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The Lewis–Mogridge Position, named after David Lewis and Martin J. H. Mogridge, was formulated in 1990. It captures the observation that the more roads are built, the more traffic there is to fill these roads. Speed gains from some new roads can disappear within months if not weeks. Sometimes new roads do help to reduce traffic jams, but in most cases the congestion is only shifted to another junction.
The position reads traffic expands to meet the available road space (Mogridge, 1990). It is generally referred to as induced demand in the transport literature, and was posited as the "Iron Law of Congestion" by Anthony Downs.
Following the Lewis–Mogridge Position it is not generally concluded that new roads are never justified, but that their development needs to consider the whole traffic system. This means understanding the movement of goods and people in detail, as well as the motivation behind the movement.
The Lewis–Mogridge Position is often used to understand problems caused by private transport, such as congested roads in cities and on motorways. It can also be used to explain the success of schemes such as the London congestion charge.
The position, however, is not confined to private transport. Mogridge, a British transport researcher, concluded also that all road investment in a congested urban area will have the effect of reducing the average speed of the transport system as a whole — road and public transport. This relationship and overall equilibria is also known as the "Downs–Thomson paradox". However, according to Anthony Downs this link between average speeds on public transport and private transport "only applies to regions in which the vast majority of peak-hour commuting is done on rapid transit systems with separate rights of way. Central London is an example, since in 2001 around 85 percent of all morning peak-period commuters into that area used public transit (including 77 percent on separate rights of way) and only 11 percent used private cars. When peak-hour travel equilibrium has been reached between the subway system and the major commuting roads, then the travel time required for any given trip is roughly equal on both modes."
- Mogridge, Martin J. H., Travel in towns: jam yesterday, jam today and jam tomorrow? Macmillan Press, London, 1990. ISBN 0-333-53204-X
- Lewis D. (1977), Estimating the influence of public policy on road traffic levels in greater London. J. Transport Econ. Policy, 11, pp. 155–168.
- Clément, L., La conjecture de MJH Mogridge : test sur l’agglomération de Lyon (PDF), Cahiers Scientifiques du Transport (30), 1995.
- Afimeimounga, H., Solomon, W., Ziedins, I. The Downs–Thomson Paradox: Existence, Uniqueness and Stability of User Equilibria, Queueing Systems: Theory and Applications archive, Volume 49, Issue 3–4 (January 2005).
- Mogridge, M. J. H., Holden, D. J., Bird, J. and Terzis, G. C. "The Downs–Thomson paradox and the transportation planning process". International Journal of Transportation Economics, 14, pp. 283–311, 1987.