Road journey speed is set by the speed of the parallel public transport, so expanding roads without transit yields no gain.
Named jointly for Downs and Thomson. Adding road capacity draws riders off transit, transit frequency falls, and the equilibrium journey time returns to where it was.
In practice
- A city widens its ring road, bus ridership falls, services are cut, and journey times return to where they started.
- A commuter route where the train is slow finds that road improvements are absorbed within two years.
- Further reading
- Wikipedia
- Subject
- Economics & Incentives
- Essay
- Rules for choosing, and the paradoxes that humbled the theory