Does closing roads cut delays?

A recent study has found that closing off certain streets can actually improve traffic congestion.

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    Rush-hour traffic in Boston.
    John Nordell / The Christian Science Monitor /FILE
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File this one under "intensely counterintuitive." A recent study has found that closing off certain streets can actually relieve traffic congestion.

Using Google Maps, a trio of scientists – Hyejin Youn and Hawoong Jeong, of the Korea Advanced Institute of Science and Technology, and Michael Gastner, of the Santa Fe Institute – looked at traffic routes in Boston, New York, and London. Their paper, titled "The Price of Anarchy in Transportation Networks: Efficiency and Optimality Control" [PDF] and published in the journal Physical Review Letters, found that, when individual drivers seek the quickest route, they sometimes end up slowing things down for everybody. [Editor's note: An earlier version misstated the name of the journal.]

It all hinges on something called Braess's Paradox (and yes, I appreciate the irony of a Wikipedia entry that challenges the wisdom of crowds), which states that adding capacity to a network in which all the moving entities rationally seek the most efficient route can sometimes reduce the network's overall efficiency.

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.

Note that this still happens even if – indeed, especially if – all the drivers have perfect information about what all the other drivers are doing, such as with a GPS that gives real-time traffic updates.

The authors compared the Nash equilibrium time to the socially optimal travel time, and dubbed the ratio between the two "the price of anarchy." In their study of the Boston area, which looked at travel times from Harvard Square to Boston Common, the price of anarchy at peak traffic times made for a journey that is 30 percent longer.

But the price of anarchy drops if you close a few roads, because individual drivers are less able to selfishly optimize their routes. In their analysis, the authors identified six streets in Boston and Cambridge: By closing those streets, they say, the optimal collective travel time would decrease between the two points.

At first blush, this study seems dissonant with findings that traffic flows can be improved by increasing vehicular anarchy. As I noted a few months back, there is considerable evidence that removing all traffic controls – lights, signs, road markings, and even the distinction between streets and sidewalks – can actually make traffic move more smoothly, as well as cut down on the number of accidents and increase the area's economic vitality. The idea behind these "shared streets," which have been successfully deployed in many European cities, is that the lack of traffic signs makes you take personal responsibility for directly negotiating with the pedestrians, cyclists, and other cars around you, instead of, say, gunning it through an intersection just because you know you have the light.

But maybe these two traffic models have more in common than it first seems. Both encourage individuals to drive more slowly so that everyone gets to his destinations faster. Both favor a holistic approach to traffic, one that designs from the perspective of the overall flow rather than that of an individual driver. And both open up more space for pedestrians.

It's not too difficult to imagine a city designed with these principles in mind. Fewer roads with slower but smoother traffic. Spaces that can easily be converted to car-free zones to suit the needs of the network. And fewer opportunities for people to drive like jerks. Sounds like a nice place to take a walk, actually.

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