This is practical and commercial kits are already available :

A system that lights up two LEDs on the dashboard of a car - one lights up when something gets too close ( less than 1 car length ) in the front and the other when something gets too close at the rear of the car.

(Incidentally why is a dashboard called as such ? Is it because it is the first thing to dash against the front seat occupants in a crash ? )

Raghu