Wash. Rev. Code § 47.10.806
At least one year prior to the date any interest is due and payable on such bonds or before the maturity date of such bonds, the state finance committee shall estimate, subject to RCW 47.10.805, the percentage of the receipts in money of the motor vehicle fund resulting from collection of excise taxes on motor vehicle and special fuels, for each month of the year which shall be required to meet interest or bond payments when due and shall notify the treasurer of such estimated requirement. The state treasurer shall thereafter from time to time each month as such funds are paid into the motor vehicle fund, transfer such percentage of the monthly receipts from excise taxes on motor vehicle and special fuels of the motor vehicle fund to the highway bond retirement fund heretofore created in the state treasury, which funds shall be available solely for payment of the principal of and interest on the bonds when due. If in any month it shall appear that the estimated percentage of moneys so made is insufficient to meet the requirements for payment of the principal thereof or interest thereon, the treasurer shall notify the state finance committee forthwith, and the committee shall adjust its estimates so that all requirements for the interest on and principal of all bonds issued shall be fully met at all times.
RCW 47.10.806