Ever notice that when a police car is driving on the interstate nobody wants to pass it? I noticed that the other day on the way to work. Traffic was going slow so I used my superior driving skills to manuever to the front of the slow traffic like Mario Andretti, whereby I noticed a police car doing the speed limit, and nobody passing.
It occured to me that since police cars are considered fleet vehicles, just like taxi cabs, that they should have the "on-duty" "off-duty" signs installed. Then the other drivers could easily see whether the policeman was working, or simply driving his squad car while off duty to get donuts, and maybe it wouldn't cause so many traffic jams on the interstate.