It's the car horn, isn't it?
Because when you honk, the cars in front, behind, and to the side of you are all going, "Who's honking? Are they honking at me? Is there something I should be aware of or did they just see someone walking down the sidewalk they recognized? Should I be moving? Oh crap, did I accidentally lean on the horn?" One honk can confuse a dozen people.
Meanwhile, there is no need to blow the horn at the person you are actually honking at, because s/he already already knows that s/he cut you off, and s/he will follow that up by flipping you off for making such a big frigging deal about it.
I am just saying, during a busy morning commute, what we need less of is LOUD SOUNDS WITH NO CONTEXT.