This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
Bicycles are prohibited on Interstate highways in Illinois. Bicycles are prohibited on all Interstate highways in Illinois under IDOT policy (625 ILCS 5/11-1426.1 authorises the Department to prohibit pedestrians, bicycles, and other slow-moving traffic on any controlled-access highway). Conventional US routes and state highways are open by default; the Illinois Bicycle Rules of the Road manual lists the signed exceptions.
What this means in practice
Stick to surface streets, US routes, state highways, and dedicated paths. If your route in Illinois appears to use an Interstate, plan a detour before you start.