(a) Impeding traffic by slow speed prohibited. -No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law.

(b) Establishing minimum speed zones; signs. Whenever the Department of Transportation or local authorities within their respective jurisdictions determine 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 Department of Transportation or local authority may determine and declare a minimum speed limit below which no person shall drive a vehicle except when necessary for safe operation or in compliance with law, when appropriate signs giving notice thereof are erected along the part of the highway for which a minimum speed limit is established. Also any minimum speed limit adopted by a municipality for a section of the state highway within the municipality shall not be effective until such minimum speed has been approved by the Department of Transportation.