What are the black round cameras on top of the traffic lights for?

Some of them just sense if there are any cars waiting to move thru the intersection. Others sense oncoming police cars %26amp; ambulances, %26amp; turn all the lights red, so they can get thru the intersections quicker. So, they are to control traffic flow for just the intersection.What are the black round cameras on top of the traffic lights for?
They are Microwave vehicle detectors to help aid the flow of traffic. They are NOT anything to do with, or in any way help emergency vehicles get through the lights. That is an urban myth. They are used in conjunction with the induction loops in the roads already.