605 ILCS 5/5-502
In case the county board deems it expedient to construct or repair a bridge, culvert, drainage structure, drainage facility or grade separation, including approaches thereto, on, across or along any highway, in the county, the county board may order the same constructed or repaired at the entire expense of the county; or the county and any other highway authority may jointly construct or repair any such bridge, culvert, drainage structure, drainage facility or grade separation, including approaches thereto, provided that the Department's participating authority shall be limited to the State highway system.
If it is decided to pay the cost of such construction or repair jointly, the county board and any other highway authority shall enter into a contract as to the proportion of the expense of such construction or repair to be borne by each. Such contracts, except as against the Department, shall be judicially enforceable.
Such improvement shall be made according to plans and specifications prepared by or under the direction of the county superintendent of highways, and the county board may undertake such work either by letting a contract for the same or may authorize the work to be performed directly by the county through and by its officers, agents and employees.
In case a bridge, culvert, drainage structure, drainage facility or grade separation is located on a public road which crosses a county line, transversely or substantially so, and such bridge, culvert, drainage structure, drainage facility or grade separation is so located that the county line passes through any part of such structure, then the adjoining counties may jointly construct or repair such bridge, culvert, drainage structure, drainage facility or grade separation, including approaches thereto.
For the purpose of such joint construction or repair, the adjoining counties shall enter into a contract as to the proportion of the expense of such construction or repair to be borne by each. Such contracts, except as against the Department, shall be judicially enforceable.
605 ILCS 5/5-502