Sec. 4. The department shall designate a county highway as the official detour route if:
(1) the executive of the county through which the county highway passes adopts a resolution consenting to the official detour route; and
(2) under rules adopted by the department, the department determines that it is not more expensive to designate a county highway as an official detour route than it is to provide other means for a motorist to reach a destination.