Red cars drive right and blue cars drive down, on a grid that wraps around. On even steps every red car moves one cell if the cell ahead is empty, and on odd steps every blue car does the same. That is the whole model, but how crowded the grid is changes everything: below about a third full the cars sort themselves out and flow freely, and above that they lock into one big diagonal jam. Use the slider to choose how many cars there are (it starts a new random road), then press Play. Click or drag to add and remove cars.

The model was proposed by Ofer Biham, A. Alan Middleton and Dov Levine in 1992. In a 128-cell-wide test of my own, a road about 20 to 30 percent full ended with almost every car moving, and one 40 percent full or more ended with hardly any moving (on this wider grid a 55 percent road still had about a tenth moving after 480 steps); around 35 percent it can go either way, so run it a few times. A jam is a still picture for the cars that are stuck, but the page does not stop by itself because the red and blue cars take turns.

This page was created with Claude Code.