Traffic jams are interesting phenomena. While we tend to think of them as the result of car crashes (and sometimes they are), far more often they're a subtle outgrowth of the way we drive, and how we apply our brakes.
Humans often overreact to small changes in traffic flow, and that reaction cascades into a massive slowdown. In the video below, CGP Grey illustrates how traffic works, and why humans are typically bad at managing traffic flow.
This video explains that endlessly frustrating phenomenon where you're stuck in a jam, assuming there's some massive wreck just up ahead, but there is no wreck and never was—you just magically speed up at some point. If you've ever wondered why that happens or what to do about it, have a look:
If you're interested in this, there's a lively discussion thread on Reddit talking about this whole business.