1400km in Europe is significantly more taxing than in the USA. The density means more junctions, traffic, variations in speed etc -- I think it's twice the mental effort of the same distance in North America.
Something like a drive between Denver to Kansas City is utter torture for me because it's nearly void of any mental stimulus, doesn't matter that I can cruise along at a high speed in a straight line the entire time, I'm worried about myself or other drivers around me falling asleep at the wheel midway through the utter nothingness that is Kansas. Give me cities and terrain to navigate so that I'm doing something with my body while just sitting there for hours and hours and hours on end.