605 ILCS 5/5-504
The Department or the Illinois State Toll Highway Authority and any county, municipality, or road district, or any one or more of them, may jointly construct, repair and maintain a bridge, culvert, drainage structure, drainage facility, grade separation or interchange and approaches thereto.
For the purpose of such joint construction, repair and maintenance, the Department or the Illinois State Toll Highway Authority and the county board, city council, board of trustees or highway commissioner in road districts (as the case may be) may enter into a contract as to the proportion of the expense of construction, repair and maintenance to be borne by each. Such contracts, except as against the Department, shall be enforceable at law or equity.
Contracts for such construction, repair or maintenance work shall be let by the Department or the Illinois State Toll Highway Authority and such municipal corporations as have so agreed and shall expressly fix the extent of liability for each of such contracting parties.
605 ILCS 5/5-504