Understanding the road traffic congestion relief impacts of public transport

The primary objective of this research is to understand the congestion relief impacts of public transport. It is suggested in this research that none of the existing measures of traffic congestion provides information on the extent to which traffic congestion is relieved by public transport. In addition, previous studies related to traffic congestion relief have not quantified the relationship between the presence of public transport and the amount of traffic congestion of a city. The research develops a new measurement index which can be used as a relative measure of the city-wide congestion relief impacts of public transport. The proposed research framework consists of three main components: a congestion relief valuation framework that compiles relevant evaluation research and explores mode shift; an experimental modelling approach that analyses changes in congestion measures as a result of alternative public transport scenarios; and a relative measure development method that constructs a congestion relief index for expressing the city wide congestion relief impact of public transport in a generalised form. An examination of secondary evidence demonstrating changes in mode split associated with changes in public transport suggests that about one third of existing public transport users would contribute to additional car trips if public transport were removed. This includes all car drivers and half of the car passengers. A comparative assessment of international research valuing the congestion relief benefits of public transport reveals that congestion relief impacts are valued at between 4.4 and 151.4 Australian cents (2008 value) per marginal vehicle-kilometre of travel with an average of 45.0 cents. Valuations are higher for circumstances with greater degrees of traffic congestion and also where both travel time and vehicle operating cost savings are considered. Experimental transport modelling analysis has been performed to understand the spatial distribution of congestion relief and the effects of different public transport modes. In addition, a new measure called the congestion relief index (CRI) has been developed for use as a comparative index of the congestion relief impact of public transport in cities. The proposed CRI integrates a number of different dimensions of city and transport characteristics into a single measure. The lessons learned from this study include (i) public transport plays a significant role in relieving road traffic congestion in cities; (ii) the wider significance of public transport in relieving congestion outside the central location of the metropolitan area challenges the traditional view that public transport and its impact on congestion is an issue only for central cities; (iii) public transport modes differ in their contribution to congestion relief, but the particular characteristics significantly influencing this impact include patronage, trip length, and spatial coverage; (iv) large-scale public transport systems in large cites provide significant high congestion reduction benefits in comparison to a small-scale systems in small cities; (v) the underlying dimensions of elements affecting the congestion relief impact of public transport in cities can be classified into three broad categories – (1) the public transport-oriented factor, (2) the car-deterrence factor, and (3) the urbanform factor; (vi) the car-deterrence factor has the strongest influence on public transport congestion relief, followed by the public transport-oriented factor and the urban-form factor; (vii) high congestion relief impacts are obtained in cities characterised by high public transport availability and patronage, high urban density and low car ownership and use.