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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 sometimes has no impact at all on congestion, especially if the new capacity just causes more people to hit the road during busy parts of the day—say, people who used to wake up an hour earlier to avoid rush hour. (There's even a mathematical basis for this paradox, see here.) That's why the growing consensus among transportation experts is that congestion pricing is often 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. Three scientists—Hyejin Youn and Hawoong Jeong, of the Korea Advanced Institute of Science and Technology, and Michael Gastner, of the Santa Fe Institute—have found that sometimes you can actually speed up traffic flow in certain cities by eliminating roads. The underlying theory here, laid out by Eoin O'Carroll, is that, given their druthers, drivers don't always reach a socially optimal equilibrium on their own:  

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.

Anarchy doesn't always lead to the most efficient outcome, and sometimes even exacts a real cost. So is there a way to fix the problem? Sometimes! In their full paper, the three researchers examine a bunch of roads in Boston and downtown Manhattan. Looking at the map below, if you shut down any of the red streets shown, you'd make travel times worse. Close any one of the blue routes, 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—and traffic could actually speed up, sometimes by as much as 30 percent, because they'd bring the equilibrium closer to the "optimal" total travel time between key points:

Weird but fascinating.

--Bradford Plumer