The following was a flow chart I found posted on Facebook for helping motorists understand when it is okay to honk at cyclists.
It is simple and to the point. DON'T HONK! I don't care if you're a moped or a semi-truck (I've had them both honk at me). I've had the long "OMG, get out of the way" honk and the quick double tap "Excuse me, but could you move over a bit more?" honk. No matter the intention, it only does one thing; scares the heck out of me! It is neither helpful nor safe.