vehicle terminating a gap as it affects the critical gap was determined. A new set of categories was developed for saturation headway cases for AWSC intersections, and the importance of the movement direction of the subject approach vehicle on the saturation headway was determined.