The plot is trying to illustrate the solution to an equation that is quite similar to Burger's equation, with a strong dependency on space and time. That's not

The plot is trying to illustrate the solution to an equation that is quite similar to Burger's equation, with a strong dependency on space and time. That's not easy to do in a single, static image. When the characteristics meet (when the lines overlap), you get a shock wave. Or, in the current context, a traffic collision.