most road cars made today have similar stopping power built into thier brakes, so the only real reason for getting rear ended is the numpty behind you following too closely. even the massive brakes kits that stop the car in less than half the DVLA stopping distance shouldnt cause an accident. if you notice a car slowing, brake, and if you are a good driver the space you have left should be ample.
what i find amusing is the new advice that many of my younger friends are being given, that you should use only the brakes to slow for a new speed limit, for years now i simply go off the power a few dozen yards before the change, maybe downshift - it saves the brakes, and gives better fuel economy, as many more inexperienced drivers slow too much and have to use the gas to get back to the limit.
apparantly this is to let the person behind you know that you will be slowing down via the rear lights, IMO an individual should not be put in charge of a vehicle if they cannot judge changes in speed and distance by perception of the remaining space alone.
more directly on topic - no idea, usually about four seconds away from impact with the car in front, by the way anyone know why a 10 year old citroen would fishtail wildly under heavy braking?