Houston's lights all have them in some regions. They could observe you down the block dismantaling another camera. They are affixed atop the traffic lights.
The cameras on top of traffic lights are usually video cameras connected to a computer which does image recognition to detect if a car is waiting in a left-turn lane.
They have the resolution of a standard NTSC TV camera.
Which is to say, not much. I've seen pictures of the output of those cameras and they don't have the resolution to show much beyond that there's cars waiting.
They replace loop detectors which are what the other option for detecting left-turn traffic is. Loop detectors have their own problems, not the least of which is that they always seem to be needing maintenance.
The red-light violation cameras are different. For one thing, they're mounted on the side of the road about 20 feet behind the traffic lights, not on top of them. Maybe that's because they need to show what color the light was when the photo was taken?