in Newcastle, Wyoming, is mainline US 16 signed as a business route.

It splits from US 16 in the Black Hills of the southwestern part of the U.S. state of South Dakota.

The eastern terminus is at an interchange with US 16 called Keystone Wye south of Rapid City.

It begins at Main Street north of Pond Court, then runs alongside the George S. Mickelson Trail.

[citation needed] U.S. Highway 16A (US 16A) is the former designation of SD 240, serving Badlands National Park.