I used to drive OTR and it's defenitly worse along the east coast, especially in the mountains up above the mason-dixon line by far.
Don't get me wrong, the south gets hot and humid but up in W.V., Pa., Va., and north it's so much worse, in the south it has to get up above 80 degrees to start really feeling it, up north with the humidity it could be 60 degrees out and you're sweating your ass off at 40% humidity levels.
Out in the midwest it gets really hot but temp wise but the humidity isn't felt as much as just the high temps.