SDCL § 32-25-5: Minimum speed on interstate highways--Violation--Exceptions.

It is a Class 2 misdemeanor at any time for a person to drive or operate a motor vehicle upon the national system of interstate highways at a speed less than forty miles per hour unless the vehicle is operated pursuant to a permit, issued by the Department of Public Safety. This section does not apply to highway equipment operated by personnel in the performance of highway maintenance duties.