Interstate 80 bypasses Chicago, but for some distance concentrates the traffic from three major Interstate highways (94, 90, 80) and picks up some local traffic from the far southern suburbs. Getting to I-80 without going through Chicago suburban expressways is more of the problem. I-39 gets you there far enough west, but connects with I-80 quite a bit to the south.
Since all motor traffic coming from north of NYC, headed to any place from San Francisco north, goes through this bottleneck south of Chicago, traffic does get dense along the southern shore of Lake Michigan. Most of the time it gets through without delay.
You can bypass the Chicago area almost entirely (maybe not, some people think Kankakee is a suburb) by going down I-39 to Illinois 18/17, then 114 out of Momence which becomes Indiana 10, across to US-421 and up to the Interstates again at Michigan City. But some people think Michigan City is a suburb, and speeds on this route, towns you pass through, will add 2-3 hours travel time compared to I-80. Also, US-421 was in pretty rough shape last time I went that way (July) with some of the roadbed still damaged by the flooding in late Spring.
The next tier of Interstate highways that goes east would be through Indianapolis and Columbus (get to I-80 before Quad Cities, take I-74 to Indianapolis, then I-70 east), Then you have to figure out where you want to work your way north, since I-70 is going to Baltimore and D.C. Probably cut north at Harrisburg, or go on in to Philadelphia and up I-95?
Unless you are planning to go through Canada (actually, from Montana doing the whole trip through Canada is not a bad idea), yes, you have to go through Illinois, whether you go through ND, SD and Minnesota and Wisconsin, or Wyoming, Nebraska and Iowa to get there.
Lake Michigan is a big physical obstacle, you either go around the south end or you go around the north end, or you go north around all the Great Lakes.
Tom Test
Itasca Spirit 29B
