Downs–Thomson paradox
The Downs–Thomson paradox (named after Anthony Downs and John Michael Thomson), also known as the Pigou–Knight–Downs paradox (after Arthur Cecil Pigou and Frank Knight), states that the equilibrium speed of car traffic on a road network is determined by the average door-to-door speed of equivalent journeys taken by public transport.
It is a paradox in that improvements in the road network will not reduce traffic congestion. Improvements in the road network can make congestion worse if the improvements make public transport more inconvenient or if it shifts investment, causing disinvestment in the public transport system.
The general conclusion, if the paradox applies, is that expanding a road system as a remedy to congestion is ineffective and often even counterproductive. That is known as Lewis–Mogridge position and was extensively documented by Martin Mogridge in the case study of London on his book Travel in towns: jam yesterday, jam today and jam tomorrow?[1]
A 1968 article by Dietrich Braess[2] pointed out the existence of the counterintuitive occurrence on networks: Braess's paradox states that adding extra capacity to a network, when the moving entities selfishly choose their route, can in some cases, reduce overall performance.
