Iron Mountain Road
This is the first photo in a series of three showing coming through a one-lane tunnel immediately onto a pigtail bridge. This is the only only one-lane tunnel that passes through two rocks. Immediately on the other side of the tunnel is a pig-tail bridge, which you can see in the next two pictures (seen in the comments below)
Probably the best drive in the Black Hills is Iron Mountain Road which is Route 16A between Keystone and Custer. We rode the portion that extends from Route 244 south to the entrance of the Wildlife Loop Road in Custer State Park. It ranges from gorgeous sweeping vistas to twisty treacherous hairpin turns.
This is also part of the Peter Norbeck Scenic Byway. This particular section is called the Peter Norbeck Memorial Byway and at its highest point is a small memorial to the governor whose vision and perseverance brought us this magnificent road and all its quirky features.
There are three one-lane tunnels on this portion of Iron Mountain Road, all of which afford a spectacular view of Mount Rushmore as you pass through (depending on which direction you are going, you either see it in front of you as you pass through, or look behind you to see it) They were designed and blasted to frame the faces as you pass through, and they even keep paths cleared through the trees so that nothing obstructs the view of it.
Iron Mountain Road is also home to three pigtail bridges. These are bridges that curve around and go under themselves. When Peter Norbeck first envisioned a highway going through the Black Hills, he was committed to having as little impact as possible on the environment. A pigtail bridge is a particular form of spiral bridge, first built for Iron Mountain Road. Its purpose is to allow for the road to negotiate sudden changes in elevation while minimizing the impact on the surrounding environment. Several of these bridges are located at the entrance/exit of one lane tunnels, which makes for a spectacular ride. They were built in the 1930s by the Civilian Conservation Corps and constructed of natural materials such as local lumber. They were expensive...and well worth it.
The third distinctive feature of Iron Mountain Road is its sections of divided roadway - where the two lanes split and you drive on a narrow one-lane section. This, again, was done because of Norbeck's insistence that the road have as little impact as possible on the environment and natural beauty.