Lewis-Mogridge Position
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The Lewis-Mogridge Position was formulated in 1990. It captures the evidence that the more roads are built, the more traffic there is to fill these roads. Speed gains from new roads tend to 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. With public transport, however, there tends to be more control on capacity and use, such as how many trains are scheduled.