Here in NC, the driver's handbook still says you're supposed to give a small honk to alert a driver if you're going to pass. The horn is required in cars as a form of communication, not a release for frustration. But then, its pretty easy to tell the difference between a short "here I am" honk and the angry "Get off the road!" honk.

Be careful with obscene gestures in responses to honking - it could be co-workers (or boss) just trying to say "hi"!