Was sticking to US and State routes marked as primary. Major buisness lined roads that traverse the highways and rivers as secondary, other major roads and county routes as tertiary, separated highways as motorway, and partially separated highways / connector highways as trunk.   At least that is what is was originally within the Chicago area until someone started playing around with an unfinished proposal. 
I've toyed a bit upgrading some of the loop area streets as primary for connectivity, part of michigan avenue, ohio street, ida b wells. but at what point do you not just mark all of the major roads as primary? Those roads dont nessesarily mean it is the prefered rout of everyone, or that its even the fastest route though the area.   The US/state routes from my experience are actually better to drive because they have more lanes and less traffic lights along those routes.   It really needs to be discussed a bit more. 
