Illinois State Route 123 was extened back into Sangamon County in October, 2003. This runs along a old alignment of Illinois State Route 124. Then a previously un-numbered highway from the junction with IL St Rt 124 to I-55 at Williamsville. Tourist traffic now can take IL St Rt 123 from I-55 to Lincoln's New Salem Historical Site. IL St Rt 123 passes through the following towns in Sangamon County. Williamsville IL St Rt 123 is multi-plexed in Sangamon County. Illinois State Route 29, near Athens.
IL St Rt 123 intersects the following highways in Sangamon County. Illinois State Route 29, near Athens.
The end and begin markers in Williamsville. The eastern end of IL St Rt 123. WB IL St Rt 123 at its start in Williamsville. A BGS on NB I-55. WB IL St Rt 123 west of Williamsville. A EB guide marker. A junction marker on NB IL St Rt 124. EB IL St Rt 123 west of IL St Rt 124. A EB guide marker at IL St Rt 29. SB IL St Rt 29/EB IL St Rt 123 near the Menard/Sangamon County Line.
