In the old days, Route 66 was the way to get from Chicago to Los Angeles. The route goes through St. Louis as you can hear in the song Route 66. Today Interstate 55 has replaced Route 66 as the direct route from Chicago to St. Louis but the old Route 66 still remains, although now it is often no more than a collection of signposts rather than an obvious road.
Initially I am working from a route developed by the League of Illinois Bicyclists which you can see here.