It's a US highway, US 129. If you ever get there, stop in at the store/restaurant at Deal's Gap and there are plenty of pictures that make you wonder why they bother with motorcycles when they should be diverting trucks off this road.
My understanding was that many of the truckers end up on this road by accident - their GPS units route them there and they have no idea what awaits them. Kinda like what happens when some of the EU truckers cross over to England and get routed through a small village where they then cannot proceed forward or even get jammed in between buildings.
There's a big sign on the north end that warns of tight twisty roads and advises trucks to seek an alternate route. No doubt there are some truckers who don't know what they're in for and give it a shot, but I suspect there are some who realize the length of the detour required to go any other way, and decide to shoot the gap. Starting out in Marysville:
-Direct route to Robbinsville, through Deals Gap, is 32 miles.
-To the east, commercial traffic through Smoky Mountains Nat'l Park is prohibited. You have to go around the east end of the park, for a total distance of 160 miles.
-To the west - 411, 360, Cherohala Skyway - is 82 miles.
So through the Gap, treacherous though it be, saves 50 miles and at least an hour.
And no, AFAIK it is not legal for anyone to cross a double-yellow line - even if you're dragging a 53-foot trailer through hairpin turns. I doubt they get cited for it very often though.