It's not really a practical rule either in most applications- especially out west and in major urban areas. I'm thinking on I-80 if Bloomington and Joliet don't qualify as big cities, than the I-80 control city for I-80 East in Salt Lake City should logically be what...Omaha? Or Denver, but that doesn't give an accurate representation of what I-80 does.
In Chicago, 95% of people don't care that the Stevenson heads to St. Louis, they're more likely to be helped by navigation by knowing it's going to Joliet, or by knowing that the East-West is heading to Naperville, not the Quad Cities. And it's certainly practical to list two cities, one local and one distant, so for I-355 it should say "Joliet-St. Louis." That way everyone wins.
Do as Seattle does and select a few reference cities and consistently use them- it's not a matter of citing every single suburb as the IDOT spokeswoman says. Seattle uses Olympia, Tacoma, Bellevue, Renton, and Everett as local control cities (while using Portland, Vancouver BC, and Spokane as distant ones) and that works perfectly.
I don't know the northern suburbs that well off the top of my head, but I know Aurora, Joliet, and Elgin are all sufficiently large to be control cities. And I don't think the people of say...Batavia are going to be offended that Aurora and Elgin are listed as control cities
That's one rule that should be immediately discarded