Illinois Route 203 follows an old alignment of Illinois Route 3, and it parallels the current Illinois Route 3 to the east in the eastern suburbs of St. Louis. Illinois Route 203 runs between Interstate 270 and Interstate 55/70 in Madison County and St. Clair County.
The northern end of Illinois 203 is at Interstate 270 where it begins heading south...
Illinois 203 turns westerly at the western end of Illinois 162...