No offense but hypothetically if we had private ownership they would agree with a standard marking. Its naive to assume otherwise. I don't agree with ayn rand on all things but corporations have standards....
The EU practically is a country. There are plenty of other counter examples, mostly countries that were formally some part of an imperialist empire, or essentially still are (many countries in the Caribbean).
The differences between countries' marking and traffic laws are small compared to the broad commonalities. Close countries tend to harmonize. And a network of interlinked but mildly competitive roads in a single country would have even stronger incentives to harmonize.
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.)
