whatCars on a ring road, every one driving forward — and below them, their history: the scrolling chart traces each car through time, position across, time running down. Tap any car (or its line) and it brakes for a moment. Watch what its mistake becomes: a dark band of stopped traffic that drifts BACKWARD round the ring, against every car in it, long after the guilty driver has sped away.

whyThe phantom jam is the classic emergence of everyday life: no crash, no bottleneck, yet traffic stops. It stops because each driver reacts a little late and brakes a little harder than the driver ahead. The jam is a wave, its backward speed of roughly fifteen to twenty kilometres per hour is measured on real motorways, and this piece measures it on the chart in front of you.

howEach car follows the Intelligent Driver Model: it accelerates toward its preferred speed and brakes by how close it is, how fast it is closing, and the safety gap it wants — nothing else, and only the car directly ahead is ever seen. The ring is loaded near the critical density where flow is possible but fragile. In the chart, each line's slope is a car's speed: steep lines are free flow, flattened lines are the jam, and the jam's edge slants the other way — the wave moving upstream. The readout gives the mean speed and the measured wave speed. One integer seeds the starting imperfections.