To secure the payment of the certificates of indebtedness authorized in R.S. 48:912, the municipality or parish, or the municipality and parish acting conjointly, may fix the rate of tolls to be charged persons passing over the bridge, and pledge the tolls for the payment of the certificates.  The rate of toll, when so fixed and pledged, shall not be changed except with the consent of the holder of the certificates.  

The funds arising from the collection of the tolls shall not be used for any other purpose than maintenance of the bridge and payment of the cost of its construction.  When the cost of construction of the bridge has been paid, tolls shall no longer be charged except for the purpose of maintenance.