Especially since, most of the time, they weren't intending to rear end you and therefore may be going far too fast to reasonably slow down in time. In my town of 1200 we had a death recently where a driver (no seatbelt) was speeding through a 45 MPH road and somehow didn't see the loaded dump truck stopped to turn left at a construction site. Full speed contact, his vehicle veers to the right and into the ditch. He was either killed instantly or when he hit the ditch.