This holiday season, many of us will take to the roads. Which means many of us are also about to face rather unpleasant traffic jams. The most frustrating ones seem to occur for no reason at all. You’re stuck, positive something catastrophic must have happened up ahead. Then for no apparent reason, the traffic clears.

Now mathematicians from England’s University of Exeter believe they have solved this mystery. They modeled the effect of various highway events, such as a truck slowly pulling out from the merge lane. If the truck’s approach causes the car behind to quickly drop below a certain speed, then the next driver back also hits his or her brakes. This continues in a braking chain that the authors call a backward traveling wave. It could stretch back miles.

So it’s not necessarily the volume of traffic, but rather a sudden interruption in the flow that causes delays. The authors advise drivers to pay attention, and, when necessary, brake gently and slowly.  If you suddenly overreact and hit the brakes, you could be causing someone to grimace in frustration in a car many miles away.

—Cynthia Graber

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