Zombieland, Jay and Silent Bob Strike Back, and Drive Angry. And if you REALLY want to stretch the definition of a road trip movie then Apocalypse Now. Otherwise just the first three.
Saw Zombieland for the first time about a month ago...AWESOME!!!
It's not my favorite, but as I grew up in Dayton, Ohio I'll give a shout out to The Dream Catcher (1999), as it was filmed by a Dayton native and filmed in large part there, despite it being the quintessential road movie. There's just not a lot of notable films from Dayton.
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I may go back to hating you. It was more fun.