Any funds required to repay the bonds authorized by RCW 47.10.790 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 for expenditure pursuant to RCW 46.68.130 and shall never constitute a charge against any allocations of such funds to 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 for state highway purposes proves insufficient to meet the requirements for bond retirement or interest on any such bonds.