Mich. Comp. Laws § 247.436

Sec. 36. All special assessment rolls and copies of rolls heretofore required to be delivered to county or township clerks under this act shall be prima facie evidence of the regularity and legality of all proceedings had to levy such tax. Any determination herein required to be made by the state highway commissioner or by the board of county road commissioners shall be deemed to be final and conclusive as to the existence of the facts therein involved.