These SBI numbers remain on IDOT district maps to this day and are used along with other designations for bid requests and other official documents.

During 1930s as the US Highway System matured, redundant state numbers were often removed from US Routes.

After the original Interstate Highways were substantially completed in the early 1970s many state (and US) routes, especially in the Chicago metro area, were removed or shortened as unnecessary.

[2] A, B and C suffixes were used for spurs of a nearby route, the N and S were legs of IL 113 on either side of the Kankakee River.

[4] Illinois has also used special routes, such as "Business", "Alternate", "City", "Bypass" and "Truck" on state-numbered highways.

These signs are not intended for navigation as route numbers are but rather as guides for IDOT crews and contractors and to identify specific intersections, culverts or other features.