IDOT designated it as a state route, Part of it is also a US route that runs though Oswego, So it is a primary route designated by those government organizations. as a Functional Classification even IDOT has it marked as primary south of US30.  Looking at google maps, it looks like a pretty popular route as I see many cars using it when street view went though.  Even routing services follow 34 as a primary route or recommend using 31 to 30 to get though the area. 