Whenever such bonds are issued by the bridge and park commission, the rates, fees, charges and tolls to be established for the use of any bridge or causeway or for the services or use of any of the facilities owned or operated by the commission, or to be received by it from concessionaires or lessees, and from the lease or sale of any lands under the control of the commission and other sources, shall be such as to produce an aggregate amount sufficient at all times to pay the cost of operation and maintenance of any bridge, causeway or other property or facility of the commission, to pay the principal of and the interest on all bonds issued by the commission as the same shall mature and accrue, and to provide adequate reserve and contingent funds.