§ 1620. Speed limits on state highways, and on Indian reservations. (a) The department of transportation with respect to state highways maintained by the state outside of cities having a population in excess of one million, and highways on Indian reservations, may by order, rule or regulation establish higher or lower maximum speed limits at which vehicles may proceed on or along such highways than the fifty-five miles per hour statutory maximum speed limit. No such maximum speed limit shall be established at less than twenty-five miles per hour, except that school speed limits may be established at not less than fifteen miles per hour, for a distance not to exceed one thousand three hundred twenty feet, on a highway passing a school building, entrance or exit of a school abutting on the highway. Absence of signs installed pursuant to this section shall be presumptive evidence that the department of transportation has not established a higher maximum speed limit than the fifty-five miles per hour statutory limit.

(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.