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The Traffic Jam With No Cause, and Why It Moves Backward
In 2008 a Japanese team produced a phantom traffic jam on a circular track with no bottleneck at all, and it drifted backward.
Somewhere on a motorway this afternoon, a few thousand people will slow to a crawl, creep forward for twenty minutes braced for the wreckage that has to be waiting at the front of it, and find nothing. No crash, no breakdown, no lane closure, no police car on the shoulder. The road simply opens and everyone accelerates away. Nobody learns what happened, because in the ordinary sense nothing did.
And yet every one of those drivers stopped. Whatever they hit was solid enough to halt a lane of two-tonne vehicles, and it has been measured often enough, by the detectors buried in motorway lanes and by cameras overhead, that its speed is known. Traffic engineers call it a phantom traffic jam, a fine name for the experience and a poor one for the physics, because the thing is not a ghost. A phantom traffic jam is a wave of stopped cars that forms in dense traffic with no crash and no bottleneck to cause it, and travels backward through the traffic at about 15 km/h. It obeys a stability criterion that fits on one line, and it can be conjured out of thirty lines of Python.
