The self-defeating nature of urban road capacity policy: A review of theories, disputes and available evidence

Martin J H Mogridge

Transport Policy, 1997, vol. 4, issue 1, 5-23

Abstract: For over 30 years, an argument has been proceeding among researchers who have analysed trends in urban traffic conditions, with notable contributions from Downs, Thomson, Smeed, Zahavi, Bly, Webster and the author. The argument has been rather theoretical, but has an important policy issue at its heart: if urban road capacity is increased, does this result in some improvement in traffic speeds (as traffic engineers have hoped), or does it make congestion worse (as many urban authorities now suspect)? Resolving this question depends on explaining the ubiquitous observations that there is a very wide variation in day-to-day running speeds for individual vehicles on particular journeys, but there is only slight long-term change in average traffic speeds, in spite of the substantial growth in car ownership and the many different transport policies which have been adopted. This paper reviews the views and empirical evidence that have been put forward to date, and defends the conclusion that the counter-intuitive argument is in fact correct: increasing road capacity in congested conditions can make congestion worse. The reason for this lies in the interaction between private and public transport, or rather between individual and collective transport. An important policy conclusion follows: a necessary condition for increasing journey speeds in towns (for both car users and collective transport users) is to improve the quality of collective transport. Sample calculations suggest that the average direct journey speed in central London may be more than doubled by such a policy.

Date: 1997
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