S.D. Codified Laws § 32-25-5.2

If the transportation commission within its respective jurisdiction determines on the basis of an engineering and traffic investigation that slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic, the commission may adopt rules setting minimum speed limits below which no person may drive a vehicle except when necessary for safe operation or in compliance with law. A violation of such minimum speed limits is a Class 2 misdemeanor.