This is kind of interesting. Found it on ebay and wondered if anyone had any experience with these.
Uniden BCT8 BearTracker Warning System with 800mhz TrunkTracker III
The Uniden BearTracker is a sophisticated, multi-band highway communication system that responds to radio waves emitted from special secondary transmitters on most highway patrol vehicles across the United States. BearTracker is preprogrammed to alert you both audibly and visually to the presence of highway patrol units within a three mile radius-even if they're not using radar.
* Highway Patrol Alert Responds with both a visual and audible warning alert.