The relationship between increases in motorway capacity and declines in urban rail passenger journeys: a case study of Sydney's M4 Motorway and Western Sydney Rail Lines

Planning and Transport Research Centre
Publication Type:
Conference Proceeding
Australasian Transport Research Forum - Transporting the Future: Transport in a Changing Environment, 2005, pp. 1 - 14
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This paper examines the relationship between the addition of urban motorway capacity and declines in rail passenger journeys on parallel alignments. The research presented here forms part of a wider study that investigates outcomes from the expansion of urban motorway capacity in Sydney and the phenomenon of induced traffic growth. I nduced traffic growth is defined as new and additional road traffic movements that occur in response to increases in road capacity. By increasing road capacity, congestion and travel times are reduced, making travel by car more attractive. This generates a rapid succession of changes in travel behaviour across the surrounding network including traffic reassignment, traffic redistribution, generated traffic and passengers switching from parallel rail and public transport services, or mode shifting. This last response is the focus of this paper. Together, all form part of the composite effect called induced traffic growth (SACTRA 1994, p.53). The effects of mode shifting and other travel behaviour responses are significant because they potentially undermine the primary benefit of supplying additional urban motorway capacity which is to reduce travel times (Thomson 1977; Downs 1992; Mogridge 1997). If road traffic volumes increase, travel time savings are quickly eroded and congestion returns. If public transport patronage falls and services sustain revenue losses, service levels may be cut, imposing additional costs on public transport users and operators (SACTRA 1994, pp.128 129). Investigating responses to urban motorway development, such as mode shifting, is therefore important as it assists in gauging whether or not additional motorway capacity has been an effective policy response for reducing congestion.
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