In all likelyhood, the road still exists out in the desert. The major cause of road damage is rain and grass; rain will get into microscopic cracks, freeze, causing the cracks to expand, and repeat the process over several years. Grass (or any plant) can then grow into a large enough crack, causing the crack to expand and... well...
At any rate, a desert has neither of these in any appreciable amounts. Therefore, barring the occasinal vandal or earthquake, the road should be relatively pristine. That doesn't mean it wasn't buried by sand over the years...
All good points. Would emp's from the nukes, carried along the telephone wires, have caused many of the vehicles on the road to stop? One more question, for the moment, would Enclave be interested in keeping the road relatively secret, as a means to better control each coast? The antagonist of my story is actually a female anologue of The Master (pheromonal attraction ensnares the mutant), not Enclave, but they should factor in somewhere. :shrug:
Imagine, crawling across the desert road for days, but finally you've come to an abandoned gas station. Once you stand, chaffed knees buckling, reaching for a crutch, you tilt your neck to http://www.legendsofamerica.com/photos-texas/Road%20Does%20Not%20End-500.jpg peeling from the collapsed sign you're leaning to.