Dynamic Modeling of VISSIM's Critical Gap Parameter at Unsignalized Intersections

Critical gaps are essential parameters in the modeling of traffic operations at unsignalized intersections and roundabouts and are the main determinants in the calculation of intersection capacity. In microscopic simulation tools, critical gaps are basic driver-related variables; the gaps' values and variations within the population are user defined. Errors or oversimplifications of these basic variables may yield unrealistic results in terms of total throughput, queue lengths, waiting times, and so forth. In this study, a method is proposed for the calibration of the critical gap parameter in microscopic simulations; the method is based on individual travel time observations. A simple formulation for the calculation of critical gap parameters takes into account the increasing risk taken by drivers on a minor street with increasing volumes on the major road. This new formulation provides a better match between the accepted gaps observed in reality and the actual traffic conditions. The new methodology has been used to calibrate and validate the minimum gap parameter of the software program VISSIM through the use of data taken from several unsignalized intersections in Leuven, Belgium. The results show that the volume-dependent model outperforms the conventional constant critical gap parameters.