Also they dont use radar, they use the same guns as police... they have to land a laser beam on your car first to get a reading...
theres also a thing called the "cos" factor, It derives from pythagoris theroem... one of the guys who works in them told me that for every 1m they are beside you... (1m off the road) it makes for a 20m difference to the car... NOT 20km/hr, but 20m, the camera then has to then do the math with that difference so you end up with a lower speed overall if the camera is further to the side.
(ie if its sitting off to the right of a 2 lane road you could probably go through in the left lane doing 20% over... they give you 10% and depending where it is sitting the "cos" factor will knock you speed down some more...
also the fixxed cameras using RADAR, you can tell they use radar because there are no loops cut into the road beside it.
They CANNOT take a picture if there is 2 cars in the beam, I have done a little work on these and we were testing the one on hume highway (southbound), everytime 2 cars went across its radar beam it read "ILL" instead of their speed...