1 A Reassessment of the Characterisation of Congestion on an Urban Road Network– Some Theoretical Suggestions and Illustrative Experiments (p. 7-8)
Truong P. Truong
School of Economics
University of New South Wales (Australia)
David A. Hensher
Institute of Transport and Logistics Studies
University of Sydney (Australia)
To the memory of Peter J. Hills
1.1 Introduction
When a road user selects a particular part of the network to use, they are in the main buying speed– or its inverse, travel time savings, in return for an outlay of money, the latter including a toll or congestion charge where applicable. To study the phenomenon of buying speed as the indicator of service levels within a particular link, it is important to consider also the issue of interconnections between different links, since speed (and all its related concepts such as travel time, link capacity, and congestion) is to be seen as the outcome, not only of what happens on a particular link, but also of available service levels on other inter-connected links. In this paper, we first establish the framework for analysing the basic activity on one link, and then extend the analysis to consider the interconnection between different links. The purpose of the framework is to establish a foundation on which more comprehensive analysis of a general inter-connected network can be carried out in a routine manner, once the basic building blocks have been created. We show that the approach can be considered as a modification and/or extension of the traditional approach (e.g., Else 1981, Evans 1992) where the concepts of‘capacity’ and‘congestion’ have been defined, not explicitly, in terms of the infra structure capacity of the network or the traffic volume density, but rather in terms of the traffic flow outcome, which is the product of these two basic variables1. We will illustrate that by separating out the two basic components of this traffic flow variable, we can see the underlying process of interaction between travel demand and capacity supply, or price (travel time, or its inverse, speed) and quantity (traffic density) more clearly as compared to the situation when both of these components are mixed together in a single‘flow’ variable.
Having defined the basic concepts of‘price’ and‘quantity’ in an economic framework describing the underlying travel activity supply and demand, we can then relate these concepts to the empirical data on how prices and quantities (or demand and supply) are related to each other in different empirical situations. These empirical observations often take the form of traffic density-travel time (or speed) curves, which are unique characteristics for each particular link. From these empirical observations, we can deduce an underlying congestion‘index’ to represent the equilibrium outcome of the demand-supply interactions in a particular circumstance. We then use these indices as criteria to plan for the future, either in the form of short run‘congestion pricing’, to reduce the level of congestion at a particular link, given a fixed capacity, and/or long run investment planning, to expand capacity to cope with expected rising demand. The congestion index is a useful way of summarising the underlying demand-supply interactions, as a guide for policy design.
The paper is organised as follows. Section 2 defines the basic concepts of‘capacity’ and‘congestion’. Section 3 establishes the basic economic framework for linking the concepts of capacity and congestion as defined in the context of the basic individual travel activity within the transport link, as described by the theory of consumption (or production) of a congested public good. Section 4 then extends the analysis to apply to the case of a heterogenous population of travellers within a given link. Section 5 illustrates how the theoretical approach can be applied, using the example of a simple network of two parallel links with data calibrated on the information obtained from the Sydney road network. Section 6 concludes the paper. |
