(a) No person shall drive a motor vehicle at such a slow speed as to impede, hinder or obstruct the normal and reasonable movement of traffic, except when reduced speed is necessary for safe driving or when negotiating a grade, or in compliance with law.
(b) Whenever the Police Commissioner determines on the basis of engineering and traffic investigation that slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic, the said Commissioner 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.