605 ILCS 5/4-408
The Department may, upon application by the proper authorities of any local governmental agency, issue a permit to such agency to temporarily close to traffic any portion of a State highway for any public purpose or for any temporary needs of such agency. Such permit shall be issued only upon the explicit agreement of the local governmental agency to assume all liabilities and pay all claims for any damages which shall be occasioned by such closing and such agreement shall be made a part of every such permit. When a State highway is closed by a local governmental agency under the terms of a permit, the agency shall maintain efficient detours satisfactory to the Department around the portion of the closed highway. Except for an unanticipated emergency as determined by the Department, the Department and the local governmental agency shall post notice of the detour locations on the Department's website and the local governmental agency's website no later than 10 days before the detour becomes active. A hyperlink on a local governmental agency's website to posted notices on the Department's website shall satisfy the requirements under this Section. A local governmental agency that does not have a website maintained by a full-time staff or a municipality with 1,000,000 or more inhabitants shall not be required to post detour locations under this Section. Such detour shall be plainly and conspicuously marked with signs by which traffic may be guided around that part of the State highway so closed.
605 ILCS 5/4-408