Mich. Comp. Laws § 280.559
Any assessment of any nature or kind determined or made under the provisions of this chapter shall be based upon benefits to the public corporation assessed and upon the extent to which such public corporation contributes to the conditions which make a project necessary: Provided, That assessments against the state shall be based solely upon the drainage or prevention of flooding of state highways, and those against the county shall be based solely upon the drainage or prevention of flooding of its county highways. This rule shall govern the director of agriculture, the commission, the board, drain commissioners and any other person in any action relating to determining public corporations to be assessed and in the making of assessments.
MCL 280.559