(a) 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) Whenever the secretary of transportation or local authorities within their respective jurisdictions determine on the basis of an engineering and traffic investigation that slow speeds on any highway or part of a highway impede the normal and reasonable movement of traffic, the secretary or any such 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 and that limit shall be effective when posted upon appropriate fixed or variable signs.