Understanding Phantom Traffic Jams

While driving you’ve probably seen it happen quiet often. Traffic slows to a crawl, then stops entirely. Minutes later, it begins to move again, and then suddenly, you’re moving at full speed. The weirdest part is that there is no construction, accident, or other possible explanation for the traffic jam. Why does this happen?

As it turns out, a few different groups of researchers have been using mathematical calculations and real-world experiments to try answering this question. And they think they have the answer. They also have suggestions on how to stop these jams.

Why Phantom Traffic Jams Occur

If there are enough cars on a road or highway, any minor disruptions to the flow of traffic can cause a self-reinforcing chain reaction: It just takes one motorist to get too close to the car in front and hit the brakes. The driver behind does the same thing, as do hundreds of motorists in succession. Within moments there is a ripple which can stretch several meters. Even when cars leave this traffic wave, though, the wave itself doesn’t disappear: it gradually drifts backward, against the direction of traffic.

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Researchers have conducted real-world experiments that come to the same conclusion. In one, they instructed 22 drivers to drive at the same speed (30km/h), and preserve the same amount of space between cars, on a small circular road. Inevitably, traffic waves formed.

Avoiding Phantom Traffic Jams

In one sense, it seems reasonable to blame these phantom traffic jams on individual drivers. The models indicate that these jams are more likely to form when people drive as fast as possible, then finally brake when necessary to avoid hitting the car in front of them, triggering a chain reaction.

If people anticipate higher traffic densities ahead, and take their feet off the accelerator pedal earlier and leave more room in front of them, instead of waiting until they have to brake, that can prevent these traffic jams from arising.

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However this doesn’t totally eliminate phantom traffic jams, it does makes them less likely to form. But if there are enough cars on the road, even if people anticipate approaching traffic to the best of their abilities, phantom traffic jams will still form.

The partial solution to this problem is all about spacing, make sure there’s a buffer of about half a car in front and behind you. This will avoid any sudden braking, as well as a snowballing traffic jam behind you. This method wouldn’t eliminate phantom traffic jams completely, it will just make them less likely to form.

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