Keep your distance behind

There are systems for cars that measure the distance to the car in front and, based on current speed, advise if the stopping distance is insufficient. It is, however, just as dangerous to you if the car behind you in not maintaining its stopping distance. I suggest therefore, a system that measures the distance to the car behind and, if the distance is insufficient, based on current speed, displays the message "Keep your distance!" on a display (e.g. an LED display) in the back window.