The thing is, not all speed cameras use doppler radar some use laser or clever software to detect changes in the output of a ccd camera in order to track motion. These produce no detectable signals and warning is provided by a closed loop system , that is your car has to tell a central database where it is via a chargeable cell-phone link, and the database then tells you if you are approaching a camera. Maps showing the location of traps can be found on the web , but a gps module would also be required along with some serious software to get it to work.
Better and cheaper option is to obey the speed limit for a given area