Between Marquette Park and Alton, approximately 20 miles, about 1/2 to 2/3 of the Sam V. trail is a paved shoulder on a divided 4 lane road. It's a bike trail, but traffic is whizzing past you in the adjacent lane. You can keep going past Alton towards St. Louis and you're pretty much on all bike paths; no shared roads.
It's the only bike path in this area on the Illinois side of the river so I am happy to have it.