> There literally are "no u turn" signage where you are supposed not to do that

These signs, you mean? https://en.wikipedia.org/wiki/Prohibitory_traffic_sign#No_U-...

And it's trivial to learn "no u turn" and a few other basic signs as a graphic, even when you can't read them (or can't read in general).

It's also trivial to do a u turn even when you can read, know what the sign says, and you feel like doing one because no car is coming anyway, and millions of people do that everyday too.