Oversaturation Delay Estimates with Consideration of Peaking

Abstract

A deterministic oversaturation queueing model that uses a generalization of the peak hour factor concept of the U.S. Highway Capacity Manual (HCM) as a simple variable demand model is described. The model is used to explore several issues related to oversaturation models. In particular, the relationship between the delay measurement methods (queue sampling and path trace) and the delay definitions used in the corresponding analytical delay models is investigated with a view to level of service assessment and performance prediction. The differences in delay definitions and delay measurement methods are negligible for undersaturated conditions (low to medium v/c ratios). However, as flows approach capacity (high v/c ratios below capacity) and exceed capacity (v/c ratio greater than 1), the selection of the duration of the flow period, delay definition, and delay measurement method affects delay estimates significantly. Substantial differences in delay and queue estimates are found between the cases of peak flow and maximum delay periods regardless of the delay measurement method. The use of the average delay experienced by individual vehicles in a maximum delay period creates problems in system performance analysis. A delay definition based on a maximum delay period reveals an inconsistency in relation to delays measured in the field. Whereas the HCM recommends that field delays be measured in the peak flow period, the maximum delay period does not coincide with the peak flow period. It is therefore important that the delay definition implied by the present HCM delay formula for signalized intersections be clarified.