Wash. Rev. Code § 47.56.170
From the money deposited in each separate construction fund under RCW 47.56.160, the state treasurer shall transfer to the place or places of payment named in the bonds such sums as may be required to pay the interest as it becomes due on all bonds sold and outstanding for the construction of a particular toll bridge or toll bridges during the period of actual construction and during the period of six months immediately thereafter. The state treasurer shall thereafter transfer from each separate toll revenue fund to the place or places of payment named in the bonds such sums as may be required to pay the interest on the bonds and redeem the principal thereof as the interest payments and bond redemption become due for all bonds issued and sold for the construction of the particular toll bridge or toll bridges producing the tolls or revenues so deposited in the toll revenue fund. All funds so transferred for the payment of principal or interest on bonds issued for any particular toll bridge shall be segregated and applied solely for the payment of that principal or interest. The proceedings authorizing the issuance of bonds may provide for setting up a reserve fund or funds out of the tolls and other revenues not needed for the payment of principal and interest, as the same currently matures and for the preservation and continuance of the fund in a manner to be provided therein. The proceedings may also require the immediate application of all surplus moneys in the toll revenue fund to the retirement of the bonds prior to maturity, by call or purchase, in such manner and upon such terms and the payment of such premiums as may be deemed advisable in the judgment of the department.
The moneys remaining in each separate toll revenue fund after providing the amount required for interest and redemption of bonds as provided in this section shall be held and applied as provided in the proceedings authorizing the issuance of the bonds. If the proceedings authorizing the issuance of the bonds do not require surplus revenues to be held or applied in any particular manner, they shall be allocated and used for such other purposes incidental to the construction, operation, and maintenance of the toll bridge or bridges as the department may determine.
RCW 47.56.170