Speed Cameras: How Do They Know?

I was driving on the highway last night through a work zone. The work zone speed limit is 50 m.p.h., and I was going 40 (the car in front of me was going slowly so I couldn't get up to 50). We passed by a speed camera, and I saw the flash of the light in my rearview mirror as it caught someone speeding (probably the guy next to me who hit the gas at just the wrong moment). However, I'm really curious as to how (and if) it knows whom to cite. The camera seems to take a still picture of a whole range of cars. My (not speeding) car must be in the picture right next to the speeding car, but you can't tell from a still picture. I guess (I hope) it is sophisticated enough to know who is speeding and who is not.

On a related note, I'm finding the speed cameras in this work zone to be very curious. First, I have noticed that speeds are generally down in the work zone, which is a good thing. When they first started with the speed cameras, the light would flash like flashbulbs at a rock concert, basically constantly. Now, they only flash occasionally.

However, the speed cameras are not there all the time (they're mounted in special trucks on the side of the road and can move around). I have noticed that they are almost always there at night and rarely there during the day. The goal is to increase safety, especially of the workers, in work zones. However, in this particular work zone, they are not working at night so it seems like they are there at the wrong time.