The impact of driver and flow variability on capacity estimates of permissive movements

The following paper describes a stochastic extension to earlier research into a generalised deterministic model of driver gap acceptance. This gap acceptance model is applicable to traffic flow at both signalised and unsignalised intersections. The proposed microscopic gap acceptance model has been implemented within the INTEGRATION traffic simulation model and is therefore equally suitable for analysis of a single isolated intersection approach for static conditions, as well as in a fully dynamic analysis of networks consisting of hundreds of signalised and unsignalised intersections. Variability in both the supply of gaps in the priority flow and the demand for gaps from the non-priority vehicles is introduced. The analysis illustrates the impact on the overall relationship between priority flow and the capacity of the non-priority flow. Cowan M3 and exponentially distributed gap supply models are considered, as are normal and log-normal gap-demand models which account for driver heterogeneity. The results are shown to be not only logical, but also consistent with several analytical capacity models, the Highway Capacity Manual and a recent National Cooperative Highway Research Program project on the same topic.