Illinois State Route 203 runs briefly in Northwestern St. Clair County. It runs from I-55/70/U.S. Rt. 40 to the St. Clair/Madison County Line. IL St Rt 203 intersects the following highways in St. Clair County. Madison
IL St Rt 203 is not multi-plexed in St. Clair County. IL St Rt 203 intersects the following highways in St. Clair County. Interstate 55, at Madison. Interstate 70, at Madison.