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If You Build It, They Will Clog

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  • Eric Britton
    Kind thanks to Robert Moskowitz for the heads-up. Eric Britton If You Build It, They Will Clog Common sense suggests that the best way to deal with traffic
    Message 1 of 1 , Nov 8, 2008

      Kind thanks to Robert Moskowitz for the heads-up. Eric Britton


      If You Build It, They Will Clog

      Common sense suggests that the best way to deal with traffic jams and gridlock is to… add extra lanes or build more roads. But common sense is often wrong: Adding extra lanes to a packed highway often has no impact whatsoever on congestion, especially if the new capacity just causes more people to hit the road during crowded parts of the day—say, people who used to wake up earlier to avoid rush hour. (There's even a mathematical basis for this paradox, see herehttp://en.wikipedia.org/wiki/Braess_paradox) That's why the growing consensus among transportation experts is that congestion pricing is a better way to unclog roads and highways than adding new capacity.

      But now here's another twist, courtesy of WorldChanging's Clark Williams-Derry. (http://www.worldchanging.com/archives/008957.html) Three scientists—Hyejin Youn and Hawoong Jeong, of the Korea Advanced Institute of Science and Technology, and Michael Gastner, of the Santa Fe Institute—found that you can actually speed up traffic flow in certain cities by eliminating roads. Here's the nut: (http://features.csmonitor.com/environment/2008/10/06/does-closing-roads-cut-delays/)

      The authors give a simple example of how this could play out: Imagine two routes to a destination, a short but narrow bridge and a longer but wider highway. Let’s also imagine that the combined travel times of all the drivers is shortest if half take the bridge and half take the highway. But because each driver is selfishly trying to seek the shortest route for himself, this doesn’t happen.

      At first, everyone will go for the bridge because it’s shorter. But then, as the bridge becomes backed up, more drivers start taking the highway, until the congestion on the bridge starts to clear up. At that point more drivers go back to the bridge, which then becomes backed up again. Eventually, the traffic flow settles into what’s called the Nash equilibrium (named for the beautifully minded mathematician), in which each route takes the same amount of time. But in this equilibrium the travel time is actually longer than the average time it would take if half of the drivers took each route.

      In other words, anarchy has a price. In the full paper, (http://arxiv.org/PS_cache/arxiv/pdf/0712/0712.1598v4.pdf) the authors examine a bunch of roads in Boston and downtown Manhattan. Basically, if you shut down the red streets shown in the maps below, you'd make travel times worse. Shut the blue ones down, and you wouldn't make much of a difference one way or the other. But close the black dotted roads—turn them into bike lanes, maybe—and traffic would actually speed up, sometimes by as much as 30 percent:

      Weird but fascinating.

      --Bradford Plumer


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