Unless reduced speed is necessary for the safe operation of the vehicle or otherwise is in compliance with law, a person may not willfully drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic. If, on the basis of an engineering and traffic investigation, the State Highway Administration or a local authority determines that slow speeds on any part of a highway in its jurisdiction impede the normal and reasonable movement of traffic, the State Highway Administration or the local authority may establish a minimum speed limit for that part of the highway. Unless reduced speed is necessary for the safe operation of the vehicle or otherwise is in compliance with law, a person may not drive a vehicle below a minimum speed limit established under this subsection. A minimum speed limit established under this subsection is effective when posted on appropriate signs giving notice of the limit. A person may not drive a Class A (passenger) or Class M (multipurpose) vehicle on a highway if the maximum speed capability of the vehicle does not exceed the posted maximum speed limit for the highway by at least 5 miles per hour.