Fans of college football teams follow the same ritual; the colleges generally play on Saturdays while Sundays are reserved for the NFL.

I think that many of the drunkest participants "sleep it off" in their vehicles before driving home. You probably noticed a number of huge and luxuriously appointed RVs parked in the tailgating area; their owners probably did not drive off immediately after the game. Same for drivers of innumerable pickup trucks, SUVs and even regular passenger cars.

There are probably a (very) few "designated drivers," and yes, there are plenty of folks driving away from the games who are less than completely sober.

As I'm sure you noticed, there is a great emphasis on transporting, cooking, and eating really huge amounts of food -- this probably serves to counteract, at least slightly, the effects of all that drink.