Adding a road can increase congestion

http://www.uh.edu/engines/epi2814.htm

BRAESS'S PARADOX
by: Andrew Boyd

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 can’t make our trip any longer. Or can they?

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 road’s been opened, and finding that your commute — and everyone else’s — just got longer. It sounds paradoxical. And, in fact, it is. It’s known as Braess’s Paradox.

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Interestingly, Braess’s paradox isn’t some bizarre theoretical construct, but has actually been observed in practice. Another way of looking at Braess’s paradox is that if we close existing roads, we may actually see people’s 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 York’s 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 weren’t 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 haven’t exacerbated traffic headaches.