Even better: your phone will check for cops on your way, using GPS. When you locate a cop, you can report him in real time, and someone driving 200m behind you, using the same site, will get an alert.
The phone network can locate your phone - so your phone can also know its location even without GPS. If your phone knows your location, and is connected to the web, it can work.
If you can tell me how to do this using only a website (no phone specific programming) it would dovetail nicely into a startup idea I'm considering.
I figured it would require getting users to install my software on their phone somehow, dealing with phone companies, and learning something called "Symbian".