Simple idea, iOS app, I’d charge $0.99. App has one button “spotted” that you press when you see a cop.
Fuzz Alert app exists and appears to be similar.
Push or audio notifications when you are near an area with a recent spot, or a typical area of spots.
When a notification happens, it asks user spotted or not spotted, which helps keep the data updated for that point, and also whether the reporting user was for real or not. Maybe give points and ability for people to thank others. Maybe reporting spotted after a notification automatically thanks people.