I would have to join in with the "dumb luck" vote, unless your vehicle is one of the newer ones with a radar sensor in addition to the speed sensors. I know Toyota has this, or is at least working on it.

The coaches we build use the Vorad system http://www.roadranger.com/products/...cruise.htm which keeps you from bumping into the guy in front of you, and warns you if you're actively driving and approaching an obstacle to the front or side.

My old car's cruise control would let me run right into the car in front, all while going a nice steady speed...

Dennis