It is maintained by the Illinois Department of Transportation (IDOT).

[2] In Williamson County, the highway turns west and passes through Creal Springs.

Route 166 turns north after passing through Creal Springs and continues through farmland until it reaches its northern terminus at a traffic light-controlled intersection with Illinois 13, a four-lane divided highway.

Illinois 166 is an undivided two-lane road for its entire length.

[3][4] A road connecting Marion to New Burnside was first marked on Illinois highway maps in 1922.