People that are afraid to pass police cars is another one that gets me. If a cop is doing 30MPH in a 45MPH zone, and you're holding up traffic by pacing the cop, you're an idiot. He won't pull you over for doing the speed limit.
If the cop sees you pacing with him at a lower speed than posted he's probably thinking to himself "This idiot has expired tabs"
If you see a license plate the the "handicap" symbol on it. More often than not, the driver is mentally handicapped.