Some work off a database and some can pick up the signals from certain cameras, but not average ones.
I got one for Christmas a few years ago. It claimed to pick up the signals from speed cameras. In that respect it worked; however,
1) if it can detect the camera then the camera can detect you, so by the time it goes off it’s too late, and
2) it also picked up every other god damned transmission in the area. So, every puffin crossing, every sensor-controlled traffic light, every time a pedestrian breaks wind, beep beep beep beep. You get so acclimatised to it that even if it was effective you’d be ignoring it by the time you actually needed it.