The Lewis–Mogridge position, named after David Lewis and Martin J. H. Mogridge, was formulated in 1990 and observes that as more roads are built, more traffic consequently fills these roads. Speed gains from some new roads can disappear within months, if not weeks. Sometimes, new roads 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. It is a special case of Jevons paradox (where the resource in question is traffic capacity), and relates to the Marchetti's constant (average commute times are similar in widely varying conditions).

Following the position, it is not generally concluded that new roads are never justified but that their development needs to consider the whole traffic system, which means understanding the movement of goods and people in detail as well as the motivation behind the movement.