Starting with CR-A1, Russel Road which is the farthest east-west road in Lake County, there appears to be a pattern among county roads that is shared by McHenry and Cook County. Lake County's East-West road numbers get gradually larger the farther south you go, until A-47, which is Deerfield Rd. Lake-Cook Rd which is signed in spots as Cook County A-50 (Cook County and IDOT maintain different parts of the road). Cook County doesn't sign most of their roads, but farther south, Bartlett Rd is unsigned B-19. In Mchenry County, V-45 is shared by Lake and McHenry and encompasses River Rd and Roberts Rd.. It is a north-south Rd The V's get larger numbers from west to east in Lake until the W's come, the first of which is W-11, Butterfield Rd. Cook County also has W-X Routes. McHenry has a couple T routes before the V's. There is an obvious pattern to this numbering, but one that as far as I can tell, only Cook, McHenry, and Lake follow. DuPage has their own (fucked up) numbering system, Will has their own unsigned system, and so does Kendall and Kane. Was there ever intended to be a more strict county route numbering system in Illinois, or did these three counties decide to copy each other one day?
The letters with numbers with the county highways was supposed to be state wide grid. Some counties did it while others have not. Du Page and Winnebago don't use that system.
Then you get counties like Grundy that use the grid, but do not sign in the field. The highways are only numbered on the county maps.
Then there is Cook county, who randomly put up signs at some intersections and don't bother at others on the same roadway
There was an article in the Tribune around the time that Cook County started to post the shields on their roads about the cost. This was at one of the many times they county was saying they are broke. So when the article ran, they stopped posting the shields, thus about half are done.