The tunnel is off of Hwy 9 going through Zion.
From Wikipedia:
There is a fee to travel through Zion National Park, but the Zion-Mt. Carmel Highway is open to private vehicles year-round. A separate fee is required for vehicles 7'10" wide and/or 11'4" tall or larger. This fee pays for a park employee to stop traffic from the other side of the Zion - Mt. Carmel Tunnel to allow the larger vehicles to pass through.[3] There is a smaller tunnel in Zion National Park that does not require an escort. Commercial vehicles are prohibited from using SR-9[4] and are directed to use SR-20 instead.