The Braess Paradox Says Closing Roads Can Actually Speed Up Traffic

Brilliance | Dec. 19, 2017

Can improving something actually just make it worse? It seems nonsensical, but it's the basic premise of the counterintuitive Braess paradox.

Doomsday Averted

On Earth Day in April of 1990, New York City closed 42nd Street—an incredibly busy crosstown route. "Many predicted it would be doomsday," then-Commissioner Lucius J. Riccio told The New York Times. "You didn't need to be a rocket scientist or have a sophisticated computer queuing model to see that this could have been a major problem." But it wasn't. In fact, traffic sped up. This was a prime example of The Braess paradox, a principle conceived in 1968 by German mathematician Dietrich Braess. It states that adding an additional road to a congested area of traffic will actually make time spent in traffic longer for the drivers, and also explains why closing a road can sometimes speed up traffic.

The reason why is that drivers act selfishly: each driver wants the shortest possible trip for themselves, and aren't thinking in a communal way that might make everyone's trip shorter. So when you open up a new street, drivers will use it—and end up clogging up both the new street and the streets that they use to access it. By the same token, if you close a street, you ease traffic on its access roads and don't make much of a dent in the already-busy streets drivers would use otherwise.

When The Paradox Is Not A Paradox

Some researchers have found instances where the Braess paradox doesn't hold. According to University of Massachusetts professor Anna Nagurney, the paradox depends on demand. When there's a lot of demand, say, at rush hour, and very low demand, such as in the wee hours of the morning, a new route may work out just fine. Nagurney tells that during busy times, drivers learn that certain roads "add so much to travel time that commuters, over time, learn to switch their paths/routes of travel, and where the Braess paradox may have occurred, is then negated..." On the flipside, when there's little traffic on the road, a faster route is just that—faster.

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