Mich. Comp. Laws § 254.330

Sec. 20. The authority may, if it sees fit, at any time, when for any reason the bridge may be out of service, acquire through purchase, lease or otherwise and operate 1 or more ferries for the purpose of conveying passengers and freight between the upper and lower peninsulas, in which event the cost thereof may be paid from the revenues of the authority, including, unless prohibited by the terms of the proceedings authorizing the bonds, the proceeds of the sale of bonds hereunder, and all revenues derived from the operation of such ferries shall be regarded and treated as revenues of the bridge for all purposes of this act.