U.S. Highway 16A (US 16A) is a 36.971-mile-long (59.499 km) scenic United States Numbered Highway. It is an alternate route for US 16. It splits from US 16 in the Black Hills of the southwestern part of the U.S. state of South Dakota. The highway's western terminus is an intersection with US 16, US 385, and South Dakota Highway 89 (SD 89) in Custer. The eastern terminus is at an interchange with US 16 called Keystone Wye south of Rapid City. Portions of US 16A are known as the Iron Mountain Road, named after the peak it summits.

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Quick Facts Route information, Length ...

U.S. Highway 16A
US 16A highlighted in red
Route information
Auxiliary route of US 16
Maintained by SDDOT
Length	36.971 mi[1] (59.499 km)
Major junctions
West end	US 16 / US 385 / SD 89 in Custer
East end	US 16 near Keystone
Location
Country	United States
State	South Dakota
Counties	Custer, Pennington
Highway system
United States Numbered Highway System List Special Divided South Dakota State Trunk Highway System Interstate US State
← US 16 → SD 17

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The route passes through Keystone, the Mount Rushmore National Memorial, the Norbeck Wildlife Management Area and Black Elk Wilderness within Black Hills National Forest, and Custer State Park (including State Game Lodge and Legion Lake), before rejoining the parent highway.

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.

This section does not cite any sources. (January 2018)

An older road through the Badlands of South Dakota (currently designated as SD 240) was designated US 16A between 1944 and 1980.