Why traffic jams appear out of nowhere

Sometimes a motorway grinds to a halt and then clears up with no crash, roadworks or merge in sight. That's a phantom traffic jam, and it starts with one driver tapping the brakes.

Each driver reacts a moment late and brakes slightly harder than the car ahead, so a small slowdown grows as it passes back through the line. In a 2008 experiment, 22 cars on a circular track produced a jam out of nothing within a minute. The good news is that one calm driver who leaves a gap can make the wave disappear.