You can surely be right about U.S. 550, honestly i don't know. I mainly use Google Maps to compare the game map with the real map, i saw that 550 it's a 4-lanes highway and various images with trucks going into there, so i supposed it was fine, but i did not think about possible restrictions for trucks.
Ps: since you mentioned tunnels and tight corners, you were perhaps referring to the U.S. 550 in Colorado?
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CDOT says the following for US550:
So trucking through there is probably not allowed.MP 90.8 Ouray -NO PERMITTED LOADS THROUGH TUNNEL-
No permitted loads through tunnel. This restriction is permanent.