Whenever the percentage of the motor vehicle fund arising from excise taxes on motor vehicle and special fuels payable into the highway bond retirement fund shall prove more than is required for the payment of interest on bonds when due, or current retirement of bonds, any excess may, in the discretion of the state finance committee, be available for the prior redemption of any bonds pursuant to applicable bond covenants or remain available in the fund to reduce requirements upon the fuel excise tax portion of the motor vehicle fund.