I am not a big fan of the Eastern Express - - too many miles of busy roads.
(I'm guessing from the place names and US 40 that you are doing that route.)
Once you get to Missouri you will be on the Katy Trail and traffic-free,
but parts of the Eastern Express in Kansas, US 36, I cannot recommend.
Illinois has a state DOT website with traffic counts. (AADT - Average Annual Daily Traffic)
Less than 500 - Super, 500-999 - Nice, 1000-1999 - O.K., 2000-2999 - Middling, 3000-4999 - Iffy, 5000+ - Tough.
https://www.gettingaroundillinois.com/Traffic%20Counts/index.htmlLooks like you were in the 4500-5000 range from the state line to Marshall.
US 40 is in the 2000-3000 range West of Marshall to Montrose,
but pops back up towards 5000 the closer you get to Effingham.
Of course, US 40 is diagonal - and diagonal roads are rare in the Midwest.
From Illinois to the Rockies, if the road is diagonal, it is likely to have a lot of traffic.
But then, zig-zagging at right angles does add miles.
Also, you need to be careful because county roads may or may not be paved.
And when wet, Illinois farm roads become quicksand.
If you use the Illinois DOT traffic counts and Google Street View,
you should be able to map out alternate routes.
Roads that have almost no traffic.
With Street View, if they are not highlighted, they are probably not paved.
Good luck and happy trails - - Jama