I doubt anyone will find USDA prime meat sold as "prime rib" in most diners and typical Roadfood places. For true USDA prime rib try the best steak house in your town and be prepared to pay a prime price.
A good example of this is a local diner I sometimes frequent. They offer a daily prime rib special: Salad, soup, choice of potato, vegetable choices and of course the "prime rib" that is anything but prime. All this for $12. I suspect this "prime rib" is USDA select or at best USDA choice. It has to be otherwise the math would make no sense.
So how is the rib at this place? I would rate it as acceptable for the place and the price. The meat is very tender and it is large. That’s the good news. The bad news is that I suspect it of being adulterated. I believe the tenderness comes from a good dose of tenderizers and the "juices" come from apple juice mixed with meat juices. Another tip off is that it is always medium no matter how you order it.
I agree with many the other posters here that say if you want it right, buy it from a butcher and cook it yourself. It is the only real deal unless you go to a premium steak house and even that is a bit of a gamble.