La. Roads, Bridges and Ferries § 48:1104
When all bonds issued under the provisions of this Part as amended secured by the tolls or revenues of any one or more bridges or ferries 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, tolls or fares may be charged by the authority for the use of such bridge or ferry only to the extent necessary to provide for the annual expense of the maintenance and operation thereof; provided, however, that if the Department of Highways shall have theretofore or shall thereafter accept such bridge or ferry as part of the state highway system, after the satisfaction of claims of the bondholders as aforesaid, such bridge or ferry shall be maintained by the Department of Highways free of tolls.
La. Roads, Bridges and Ferries § 48:1104