Oh awesome, I didn't know Madison Co had that resource! Great news.

Yeah for highway ref= tags, it (generally) needs to be a reference number that's signed and used for navigation. The secondary route numbers in NC aren't used this way (you never tell someone to turn right on SR 1411, you use the actual road name). There are a small handful of exceptions where highway exit signs actually use the SR number but I've only seen a couple out east. SR numbers also change at county borders or intersections, which makes them very unhelpful for navigation (they also can get updated over the years, which is annoying for us).

But they do exist and in many cases are physically marked (mostly with tiny white signs on stop sign poles, but they're starting to print them on street signs). Another alternative is official_ref=, but this is for when there is a government reference number that's not really intended for public use and usually not signed. So unsigned_ref= is perfect: it's for reference numbers that may be consistently posted, but not prominently enough to be useful for navigation. The wiki page has some other examples for comparison https://wiki.openstreetmap.org/wiki/Key:unsigned_ref
