Any funds required to repay such bonds, or the interest thereon when due shall be taken from that portion of the motor vehicle fund which results from the imposition of excise taxes on motor vehicle and special fuels and which is distributed to the state under the provisions of *RCW
46.68.090(1)(c) for construction of state highways in urban areas, and shall never constitute a charge against any allocations of any other such funds to the state, counties, cities, and towns unless and until the amount of the motor vehicle fund arising from the excise taxes on motor vehicle and special fuels and available to the state for construction of state highways in urban areas proves insufficient to meet the requirements for bond retirement or interest on any such bonds.