Johny, I don`t know if it interrests you or not, but the Great Divide Trail, which more or less follows the spine of the Rocky Mountains on dirt roads is bicycle legal. There are a few other well known offorad routes that are mostly bike legal- Arizona Trail and I think the Colorado Trail, too. And the Adventure Cycling Utah Cliffs route is mostly offroad, I think. The Pacific Crest Trail, folowing the Sierra Nevada from border to border is closed to bicycles.