605 ILCS 5/9-101
Nothing in this Code shall prevent the execution of cooperative agreements among governmental agencies.
Any municipality may negotiate an agreement with the Department whereby the municipality may use such funds as are available to it for that purpose for the construction or maintenance of a State highway within its boundaries or with the corporate authority of a county or road district for the construction or maintenance of a highway on the county highway system or township or district road system outside of its municipal boundaries.
The county board may negotiate an agreement with the Department whereby the county may use such funds as are available to it for that purpose for the construction or maintenance of a highway on the State highway system or with a municipality for the construction or maintenance of streets on the municipal street system of such municipality.
605 ILCS 5/9-101