Bonds issued under the provisions of RCW
47.60.560 must distinctly state that they are a general obligation of the state of Washington, must pledge the full faith and credit of the state to the payment of the principal thereof and the interest thereon, and must contain an unconditional promise to pay such principal and interest as the same becomes due. The principal of and interest on such bonds must be first payable in the manner provided in RCW
47.60.560 through
47.60.640 from the proceeds of the state excise taxes on motor vehicle and special fuels imposed by chapter
82.38 RCW. Proceeds of such excise taxes are hereby pledged to the payment of any bonds and the interest thereon issued under the provisions of RCW
47.60.560 through
47.60.640 and the legislature hereby agrees to continue to impose the same excise taxes on motor vehicle and special fuels in amounts sufficient to pay, when due, the principal and interest on all bonds issued under the provisions of RCW
47.60.560 through
47.60.640.