Braess's Paradox

Braess’s Paradox describes how adding capacity to a congested network can make everyone slower. It is a mass multi-player Prisoner’s Dilemma: every actor optimizes rationally for themselves, yet the sum of selfish choices leads to a Nash equilibrium that is globally worse than the state before the expansion.

Add a new, seemingly faster connecting road to a traffic network. This shortcut acts like the temptation payoff in a huge multi-player Prisoner’s Dilemma: for each individual driver, switching to the new road is rational. But when everyone switches at once, the shortcut clogs — and travel time becomes longer than without the road. The system settles into a Nash equilibrium where no individual gains by switching, yet which is collectively worse for all. The bottleneck arises not despite the added capacity, but because of it.

In urban and traffic planning, in electrical grids, in data-packet routing across networks, and in process design. Whenever many actors independently share a common bottleneck and adding a resource might counterintuitively degrade overall performance.

Sometimes the fix is to remove the tempting link — actual road closures have been observed to speed up traffic. Alternatively, congestion pricing that aligns the private temptation payoff with the social cost, or coordinated routing that replaces individual choice with a system-wide optimal allocation.

In Seoul, tearing down an urban motorway sped up traffic rather than worsening it; similar effects were observed in Stuttgart and New York when streets were closed. In data networks, adding a high-speed link can raise latency if all flows shift onto it.