whatTwo crowds walk a corridor in opposite directions, squeezing through one doorway in the middle. No one is told where to walk; each walker only wants to keep moving and not to touch anyone. Watch the opposing streams comb themselves into lanes, and watch the doorway breathe: one side pours through while the other waits, then the flow reverses. Click to make everyone hurry: the flow briefly rises, then the door clogs, and over the next minutes fewer people get through than if no one had rushed.

whyLane formation, doorway oscillation, and faster-is-slower are the signature discoveries of crowd physics: order that no walker intends and no planner designed, plus the unsettling fact that impatience reduces throughput. The same mathematics guides real evacuation design, which is why fire codes care about door widths and why stampedes kill in crowds where every individual is behaving sensibly.

howEach walker follows Helbing's social-force model: a steady pull toward its own walking speed in its own direction, an exponential psychological repulsion from nearby bodies and walls, and, on contact, a hard pushback plus sliding friction. Walkers aiming at the wall steer for the doorway, and the corridor wraps around at the ends, so the two populations never thin out. Lanes appear because head-on encounters are costly and drifting behind a same-direction walker is cheap; the doorway alternates because whichever stream holds the gap blocks the other until its pressure wins. Hurrying raises the desired speed, which packs the doorway harder; sliding friction between pressed bodies then dominates, and after a brief spurt the jam swallows the gain — measured here as crossings over two minutes, nearly halved. The readouts track lane order, flow through the door, and the crowd's clock. One integer seeds the start.