Keeping distance between cars according to cars speed

A device that was installed on rear windows off a car to inform the driver in the tailing car that the distance between his car and our car is to small according to his car speed ( the message is written in hebrew because it was done for the people in Israel and it is - " You are too close" ' it can be done easily in any language). 

From the trailing car, very close. On the sign is written "You are too close" in hebrew laguage

Outside the trailing car

The speed is higher so the sign is activated from a greater distance

The speed is lower so the sign is activated from a smaller distance