I think the 12'7" might be measuring at the outside edge of the tunnel. That happens quite often. You just need to drive down the center. There was certainly no traffic on that beautiful October day. Same goes for the Zion Nat'l Park tunnel. That's why they stop all traffic for you - so you can drive down the center.
LOL - ice storm? We just hunker down in instances like that. A worse ice storm we encountered was going from Beaumont, TX to Livingston, TX. It was pure ice and absolutely no place to pull over on the two-lane roads. We couldn't even pull into a fuel station because their drives were uphill. So we just kept going - very slowly. We pulled into the Escapees RV park and someone handed us a bottle of champagne for succeeding the drive! We could write a book on things that happened to us in 16 years of full-timing - like falling through a septic tank with our motorhome and having to build a road under us so we could drive out! :) Lots of good memories and we enjoyed every minute of it.