Uploaded July 2026 | Updated September 2026, 2 weeks ago

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US Highway 16A is a 37-mile scenic highway in the Black Hills region of South Dakota. The highway's western terminus is an intersection with US 16, US 385, and SD 89 in Custer. The eastern terminus is at an interchange with US 16 called the Keystone Wye south of Rapid City. Portions of US 16A are known as the Iron Mountain Road, named after the peak it summits.

US 16A is famous for its scenic, one-lane tunnels aligned to frame the faces on Mount Rushmore, its "pigtail bridges", and its sections of divided highway but with single (and narrow) lanes on each roadway. It is the only route which can be used to drive through Custer State Park without having to pay an entrance fee for the park, provided the traveler does not stop in the park.

The route includes most of the tunnels on the South Dakota state highway system, including the only three-lane tunnel in the state, just north of Keystone. Part of the highway is also a boundary of the Black Elk Wilderness. The Iron Mountain portion of the road is not maintained in the winter. The road, like several other scenic roads in the Black Hills, was originally laid out by Governor Peter Norbeck, specifically to create a very scenic, slow-speed road for tourists. The section of US 16A from SD 89 to SD 244 is known as the Peter Norbeck Memorial Byway in honor of the governor. At the highest point of the byway, on the summit of Iron Mountain, there is a small memorial to Governor Norbeck.