For the purpose of refunding present and future bonded indebtedness issued pursuant to the Nebraska Highway Bond Act, the commission may issue, without further legislative authorization, refunding bonds with which to call and redeem all or any part of such outstanding bonds at or before the maturity or the redemption date thereof, may include various series and issues of the outstanding bonds in a single issue of refunding bonds, and may issue refunding bonds to pay any redemption premium and interest to accrue and become payable on the outstanding bonds being refunded. The refunding bonds may be issued and delivered at any time prior to the date of maturity or the redemption date of the bonds to be refunded that the commission determines to be in the best interests of the state. The refunding bonds shall, except as specifically provided in this section, be issued in accordance with such act. The proceeds derived from the sale of refunding bonds issued pursuant to this section may be invested in obligations of or guaranteed by the United States Government pending the time the proceeds are required for the purposes for which refunding bonds are issued, and to further secure the refunding bonds, the commission may enter into a contract with any bank or trust company, within or without the state, with respect to the safekeeping and application of the proceeds of the refunding bonds and the safekeeping and application of the earnings on the investment. Such contract shall become a part of the contract with the holders of the refunding bonds. Bonds refunded by such refunding bonds, which have been called for redemption or with respect to which the bond trustee or paying agent has irrevocable instructions to call such bonds for redemption on a date certain in accordance with the terms thereof, and which have sufficient funds or obligations of, or guaranteed by, the United States Government set aside in safekeeping to be applied for the complete payment of such bonds, interest thereon, and redemption premium, if any on the redemption date, shall not be considered as outstanding and unpaid bonds with respect to the limitations set forth in section 39-2205.