This thesis investigates the implications that urban motorway development has for the sustainability of cities. It does this by focusing on the sudden increase in road traffic that follows after the opening of additional motorway capacity, known as induced traffic growth, and asking whether induced traffic growth affects the ability of an urban system to sustain its essential economic functions. The investigation also addresses how urban systems impact on the biosphere.
Induced traffic growth, and the urban motorway development responsible for it, are often cited as a threat to sustainability because they are seen to increase fuel consumption and air pollution without necessarily improving accessibility within a city. Opponents to urban motorway construction claim that it merely represents a reshuffling of system elements, such that the spatial relationships between transport and land-use are changed, but the amount of time spent travelling, and the number of economic exchanges made by people, remain much the same. Motorway development advocates refute these claims, arguing that motorway construction reduces travel times, cuts emissions and fuel consumption and increases economic activity, thereby enhancing sustainability.
While it should be possible to resolve these issues through a program of empirical analysis, the phenomenon remains contested, raising questions about why and how its contested status affects transport decision-making and transport science. These questions are answered in this thesis by first investigating the social and political context in which debate over induced traffic growth has taken place. To do this, Soft Systems Methodology is used to investigate the way in which conflicts over urban motorway development have been resolved in London, Sydney and Zürich. The comparative analysis highlights differences between the rules of the political decision-making systems in each of the cities, and how these distribute power to different groups within society. While the history of conflicts is similar in each of the cities, more power is given to special interest groups from industry in London and Sydney. By contrast, the system in Zürich gives more power to resident populations through its system of direct democracy. Consequently, urban motorway development, the induced traffic growth it gives rise to and the impacts they have on city operations are acted upon in Zürich to the extent that transport policy has focused more on the development of comprehensive public transport systems. This leads to the conclusion that the contested status of induced traffic growth is more a product of the socio-economic goals of particular interest groups within society than it is of shortcomings in the empirical record or essentially unresolved theoretical issues.
With the political context as background, the thesis then reviews the empirical analyses and theoretical explanations for the phenomenon. First, a review of past empirical analyses is undertaken to identify the grounds that have been cited to refute the induced traffic growth hypothesis. Two key areas are identified. The first involves difficulties with distinguishing the sources of induced traffic growth from traffic reassignment. The second concerns the absence of traffic data for routes that are potential alternatives to a new motorway from which traffic reassignment may have taken place. A case study of the M4 Motorway in Sydney is presented with data for all arterial through-routes that cross relevant screenlines, thereby overcoming several of the shortcomings identified in the review. This case study adds to the general literature of case studies that corroborate the induced traffic growth hypothesis, but provides the first substantial documented case for an Australian city.
A review of the theoretical explanations for the phenomenon finds that while both microeconomic evaluation and standard modelling procedures provide accounts for the phenomenon that meet institutional expectations of technical veracity, neither constitutes a substantial description of the causal mechanism for the phenomenon, leaving unanswered questions about some findings in the empirical record. This conclusion prompts the development of a systems-based explanation for induced traffic growth that defines it as a form of multiple system feedback processes controlled by a travel budget time constant. By accounting for the phenomenon and its effects in this way, an explanation is provided for changes to travel behaviour and patterns of land-use development that reveals how urban motorway development affects urban systems in an holistic way.
The final section of the thesis combines the insights gained by examination of the politics of the transport decision-making system with empirical analyses and theoretical explanations for induced traffic growth, to produce a general systems view of cities and their place within the earth’s biosphere. This treatment considers the problems of oil depletion and global climate change, and the effects that urban motorway development has on the ability of urban systems to adapt to changes in the system environment brought about by these problems. The thesis concludes that urban motorway development and the processes that it triggers, which are embodied in the phenomenon of induced traffic growth, can undermine a city’s comparative ability to sustain the accessibility needs of its residents.