Ok, perhaps this has been covered, but I have to go to work now so I haven't looked hard.
But will RDs really help a lot with Photo Radar that has a mat trigger. In other words, it doesn't go off, until you roll over a pressure mat. I guess that you could get an alert from the car ahead of you, but in the city, many would just count it as a 'false'.
I guess the question really is, is the radar instant on when you hit the mat, or just the camera.