Green-wave speed advisory signal

The device displays three possible speeds: 50, 40 and 30. It is a green-wave speed advisory signal, used on roads with coordinated traffic lights. It advises drivers of the speed likely to allow them to reach the next traffic light while it is green. Only one speed is illuminated at a time; the other indications remain off. The displayed speed is advisory rather than compulsory. It is neither a maximum speed limit nor a required following distance.