The commission is hereby authorized to provide by resolution for the issuance of tunnel revenue refunding bonds of the county for the purpose of refunding any tunnel revenue bonds issued under the provisions of this chapter and then outstanding. The issuance of such tunnel revenue refunding bonds, the maturities and other details thereof, the rights of the holders thereof, and the duties of the county and of the commission in respect to the same, shall be governed by the foregoing provisions of this chapter in so far as the same may be applicable, and by the following provisions:

a. No tunnel revenue refunding bonds shall be delivered, unless delivered in exchange for tunnel revenue bonds to be refunded thereby, except in the amount necessary to provide for the payment of matured or redeemable tunnel revenue bonds or tunnel revenue bonds maturing or redeemable within three months, including any redemption premium thereon.

b. No tunnel revenue refunding bonds shall be issued, unless issued to refund tunnel revenue bonds which have matured or will mature within three months, or unless the interest rate of the tunnel revenue refunding bonds shall be at least one-fourth of one per cent less than the interest rate borne by the tunnel revenue bonds to be refunded.