Does building more roads get everyone there faster?
Every morning 4,000 drivers travel from Start to End, and each one takes whichever route is quickest for them. Open a connector road between A and B, then watch what happens to the average trip.
Connector road, A → B
- Via A
- Via B
- Via the connector
Average trip time, day by day (minutes)
- Connector opened
- Connector closed
How the model works
- The two highways are long but never jam: they always take 45 minutes
- The two local roads are short but congested. Each takes five minutes when empty, plus one minute for every 100 cars, so a road carrying 2,000 cars takes 25 minutes
- The connector between A and B is short and never jams: it always takes 10 minutes
- Each day, drivers on a slower route switch to a faster one: 24 drivers for every minute they would save. Traffic has settled when every route in use takes the same time, so nobody can do better by switching
- Simulation based on Braess's paradox for a four-road netork. Equilibrium algorithm based on J. G. Wardrop (1952) and Beckmann, McGuire and Winsten (1956)
- The Conversation multimedia team used Claude AI to write the underlying code of the model for this simulation.