If you're wanting to go E-W across the central part of the state, take a look at the old Route 36 pavement. We have driven on that road from Decatur, IL over to Indianapolis, and there is very little traffic. It also goes west of tehre through Springfield, Jacksonville and beyond. The traffic picks up around the small towns, but not by much. There is also some great scenery (and hefty hills) at the Illinois/Indiana border.

There are some bike maps at the www.illinois.gov site under Agencies, IL Department of Transportation. Bicycling. You can order printed copies free of charge or check out the maps online.