I find this diagram _incredibly_ hard to read. I’m pretty sure I understand the phenomenon he’s describing — without safe following distances, car n+1 must break more aggressively than car n in order to avoid a collision, so given a long enough chain of non-safe followers, a collision is guaranteed. But either I am dumber than I’d like, or that plot is terrible.

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.