There is hereby created and set up the “State Road Bond and Interest Sinking Fund” into which shall be paid the balance remaining in the state highway department fund as provided in section 226.200. From this fund shall be paid the interest and principal upon all outstanding state road bonds as they become due. Any balance in excess of the amount necessary to meet the payment of the principal and interest of any state road bonds for the next succeeding twelve months shall be transferred to the state road fund.