You’re driving along at highway speeds when you come over a hill and find a massive traffic jam sprawled before you. You slam on the brakes and nervously look at the rear-view mirror, wondering if the car behind you is going to stop fast enough.

It’s a frightening position that we’ve all found ourselves in at one time or another. It’s especially concerning if you know there is a large, heavy semitruck coming over the hill behind you. That fear is warranted, as these backups — along with other types of traffic congestion — can and do cause accidents.

Why do these backups happen? Sometimes there’s a clear indicator of what happened. Maybe there was a car accident. Maybe there’s a construction zone. Maybe traffic is entering the highway and there are just too many cars for the road, causing everyone to slow down.

Sometimes, though, you don’t ever see a clear reason for the traffic jam. In heavy traffic, experts note that it can happen if someone just hits their brakes for any reason. That forces all of the cars behind them to keep hitting their brakes in a wave that moves back through the vehicles. Since cars can slow down faster than they speed up, the overall speed on the road drops and can even grind nearly to a halt, all because of one person touching their brakes at the wrong time.

Regardless of the reason for the backup, the risks you face from traffic behind you are the same. If you are injured in an accident, be sure you know what legal rights you have.