If Texas wants the red, white, and blue shield, they've gotta build them to interstate standards. If they don't want to build interstates, they can stay US routes.
Sure, the crossings are only used once in a while, but that only means the freeway traffic will not be looking for cross-traffic at all. It would be entirely on the cross-traffic to judge how quickly the freeway traffic is approaching, and it's well-known that it's hard to estimate how quickly someone is coming almost directly toward you.