605 ILCS 10/18
The sums of money appropriated by the General Assembly for the payment of ordinary and contingent expenses of the Authority or the payment of compensation of the members of the Authority expended as a part of the cost of a toll highway financed by revenue bonds issued and sold by the Authority under this Act shall be repaid to the State Treasury out of the proceeds of the sale of such bonds, for deposit in the fund from which such sums were appropriated. Any such sums remaining unpaid because expended for preliminary investigation of toll highway routes not constructed shall be repaid by the Authority out of the proceeds of the sale of any of such bonds issued to finance additional toll highways or extensions of existing toll highways. If no such new bonds are issued, and the money appropriated by the General Assembly has not otherwise been repaid in full, then the Authority shall, after payment of all existing bonds and interest thereon, continue to collect tolls for the privilege of using the toll highways constructed pursuant to the authority of "An Act in relation to the construction, operation, regulation and maintenance of a system of toll highways and to create The Illinois State Toll Highway Commission, and to define its powers and duties and to repeal an Act therein named", approved July 13, 1953, as amended, until such time as the tolls collected are sufficient to repay any such unpaid money. The tolls so collected shall be paid by the Authority to the State treasury for deposit in the fund from which such sums were appropriated.
605 ILCS 10/18