Except as otherwise provided in RCW 47.56.850, the commission is hereby empowered to fix the rates of toll and other charges for all toll bridges built under the terms of this chapter. Toll charges so fixed may be changed from time to time as conditions warrant. The commission, in establishing toll charges, shall give due consideration to the cost of operating and maintaining such toll bridge or toll bridges including the cost of insurance, and to the amount required annually to meet the redemption of bonds and interest payments on them. The tolls and charges shall be at all times fixed at rates to yield annual revenue equal to annual operating and maintenance expenses including insurance costs and all redemption payments and interest charges of the bonds issued for any particular toll bridge or toll bridges as the bonds become due. The bond redemption and interest payments constitute a first direct charge and lien on all such tolls and other revenues and interest thereon. Sinking funds created therefrom received from the use and operation of the toll bridge or toll bridges, and such tolls and revenues together with the interest earned thereon shall constitute a trust fund for the security and payment of such bonds and shall not be used or pledged for any other purpose as long as any of these bonds are outstanding and unpaid.