I was in a crash some time ago where the car in front of me jumped out of the way because the guy in front of him was at a dead stop. A "safe distance" for following a car that will have to decelerate to stop moving is a lot different if you are approaching a stationary object. It's not fair to expect either a human or computer to meet the latter standard on a highway were you expect everything to be moving at a speed close to the speed limit. There is a reason you can get a ticket for driving too slow.