Illinois State Route 166 is a north-south highway in Southeastern Williamson County. This runs from Illinois State Route 13, near Pittsburg to the Williamson/Johnson County Line, near New Burnside. IL St Rt 166 passes through the following town in Williamson County. Marion
Creal Springs IL St Rt 166 is not multi-plexed in Williamson County. IL St Rt 166 intersects the following highway in Williamson County. Illinois State Route 13, near Pittsburg.
A junction marker on EB IL St Rt 13.
A Williamson County mile marker.
The northern end of IL St Rt 166 at IL St Rt 13, near Pittsburg.