I. No person shall drive a 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. Under this provision a minimum speed limit of 45 miles per hour shall be posted and prevail on the interstate highway system in the state.
II. Whenever the commissioner of transportation determines on the basis of engineering and traffic investigation that slow speeds on any part of a way consistently impede the normal and reasonable movement of traffic, said commissioner may determine and declare a minimum prima facie speed limit.