i'm not trying to flame you, or insult you, i want to know why you would feel justified in assaulting someone who yells at you.
In your question to Publius's post, it depends on case law of the state. Although I would think it improper or imoral to do this, you have a right to protect your space with force if necessary. When I was a constuction worker, we were instructed by someone who was instructed by a lawyer (second hand info IOW) that if you got in someones face and yelled at them, then you had a legal right to protect yourself. So, they told us that if we had to yell at someone, step back 3 ft and do it.
Some states may (or may not?) say that you may not defend yourself by punching them until they touch you. And when I say touch, it could be a simple tap of the finger on your chest. But, having said that, I would not do what Publius said and I would take the abuse or yell back in kind, instead of punching him. Especially if it was possible I was wrong and may have violated the range rules, I woud have not answered back and played by his rules just to avoid further conflict. But that is just me and RCPractitioner may have had another reason to answer back to this rude dude.