Wow, Mr. S., you are Johnny-on-it!  It was only a few hours ago I exchanged email with Kerry Irons (ACA, USBRS architect) about this and he now has an agenda item (autumn round?) to ask WSDOT about these three "parallel" segments on USBR 10 if they are possible "alt" or "belt routes" that might improve the route for the future.

What happens is "essentially rogue" OSM editors quite incorrectly add these parallel segments to route relations (more than in Washington, but Washington is sorta "hot" with these now), "making it seem" like this is / these are actual segments of the route.  But in truth, neither WSDOT applied for them to be in USBR 10, nor has AASHTO Approved them.

So, before deleting them, I told Kerry a few days ago that I see these added (sometimes with much-more-likely-to-be-Approved parallel CYCLEWAY infrastructure) to USBRs, knowing they are incorrect, but perhaps OSM can "maximally wring out" potential beneficial route data to present to a DOT to present to AASHTO if the DOT agrees these might make "better route" segments.  Kerry agreed that this is yet another way OSM can contribute to the bettering of the USBRS and agreed to present these (OSM volunteer errors to the route) to DOTs and say "mappers think these might make valid, better or parallel routing along this USBR in the future...what do you think, DOT?"

We'll see.  As they are incorrect, I added the note tag, and as I do or don't hear back from Kerry (who will hear from WSDOT), they are very likely to be deleted from the route relation.  Or, on the off-chance they like the mapper-added segments, the DOT will turn it into an application to AASHTO.  But this will take some months.

As the USBRS is "only partially signed" (Kerry estimates ~25% of the System is signed), you MIGHT see USBR 10 signs on the Winthrop-Twisp segments that are REALLY part of USBR 10 (Highway 20), but you won't on the Main Street, Perry Street, Castle Avenue, Twisp-Winthrop East Road, East Twisp-Winthrop Road, Wagner Street segments, as they are incorrectly "conjured" or "imagined" into being into USBR 10 by an OSM mapper.  If those latter segments were surveyed as to their "bicycle suitability" and say, you improved tags to where you might add cycleway=lane if it exists (I'm speculating), that would be fantastic and would add to the map, sure, but I'm not sure it would specifically help the situation beyond where Kerry intends to take it with WSDOT in the autumn.  So, maybe, but not really.  I certainly appreciate your good spirit to help here!

And, you'd be traveling between Winthrop and Twisp (9, 10 miles?) for what I might call "very little value," except I've never been to the Methow Valley, and it could be quite pretty in July.

Nice to hear from you.