Illinois State Route 40 is a north-south highway in Southeastern and South-Central Carroll County. It runs from the Whiteside/Carroll County Line, near Coleta, to Illinois State Route 78, at Mt. Carroll. IL St Rt 40 passes through the following towns in Carroll County. Milledgeville Chadwick Mt. Carroll IL St Rt 40 is not multi-plexed in Carroll County. IL St Rt 40 intersects the following highway in Carroll County. Illinois State Route 78, at Mt. Carroll.