system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why, when I'm driving along even a slightly busy highway, does the traffic build up to a complete stop, then slowly get back up to speed for no reason?

It's the oddest thing!
gpt
Traffic congestion moves in waves backward from the source. If traffic stopped a few miles up the road an hour ago, the wave will have moved backward, and when you arrive at it, there will be no visible source for the problem.