Wow, lot of hate going on. Glad I never started commuting in the US. Threats of guns, racial slurs, spitting. I guess I'll stay over here in Japan and not perpetuate the stereotype of the "ugly American."
Never had anybody give me the finger and cut me off, then again I never responded with spitting. Kind of a chicken and the egg problem, but maybe if everybody relaxed this kind of thing wouldn't happen.

You behaved badly, you know it, you said it. Chalk it up to losing your temper and move on. Try not to do it again.