South Dakota Highway 40 (SD 40) is a state highway in southwestern part of the US state of South Dakota. The highway is just over 37 miles (60 km) long and runs from U.S. Route 16A (US 16A) in Keystone to Bureau of Indian Affairs Highway 41 (BIA 41) in Red Shirt. The highway runs near Mount Rushmore National Memorial in Keystone, and its eastern terminus is on the Pine Ridge Indian Reservation near the edge of Badlands National Park.

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.