Mich. Comp. Laws § Article VII § 16: Highways, bridges, culverts, airports; road tax limitation.

Sec. 16.

The legislature may provide for the laying out, construction, improvement and maintenance of highways, bridges, culverts and airports by the state and by the counties and townships thereof; and may authorize counties to take charge and control of any highway within their limits for such purposes. The legislature may provide the powers and duties of counties in relation to highways, bridges, culverts and airports; may provide for county road commissioners to be appointed or elected, with powers and duties provided by law. The ad valorem property tax imposed for road purposes by any county shall not exceed in any year one-half of one percent of the assessed valuation for the preceding year.