Traffic stopping at a red light

The animation below is of a line of traffic approaching and then stopping behind an existing line of stationary traffic at a red light.

The blue line is the traffic density which is constant in both the stationary and moving traffic. The jump in density, or shock, moves backwards through the line of traffic. This jump in density models a situation in which the moving traffic comes to rest quite quickly.

The black dot indicates the last car that is stationary. Again, this moves backwards through the line of traffic as more cars come to rest.

(To play the animation again, click on the bottom-left button.)