We have had this in Spain for decades. At the entrance of every village you have two traffic lights: a blinking one, where the measurement is performed, and the actual red-amber-green. The main light is always green until a car crosses the detection line, then it turns red for the exact time that it takes to cover from the measurement area to the traffic light at 50km/h…
Problem is of course enforcement. No one prevents you from just disregarding it unless you pair it with a speed/traffic light trap.