65 ILCS 5/11-85-1
Municipalities lying within one or more townships, or whose boundaries are coextensive with the boundaries of a township, have the power to enter into a contract with any township within which the municipality lies, or with any township lying immediately contiguous to the municipality, and such township has the power to enter into a contract with such municipality, for the construction of any public improvement consisting of the construction of any bridge, subway, elevated way, or viaduct which may lie partly within the municipality, and partly outside the municipality and within the township, or consisting of the improvement of the roadway of any highway or street upon and along which runs the line of the corporate limits of the municipality, so that the improvement as proposed would lie partly within the municipality and partly within the township, in the manner authorized in this Division 85.
65 ILCS 5/11-85-1