Phantom Traffic Jams: What Causes Mysterious Highway Backups?

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Sometimes highway traffic jams up for no apparent reason. There's no accident, and no real reason to step on the brakes — except to avoid hitting the car in front of you. What causes these backups? An MIT scientist thinks he's found a way to prevent these tie-ups.


If you find yourself stuck in holiday traffic this weekend, our next story won't help you much, but it does bring hope for a less stressful future. A computer scientist at the Massachusetts Institute of Technology thinks he's found a way to eliminate at least one kind of really annoying traffic jam. NPR's Joe Palca explains.

JOE PALCA, BYLINE: If you've done even a modicum of highway driving, you've probably experienced this: You're moving along at a pretty good clip when all of a sudden traffic slows to a crawl. You inch along for a while, and then just as suddenly, the traffic starts moving again.

BERTHOLD HORN: There's no accident. There's no one speed-trapping, and you're puzzled. What's going on?

PALCA: That's Berthold Horn, the MIT computer scientist. So what is going on? Horn says it just takes one driver to cause the problem.

HORN: A typical case would be someone slams on the brakes just for a short moment. The car behind them is forced to slam on the brakes and so on back upstream.

PALCA: Horn says it's like a wave flowing backwards. People who study this talk about chaotic systems and positive feedback, but the practical consequences are that the amount drivers have to slow down increases the further back you are from the original incident.

HORN: There may be someone who is forced to actually stop.

PALCA: But Horn thinks he's onto a way to smooth out that wave and keep traffic flowing. His method involves not only keeping track of the car in front of you but the car behind you, as well. It may seem counterintuitive, but according to his calculations, that can prevent these phantom tie-ups.

HORN: The easiest way to think about it at first is you're trying to maintain a distance halfway between the car ahead and the car behind.

PALCA: Doing that will smooth out the traffic. Now to make the scheme really work, Horn says you need cars that can do the forward and backward monitoring on their own and make the necessary adjustments automatically. But Horn says there would be some benefit if lots of people kept track of their spacing using their rear-view mirrors and didn't tailgate.

HORN: Once I came up with this idea, I changed my own driving behavior.

PALCA: Horn says he's actually measured a small improvement in his commute as a result. Of course you can't spend too much time looking in the rear-view mirrors; that will cause its own set of problems. Joe Palca, NPR News.

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