A conceptual framework for assessing congestion and its impacts

Abstract

In urban areas, intersections are the main constraints on road capacity while traffic flows do not necessarily directly conform to the speed-flow relationship. It is rather the signal timing and the interplay between the clearing rate of each intersection which determines the formation and duration of congestion. Junctions often differ in their design and throughput. General conclusions on the relationship between vehicle speed and traffic flows on a junction link are rarely possible. Welladapted models are required for a comprehensive study of the behaviour of each intersection as well for the interactions between junctions. This chapter assesses the potential benefits of adaptive traffic plans for improved network management strategies, under varying traffic conditions. Queueing analysis in association with advanced simulation techniques reveal congestion mitigation actions when the pretimed actuation plan is replaced by the max-pressure feedback control. The case of unpredicted local demand fluctuation is studied, where uncontrolled congestion is progressively propagated to the entire network under the open-loop policy. Traveltime variability is measured under both plans and within all traffic schemes while frequency of stop-and-go actions are also encountered. Reliability of predictable trip durations is a major factor to be considered when ensuring "on time" arrivals and the related costs when the time is converted into benefits.