Wis. Stat. § 182.48

When all turnpike revenue bonds issued under the provisions of ss. 182.30 to 182.48 in connection with any turnpike project or extension or sections thereof and the interest thereon shall have been paid or a sufficient amount for the payment of all such bonds and the interest thereon to the maturity thereof shall have been set aside in trust for the benefit of the bondholders, such project or extension or sections thereof, shall become a part of the state trunk highway system and shall thereafter be maintained by the department of transportation and shall be free of tolls.