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For the first time, Japanese researchers have conducted a real-life experiment that shows how some traffic jams appear for no apparent reason. They placed 22 vehicles on a single track and asked the drivers to cruise around at a constant speed of 30 kilometres an hour. At first, traffic moves smoothly, but soon, the distance between cars started to vary, and vehicles clumped together at one point on the track, but the jams spread backward around the track, like a shockwave at a rate of about 20 kilometres an hour. Real-life jams move backward at about the same speed.