I would have probably done the same thing you did--apologize with a wave and continued driving.

If the guy really went crazy and tried to block my way, I'd probably try and stand my ground. What I would probably have done is, stop the car on the roadside, pop the trunk, open my door, and prepare to get out of the car. I wouldn't get out just yet but I would try to size out the guy and make sure he doesn't have too much of an advantage, either in size, or in terms of weaponry.

If he doesn't look too scary (too big, muscular/athletic, or doesn't have a bladed weapon or gun), I'd try to return his aggression, bringing back to mind how he had been driving too slowly, hindering my progress like he owned the road. (Hopefully, this would help me muster the courage.) Sometimes, I notice people become more aggressive if you show signs of weakness, and it'll sometimes help the situation if you let them see how you can "return fire".

The reason for popping the trunk open is because I usually have something in there that I could use as a weapon, just in case I start losing control of the situation.

But then again, this is just "hypothetical" thinking for me, cause I really haven't been in such a situation yet. I'm not really that much of a fighter but I'm kinda average in build (5'7", 200 lbs.) and I figure I could also create some damage, just as long as the opponent is not a seasoned combatant.