Saturday, March 15, 2008

Shockwave Traffic Jams

If you've driven along the freeways of a large metropolitan area for any length of time, you've probably encountered them: traffic jams that seem to appear out of nowhere, for no apparent reason. These rogue slowdowns are especially frustrating to drivers, since the only thing more annoying than a traffic jam is a traffic jam that seems to occur for no reason, i.e. it's not rush hour and there's no accident in sight.

Thanks to some research being done in Japan, however, we now know what causes these traffic jams. We also know what to call them:

The mathematical theory behind these so-called "shockwave" jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down.

After that first vehicle brakes, the driver behind must also slow, and a shockwave jam of bunching cars appears, travelling backwards through the traffic.

The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now.

Here's a video of the shockwave traffic jam being recreated:

The culprit of these rogue traffic jams is simple human behavior. Consider that the freeway full of cars is also full of drivers with different levels of comfort and different skill levels. In situations where traffic is heavy but otherwise free-flowing (a traffic engineer would call it Level of Service C), all it takes is a single driver who is not comfortable about something like his or her speed or the distance between him or her and the vehicle in front to slow down for a moment, which causes a chain reaction: the person behind hits the brakes, causing the person behind that person to slow down as well, et cetera. People in adjacent lanes begin slowing down as well, and before long the entire traffic flow is interrupted.

Now that we know what causes shockwave traffic jams and have successfully recreated them, can we do anything about them? A possible solution would be simply to slow down the overall flow of traffic during conditions favorable to the creation of these traffic jams. As speeds are reduced, the level of comfort of individual drivers is increased such that drivers won't feel the need to slam on the brakes and cause backups. As the linked article notes, this technique is being tested on the M25 motorway that circles London.

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