Engines of Our Ingenuity

No. 2814: BRAESS'S PARADOX

by: Andrew Boyd

Today, taking a new road. The University of Houstonís College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

As we all know from experience, when too many cars are on the road, traffic slows down. So we build more roads. New roads give the road network more capacity and help speed things up. The new roads may help a lot, or just a little, but they certainly canít make our trip any longer. Or can they?

traffic jam

In 1969 German mathematician Dietrich Braess showed that adding a road could actually increase travel time for everyone involved. Imagine waking up for a morning trip to work after a new roadís been opened, and finding that your commute — and everyone elseís — just got longer. It sounds paradoxical. And, in fact, it is. Itís known as Braessís Paradox.

Braessís paradox hinges on the very reasonable assumption that drivers will try to find the route to work that minimizes their own, personal travel time. When a new road opens, drivers may flock to it. But if everyone does the same thing their new route may be clogged. As a result, drivers may try a different route the following day. After a few days or weeks, people find the route that seems fastest for them. Traffic settles into a state of equilibrium. Braess showed that there are examples where, when all the dust settles, thereís a unique equilibrium where everyone is now taking longer to get to work than they did before.

Interestingly, Braessís paradox isnít some bizarre theoretical construct, but has actually been observed in practice. Another way of looking at Braessís paradox is that if we close existing roads, we may actually see peopleís travel times decrease.

In the early two-thousands, when city planners in Seoul, South Korea, replaced a six lane highway with a five mile long park, traffic flow improved. In 1969, congestion in Stuttgart, Germany improved only after a newly built section of road was shut down.

Closer to home, New Yorkís Mayor Bloomberg spearheaded experimental road closures in 2009. At issue were two sections of Broadway where it obliquely crosses major north/south boulevards at Times and Herald Squares. The experiment was pitched as a way to reduce congestion. Projections were high, with travel times estimated to fall up to forty percent on some streets. When the numbers came in they werenít as lofty as advertised. But overall congestion appeared to have dropped. The experiment was deemed a success and the road closures were made permanent. Times and Herald Squares are now home to successful pedestrian plazas — plazas that havenít exacerbated traffic headaches.

before pictureafter

Braessís paradox doesnít tell us we should stop building more roads. Cars need roads to get from point A to point B. What it does tell us is that adding more roads doesnít always make things better. Sometimes less really is more.

Iím Andy Boyd at the University of Houston, where weíre interested in the way inventive minds work.

(Theme music)


Notes and references:

L. Baker. Removing Roads and Traffic Lights Speeds Up Urban Travel. Scientific American, January 28, 2009. From the Scientific American website: http://www.scientificamerican.com/article.cfm?id=removing-roads-and-traffic-lights. Accessed July 24, 2012.

K. Button, H. Vega, and P. Nijkamp. A Dictionary of Transport Analysis. UK: Edward Elgar Publishing, Limited, 2010. See page 459. See also the Google Books website. Accessed July 24, 2012.

M. Grynbaum. New York Traffic Experiment Gets Permanent Run. New York Times, February 11, 2010. From the New York Times website: http://www.nytimes.com/2010/02/12/nyregion/12broadway.html. Accessed July 24, 2012.

All pictures are from websites of the U.S. Department of Transportation.

This episode was first aired on July 26, 2012



The Engines of Our Ingenuity is Copyright © 1988-2012 by John H. Lienhard.