Not a police scanner, but one that taps into your GPS that searches for known speed traps. Lookup "trap detector." That should get you something useful.
Trapster is a program to help find speed traps. It does rely on other people to keep it up to date. Usually it works best on the highway. I don't use it much, here in NH the backroads are too sparse to rely on it.
I picked this up last night. A little buggy, but it works well. I wish it had a favorites feature to save the streams you listen to often. Hopefully there will be a new update soon.