S.D. Codified Laws § 32-25-18
The transportation commission upon request from any local authorities shall, or upon its own initiative may, conduct an investigation of any public bridge, causeway, or viaduct, and if it finds that such structure cannot with safety to itself withstand vehicles traveling at the speed otherwise permissible under this chapter, the commission shall determine and declare the maximum speed of vehicles which such structure can withstand and shall cause or permit suitable signs stating such maximum speed to be erected and maintained before each end of such structure. Violating such posted speed limits is a Class 2 misdemeanor.
The findings and determination of the commission shall be conclusive evidence of the maximum speed which can with safety to any such structure be maintained thereon.
SDCL 32-25-18