I thought Chgo had less than 3 million. When I was a kid in a grammar school in Chgo (Howland), it had over 3 million, but since then it lost many to the suburbs.
Since then, I lived in Peoria, Champaign, and Springfield, not necessary in that order. In the country, road signs indicate the section number E & W and N & S. Illinois is in the Northwest Territory, which uses the grid system of townships, ranges, and sections. A section is a mile square. They are numbered from the NE corner of the twp. You will see, for example, once you get out of Urbana or Champaign, signs reading “2S, 3W.” That means that intersection is 2 sections S of the twp line and 3 sections W. (You may substitute “miles” for sections.)
I do not know if that is what those signs refer to, as I never noticed any around Chgo, but it is easy enough to see the section lines in Chgo. They are the mile markers: Madison, Roosevelt, Cermak, etc.
Thank God I now live in Charleston. SC, that is, not Ill.