This is as good a thread to mention this as any.
My friend has an old car radio which picks up interferance from cop cars. Anytime he is within ~500 feet of any cop cars, his radio makes a "Tck tck tck tck tck tck tck" noise and the audio gets messed up. The closer they get, the more screwed up the audio gets and the louder the pulsing Tck gets.
We've observed it in action several times, and it has actually come in useful before, too. I dont know it would be replicable 100% of the time, but it's an interesting idea... there is obviously some radiation being given off specifically common to cop cars which could be picked up. Perhaps a more refined system could be built?
Just something to consider.