605 ILCS 5/4-206
When advisable to serve traffic needs, any State highway route in or through a municipality may be relocated upon other streets in the municipality and the jurisdiction, maintenance and control of the streets upon the abandoned route shall be assumed by the city, town, village, park district or other municipal corporation. If any municipal corporation notifies the Department that it is about to construct a subway for railway or motor vehicle transportation along and under any street upon which any State highway route is located and that its use as a State highway route will interfere with such construction, the Department shall relocate such route or such portion thereof as is necessary to prevent such interference. Except as allowed by Sections 4--407, 4--408 and this Section no State highway route shall be relocated except to better serve traffic demands and the route abandoned must be left in reasonable condition for traffic.
605 ILCS 5/4-206