S.D. Codified Laws § 32-25-7
The Transportation Commission may establish, by rules promulgated pursuant to chapter 1-26, a maximum speed limit of less than that established by §§ 32-25-1.1 and 32-25-4 upon any highway or portion of highway under the jurisdiction of the Department of Transportation, and any portion of highway under the jurisdiction of a state or federal agency if requested by the agency. The speed limit established by the commission is the maximum speed that any person may drive or operate any vehicle or class of vehicle upon that portion of highway. The Department of Transportation shall conspicuously post signs at the beginning and end of a portion of highway to show the maximum speed limit established by the commission on that portion of highway. A violation of any maximum speed limit established by the commission pursuant to this section is a Class 2 misdemeanor.
SDCL 32-25-7