Do you suppose cops ever get annoyed that everyone on the road slows down when they're around?
You've seen it happen (or done it yourself) a thousand times: everyone's cruising along comfortably on a major throughfare doing 10-15 miles per hour over the posted speed limit when suddenly they see a police cruiser on the road either in front or behind. Immediately, everyone hits the brakes and slows down to exactly the speed limit, which, let's face it, is probably a lot slower than it ought to be anyway.
I'm wondering if cops ever think to themselves "Oh, c'mon people, I'm on my way home just like you! I'm off duty. This is a big road, and we don't really need to do 40. Can't we pick it up to 50 or so? I've got things I need to do!" The officers know it goes on, because they themselves witness the sudden "oops, there's a cop" slowdown when they're out driving as civilians.
They'd never be able to admit it, which is probably why they don't honk and yell when you actually slow down to the speed limit. If you asked them, I'm sure they'd have to stick to the just-the-speed-limit-ma'am story. But I'll bet at times they secretly hate it.