Traffic Cameras Help?

i was just wondering what are those little black or grey boxes on top of traffic lights not all of them have then :s are they sensors or not?

thanksssTraffic Cameras Help?
Pedestrian crossing cameras..Traffic Cameras Help?
They are a type of calculator. They monitor the traffic at the lights, and they decide which traffic to give precedence to, and change the lights accordingly. Normally in use at Rush hour times.Traffic Cameras Help?
Those will likely be Traffic Cameras. They are used by cities to track traffic violators and to respond quickly if there are some accidents.
Without a better description they are either emergency vehicle pre-emption detectors or video detection system cameras to detect the presence of a vehicle that needs a green light. A picture link would help. They are generally not red-light cameras.