Speed - Flow and Bunching Models for Uninterrupted Flows

Abstract

Akçelik's time-dependent speed - flow model based on queuing theory concepts is used to develop alternative versions of the HCM speed - flow models for basic freeway segments, multilane highways and urban streets. The corresponding travel time - flow models show that higher-quality facilities have lower levels of bunching delays. A version of the speed - flow model that describes in-stream vehicle interactions and resulting queuing in terms of traffic bunching characteristics is introduced. Speed - flow and headway distribution models for uninterrupted traffic streams are integrated using a common traffic delay parameter. A new model of the proportion of bunched vehicles is proposed for the bunched exponential model of headway distribution. The driver response time parameter at capacity flow is discussed. A model for forced flow conditions is developed, and unsaturated and forced flow conditions are contrasted in relation to determining headway distributions.