#845 Traffic Jams

For the first time Japanese researchers have ___ 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 ___ around at a constant speed of 30 kilometers an hour. At first, traffic moved ___ but soon the distance between cars started to vary and vehicles clumped together at one point on the track. But the jams ___ backwards around the track like a shock wave at a rate of about 20 kilometers an hour. Real-life jams move backwards at about the same speed.