This paper presents an examination of traffic volumes on Sydney’s western road network before and after the opening of the Mays Hill to Prospect section of the M4 Motorway. The aim of the examination is to identify any possible traces of induced traffic growth, or new motor vehicle trips generated in response to the quicker travel times made possible by the increase in road capacity.

The data show that average daily volumes on the M4 Motorway and Great Western Highway grew from just under 80,000 vehicle movements per day in 1991 to around 100,000 in 1992 after the new motorway section opened. Once business-as-usual growth (3,000 vehicles per day), road traffic reassignment (7,000 vpd) and possible mode shifting from the rail network (6,500 vpd) are taken into account, a residual volume of around 3,500 is left. This is likely to be redistributed or induced traffic growth.

Induced traffic growth has significant implications for the sustainability of urban systems. These are discussed in tandem with a brief review of the debate that has taken place around induced traffic growth.