S.D. Codified Laws § 31-3-1
Whenever any road shall have been used, worked, and kept in repair as a public highway continuously for twenty years, the same shall be deemed to have been legally located or dedicated to the public, and shall be and remain a public highway until changed or vacated in some manner provided by law.
Such highway shall be sixty-six feet wide and shall be taken equally from each side of the roadbed center line. Nothing herein contained may prevent the highway authority charged with the construction, reconstruction, or repair of any public highway from purchasing or condemning right-of-way for widening the highway to more than sixty-six feet or from purchasing or condemning more right-of-way on one side of the roadbed center line than on the other, provided they deem it necessary so to do in order to provide a better highway, to avoid destruction of trees or valuable buildings or to avoid unsuitable terrain.
SDCL 31-3-1