Lewis-Mogridge Position

The Lewis–Mogridge Position 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).

The Lewis–Mogridge Position 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

Induced demand

Induced demand is the phenomenon that after supply increases, more of a good is consumed. This is entirely consistent with the economic theory of supply and demand; however, this idea has become important in the debate over the expansion of transportation systems, and is often used as an argument...

 in the transport literature, and was posited as the "Iron Law of Congestion" by Anthony Downs

Anthony Downs

Anthony Downs is a noted scholar in public policy and public administration, and since 1977 is a Senior Fellow at the Brookings Institution in Washington D.C.....

.

Following the Lewis-Mogridge Position it is not generally concluded that new road

Road

A road is an identifiable route, way or path between places. Roads are typically smoothed, paved, or otherwise prepared to allow easy travel; though they need not be, and...

s are never justified, but that their development needs to consider the whole traffic

Traffic

Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...

 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

Private transport

Private transport, as opposed to public transport, is transport in one's own vehicle , or through self-power . Private transport differs from public in that it lacks timetables and fixed itineraries. It also tends to be more convenient to the commuter as it is readily available on demand...

, 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

London congestion charge

The London congestion charge is a fee for some motorists travelling within those parts of London designated as the Congestion Charge Zone . The main objectives of this charge are to reduce congestion, and to raise funds for investment in London's transport system...

.

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

Public transport

Public transport comprises passenger transportation services which are available for use by the general public, as opposed to modes for private use such as automobiles or vehicles for hire.Public transport services are usually funded by fares charged to each passenger, with varying levels of subsidy...

. This relationship and overall equilibria is also known as the "Downs-Thomson paradox

Downs-Thomson paradox

Downs-Thomson paradox, also referred to as the Pigou-Knight-Downs paradox, states that the equilibrium speed of car traffic on the road network is determined by the average door-to-door speed of equivalent journeys by public transport.It follows that increasing road capacity can actually make...

". However, according to Anthony Downs

Anthony Downs

Anthony Downs is a noted scholar in public policy and public administration, and since 1977 is a Senior Fellow at the Brookings Institution in Washington D.C.....

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."