(b) The department of transportation, whenever it determines on the basis of an engineering and traffic investigation that slow speeds on any part of a controlled-access state highway maintained by the state outside of cities having a population in excess of one million consistently impede the normal and reasonable flow of traffic, may establish minimum speed limits below which vehicles may not proceed on or along such highway,
(c) The department of transportation may determine the maximum speed which may be maintained without structural damage to bridges and elevated structures that are a part of any state highway maintained by the state, and, if such maximum speed is lower than the maximum speed limit otherwise applicable, may by order, rule or regulation establish such lower maximum speed limit at which vehicles may proceed on any such bridge or structure.