605 ILCS 5/10-304
Any ordinance authorizing the issuance of bonds under this Division of this Article shall provide for the creation of a sinking fund into which shall be payable from the revenues of such bridge, from month to month, as such revenues are collected, such sums in excess of the cost of maintenance and operation and the sums necessary to maintain any reserve accounts created by such ordinance, as will be sufficient to pay the accruing interest and retire the bonds at or before maturity. The moneys in such sinking fund shall be applied solely in the payment of matured interest on bonds authorized under the provisions of this Division of this Article and for the retirement of such bonds at or prior to maturity in the manner herein provided. All sums in the sinking fund in excess of the amount required for the payment of interest and principal of all outstanding bonds for the current year shall be paid out upon the order of the governing body for the purchase or redemption of bonds issued under the provisions of this Division of this Article, for the account of which such sinking fund has been accumulated, where it is possible to purchase or redeem the same at not more than par and accrued interest. If such bonds cannot be purchased or redeemed, such funds shall be used to pay the principal or interest on bonds issued under the provisions of this Division of this Article as the same become due. Any excess sums in the sinking fund which cannot be applied to the purchase or redemption of bonds may be invested in securities of the United States maturing not more than 6 months after the date such sums can be applied to the retirement of the bonds at maturity.
Upon the issuance of any bonds as in this Division of this Article provided and while any of such bonds are outstanding, all revenues, in excess of the sums required for maintenance and operation of the bridge, and the maintenance of the reserve accounts created, shall be deposited in the sinking fund, and shall be used for the purchase, redemption or payment at maturity of the interest and principal of bonds as provided in this section, that have been issued in accordance with the provisions of this Division of this Article.
605 ILCS 5/10-304