A few years back, I was in a friend's car and we were rear-ended when a couple cars ahead of us slammed on their brakes. My friend stopped in time, but the car behind us didn't even notice traffic had slowed. There were no squealing tires, or horns. We came to a fast stop, and then were hit from behind by some guy going at least 45 mph.
Incredibly enough, apart from a ruined pair of pants and a big mess in the car (my coffee spilled everywhere, what were you thinking?), we were OK.
But for the next 6 months, I got extremely nervous any time I was in a car coming to a quick stop. I always expected a neck-snapping jolt and hot coffee in my lap.
Getting rear-ended on a bike, I've heard, is no laughing matter. There's no telling where you might end up because someone else didn't understand basic physics. Personally, I'd prefer not to experience such things.
As such, I keep an eye on my mirrors at stop-lights, watching for rapidly approaching vehicles. I'll flash my brake light a few times if they're moving too quickly for my liking. Usually, that slows them down (Why does that work? Does anyone know?). I also stay in gear at intersections. Maybe it's more superstition than effective plan, but I'd rather have the slim chance of noticing I'm about to be hit and getting out of the way than the alternative.