Having just returned from Texas, I have to say your problems in that regard are entirely self-inflicted. And frankly, probably by design. You have wide open freeways with ample space in every lane, absolutely inviting people to cruise at high speed. Want to dial it back? Make the lanes narrower. The vast majority of drivers choose their speed based on comfort, not what the sign says. That won't change, so the strategy for how you slow down traffic has to.