PS: There is a reason you can get an International Driver's License.
But also: a trend in traffic control is to reduce the markings, so that people pay closer attention, using their innate intelligence to drive cooperatively. (The idea goes by the names 'psychological traffic calming' or 'naked roads'.) Speeds go down a little; accidents go down a lot. It seems traditional markings lead to overconfidence -- a destructive abdication of personal responsibility to the authoritative sign-makers.
So even if a patchwork of private roads couldn't agree on any signage standards, and just tore out all the signs (as the safest, liability-reducing option), we might wind up better off. (But of course, freeways and major thoroughfares/intersections would just work the same as they do absolutely everywhere else, under any ownership system.)