605 ILCS 5/5-405: County highways may be constructed or improved on county lines.

In case two counties desire to secure the construction or improvement of a county highway situated upon or near the boundary line between them, the respective county boards thereof may, by appropriate resolutions, initiate proceedings therefor. To this end such county boards may, by concurring resolutions, fix the portion of the total cost of construction which should be borne by each county.

In all proceedings contemplating the construction or improvement of a county line highway as herein provided, all acts of each county board relative thereto, together with the result of any vote upon the question of levying a tax or issuing bonds as provided herein, shall be communicated by the county clerk of each county to the county clerk of the other county.

In case either county refuses to take the steps necessary to secure the construction or improvement of such county line highway as herein provided, then all prior proceedings relative thereto on the part of the other county shall be regarded as suspended.