Illinois State Route 15 runs east-west across St. Clair County. At Belleville, this highway becomes a multi-lane divided highway. Most of it is a expressway, with the exception of a few driveways and a stoplight near Belleville. IL St Rt 15 passes through the following towns in St. Clair County. St. Libory
East St. Louis
IL St Rt 15 is multi-plexed in St. Clair County. Illinois State Route 4, from near St. Libory to Fayetteville.