While I was surfing the net last time I saw this news and it has something that we need to know. I guess, I will share it with you. Here it is:
Is the sensor on top of the traffic light a camera?
Generally, the writer believes they are passive infra red (PIR) devices to detect the presence of a vehicle stopping at the red traffic light. This is to notify the traffic light computer that a car is present and the lights should be changed. The advantage of the PIR is that its a lot cheaper and simpler than burying weight sensors in the road surface.
Many states have instituted a system to aid emergency response vehicles. This system consists of a sensor (which you have noted), the control circuitry in the light control housing, and a programmed strobe on an emergency vehicle.
Watch next time an ambulance drives past you, there is a strobe usually on top of the cab which is flashing a pattern. When this sensor picks this up, the light goes red in all directions to allow the ambulance to maneuver through without getting hit.
The sensors that others are referring to which tell the traffic lights who is where are not pir, they are loops of wire buries below each lane where the cars come to stop at a red light. They work by induction and are responsible for red light cameras, stop light function, the whole shebang.