not necessarily. you can't just apply a single policy to every road, you have to consider their context in the nearby highway network.

near Oswego, for instance, you have two state routes (IL 31 and IL 25) on opposite sides of the river. when you compare how they connect to other roads and their annual average daily traffic numbers, IL 31 is more important to the region than IL 25. and just west of there, Orchard Road (a county route) has far greater traffic and connectivity than either.

following the model of other states, part of the classification effort is to identify "major population centers", not just town centers in general. not every town in the state would be important enough to warrant including in the motorway/trunk network.