Study Says Closing Roads Might Cut Congestion. Huh?

In a study that at first glance doesn't seem to make much sense, three scientists have discovered that drivers choosing between multiple routes to reach their destination most quickly can actually end up slowing everyone down. Limiting their options by closing off certain streets could actually reduce congestion.

The study, titled The Price of Anarchy in Transportation Networks: Efficiency and Optimality Control challenges the idea that more roadway always equals less congestion, and argues that giving drivers fewer choices forces them to work in a coordinated way with other drivers, reducing "selfish" behavior that makes traffic worse. 

It's pretty heavy reading, but the study's central argument is built on something called Braess' Paradox, which says that adding capacity to a network where everyone in that network seeks the most efficient route can reduce the network's overall efficiency. If every driver takes the road they think will get them somewhere the fastest, then that route gets bogged down.

Here's an example, as provided by the authors of the study. Say you have two options for reaching destination X. One of them is a narrow bridge, the other a longer but wider highway. The combined travel time for all drivers is shortest if half take the bridge and half ride on the highway. But drivers only know which route is most direct, so they all head for the bridge. The crush of vehicles creates a traffic jam, so the next batch of drivers choose the highway. Bridge traffic clears as a result, causing more drivers to choose this route until it clogs again.

This back and forth continues until each route ends up taking the same amount of time, a phenomenon known as the the Nash Equilibrium. The problem is that with this equilibrium, average travel time ends up taking longer than it would if the drivers had split the two routes 50/50.

By comparing Nash Equilibrium time to optimal travel time, the researchers came up with a ratio they call "the price of anarchy." Using the journey between Boston Common and Harvard Square in Cambridge as an example, they found that the price of anarchy is a journey that's 30-percent longer. But close some roads, and drivers are less likely to bounce around, which actually cuts the collective travel time.

These findings seem to contradict the theory that traffic moves more quickly when drivers are given more control. For example, some European cities have removed traffic lights and signs, which has smoothed traffic and reduced accidents by forcing drivers to take more personal responsibility (blowing through an intersection because you have a light doesn't work if there aren't lights in the first place). Narrowing drivers' options by closing roads seems to run counter to this.

But a piece in the Christian Science Monitor points out that the two traffic models actually share some important features. They both focus less on the individual driver and more on the larger traffic system, and both suggest that a slow and steady approach can get drivers where they need to go more quickly.

By Dave Demerjian for Wired.com

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