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 or the next best alternative.
Although consistent with economic theory, it is a paradox in that it contradicts the common expectation that improvements in the road network will reduce traffic congestion.
The general conclusion, if the paradox applies, is that expanding a road system as a remedy to congestion is ineffective and often even counterproductive.
Increasing the size of the network is characterized by behaviors of users similar to that of travelers on road networks, who act independently and in a decentralized manner in choosing optimal routes between origin and destination is an extension of the induced demand theory and consistent with Downs' 1992 theory of "triple convergence", formulated to explain the difficulty of removing peak congestion from highways.
In response to a capacity addition three immediate effects occur: drivers using alternative routes begin to use the expanded highway; those previously traveling at off-peak times (either immediately before or after the peak) shift to the peak (rescheduling behavior as defined previously) and public transport users shift to driving.