A further investigation on critical gap and follow-up time

Abstract

The 1997 update to the Highway Capacity Manual (HCM) includes a new set of critical gap and follow-up time values calibrated based on the U.S. conditions. This paper is to present the detailed investigation on various factors affecting the critical gap and follow-up time, so that users can have a better understanding of the recommended critical gap and follow-up time values. The investigation was based on the comprehensive database established during the NCHRP 3-46 project, Capacity and Level of Service at Unsignalized Intersections. The paper first identifies various factors that might affect critical gap and follow-up time, such as intersection geometry, traffic movements, vehicle type, speed limits, and delay. Step-wise regression technique is then applied to identify the significance of these various factors. The relationship between critical gap and follow-up time values is also investigated. It is found that the major factors affecting critical gap and follow-up time include intersection geometry, vehicle type, approach grade, and traffic movements. While an independent set of follow-up time values are recommended, the follow-up time to critical gap ratio is found to be approximately 0.60, which has been reported in previous studies. The newly recommended set of critical gap and follow-up time values form the basis for the procedures for two-way stop-controlled intersections included in the 1997 update to the HCM and the 2000 HCM.