Maryland Code, Transportation § 21-804

(b)(1) If, on the basis of an engineering and traffic investigation, the State Highway Administration or a local authority determines that slow speeds on any part of a highway in its jurisdiction impede the normal and reasonable movement of traffic, the State Highway Administration or the local authority may establish a minimum speed limit for that part of the highway.

(2) Unless reduced speed is necessary for the safe operation of the vehicle or otherwise is in compliance with law, a person may not drive a vehicle below a minimum speed limit established under this subsection.

(3) A minimum speed limit established under this subsection is effective when posted on appropriate signs giving notice of the limit.