If what you say about your speed is correct, the camera is probably faulty/incorrectly set.
Fixed speed cameras use radar to determine whether a vehicle has passed at a speed greater than a preset value. If triggered, camera then flashes twice � it is this that is used to determine the vehicle speed. You should notice white markings on the road, just after the camera. Since the time interval between the camera flashes is constant, the vehicle�s speed can be determined by how far it has traveled between flashes (using the markings on the road).
I would suspect that you will hear nothing once those responsible realise that you were traveling at less than 40mph.