ExplainerTools

Short · Roads

Why do traffic jams appear out of nowhere?

One driver taps the brakes. On a quiet road the gaps soak it up; on a busy one each driver brakes harder until someone stops, and the jam drifts backward at about 20 km/h.

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Opens Traffic Wave Lab. Change the same thing yourself.

Narrated · 32 seconds · captions includedDownload the film

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Transcript

Why do traffic jams appear out of nowhere? No crash, no lane closure. Just one driver tapping the brakes. On a quiet road, the gaps soak it up. Now add cars. The car behind brakes a little harder, and the next one harder still, until someone stops. And the jam drifts backward, against the traffic, at about twenty kilometers an hour, while every car still drives forward through it. Try it yourself at Explainer Tools dot com.