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.

Whenever the Commissioner of Highways or local authorities within their respective jurisdictions determine on the basis of a traffic engineering and traffic investigation that slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic, the Commissioner or such local authority may determine and declare a minimum speed limit to be set forth on signs posted on such highway below which no person shall drive a vehicle except when necessary for safe operation or in compliance with law.