Mich. Comp. Laws § 207.1161

Sec. 161. In January of each year, there is appropriated from the proceeds of the tax levied by this act up to $3,500,000.00, that shall be used to pay the principal, interest, and incidental costs for the outstanding bonds, previously issued by the Mackinac bridge authority. The unexpended amount shall lapse to the Michigan transportation fund at the end of each fiscal year. Upon retirement of all outstanding bonds and any refunding bonds hereafter issued, this appropriation shall cease.