This IS the Technology forum, so I have some ideas - though it would take a concerted effort to pull it off. Not sure who's going to pay for it, but you never know...(I'm thinking interstate here - not cities).
We develop "remote" detectors which are battery/solar powered, and are deployed in a stealth manner along roadways. In your vehicle you have a special receiver which can listen to these remote devices. Basically they are "radar repeaters" so your radar detector can pick them up, and encoded within the signal is a "distance from source". When a LEO parks out in the desert and turns on his radar just once, a remote unit detects it and begins to pass the data down the chain several miles, say up to 10 miles.
Perhaps you pay a yearly subscription to a company who deploys and maintains these devices.
Perhaps this can become an extension of Trapster, instead of waiting for a user to dial it in, the remotes auto-dial this information to them.