The traffic-light problem

The animation below is of a line of traffic pulling away from a traffic light after it has turned green.

The blue line is the traffic density which is initially zero ahead of the light and uniform behind the light. The jump in density "flattens" as the traffic begins to move with each non-zero density value moving backwards through the line of traffic.

The black dot indicates the leading car that is stationary. Again, this moves backwards through the line of traffic.

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