Illinois was an early leader in providing a numbered highway system. First authorized in 1918, only Wisconsin preceded Illinois, and only by a few months. Michigan followed soon after that.

The original Illinois state highways were simply numbered in order of origination and consisted of little more than numbers placed on poles of existing trails. Before that, important routes had names, such as the "National Home Trails" or the "National Road." 

It was in 1926 that a nationwide system was established. In the first 20-25 years of the 20th Century, private auto and bicycle clubs and civic organizations marked several "Auto Routes" or "Trails." These Trails could have been more efficient because the roads were routed to and through the towns or businesses that paid to be included on the route.

In 1918 the state legislature let a series of bonds that paid for individual highways between city pairs. While some roads already existed partly, this was the first time an organized road system was built in Illinois. The 1918 "State Bond Issue Routes (SBI)(SBI 1 thru 46) were laid out in somewhat of a regional pattern. SBI 1, 2, and 3 were N-S roads that ran the length of the state, 4 was a diagonal road from Chicago to St. Louis, and Routes 5 thru 20 were E-W roads throughout the state. The rest (21-46) mainly were connector or regional roads of some importance.

Later, in 1924, another set of bonds was issued, from 47 thru 185. These were used for fill-in routes, connectors, and roads to satisfy local politicians to ensure that every community was well served. As the bonds were let and purchased, the roads were built, and the State Bond Issue number was used to designate the road. The first Illinois Route Marker was a simple state outline with a 1 or 2-digit number inside. There was also a batch of lettered routes, mostly "A" but a couple "B's" and "C's" and one set of "N" and "S" roads. These letters were added to the marker below the number. These letter-appended routes were spurs from the main road; the N-S roads were on opposite sides of a local river.
Soon it became necessary to reroute roads, change numbers and add new ones. While the SBI numbers remained the backbone of the route numbering system, there were enough changes to split it into two systems. The SBI numbers were retained as an inventory of roads initially financed from them. While keeping many SBI numbers, the State highway numbering system evolved, grew, and eventually retracted.

Neighboring states also had their numbering systems. While Wisconsin preceded Illinois by a few months, other Midwest states followed suit and established procedures. Michigan followed Illinois in short order, and Indiana was just slightly behind.

It took the Feds a few years to get into the act. By 1926 many states had their own systems, and occasionally state numbers crossed over the border and were retained by the next state. Most of the time, however, a route that crossed state lines usually changed numbers as it did. The state highway departments and the US Department of Transportation (then called the Federal Highway Administration) cooperated on a standard numbering system that we now know as US Highways. It was referred to as Interstate highways, not to be confused with the modern-day Interstate Highway System. The US Route System is not a federally built, financed, or controlled system; it is merely a standard numbering system for specific state highways.

US Routes appeared on Illinois roads in 1927 after being approved by the state in 1926. They first appeared on official Illinois maps in 1928. 

Since Illinois and other Midwest states already had an excellent (for the times) paved road system, most US route numbers were just applied to existing State highways as additions to the existing state number. Some of the numbers were changed before posting, most notably US Route-66. Initially set to be US-60, it was altered before publication but only after many maps were printed.
In Illinois, several of the new US Routes were posted. US-66 replaced, for the most part, IL-4 from Chicago to St. Louis, US-40 replaced IL-11, US-30 replaced IL-6, and so on. Mainline US Routes in Illinois were US-6, 12, 14, 20, 24, 30, 32 (replaced mainly through 34), 36, 40, 41, 45, 50, 51, 52, 54, 60, 62 (60 and 62 were and are only in IL for a matter of blocks), 66 and 67. There were several 3-digit US routes, most of which did not survive the 1930s. Illinois had more 2-digit US routes than any other state, with 21. While US-66 is gone and a couple has been contracted (most notably US-54), the rest remain where they were.

Over the next decade, the redundant state numbers were mainly removed from the newly minted US highways. Other roads were renumbered by extending a different route number to replace a shorter route. It was common for state numbers to be reused after they were pulled. Later some state highway numbers were changed to avoid conflicts with newer US and Interstate numbers. For example, in 1941, IL-20 was changed to IL-120 since US-20 was built a few miles to the south. The original IL-120 in the Mason City area was changed to IL-119 in 1937 as that road was extended. In turn, that became US-136 in 1956.
On Harlem Avenue near Irving Park Road in Chicago, there was a Route 42A sign with the route number within an outline of the state. These signs were around in the 1950s and perhaps into the early 1970s. The Rt 42A signs were eventually replaced by square Illinois Route 43 signs.

In the 1950s, the Interstates were being planned. Now known as the Eisenhower Interstate System, Illinois was a nexus for these new super-highways. While several controlled access roads around the state already existed, including parts of the Illinois Tollway system, most of the Interstates were newly built. Mostly, they replaced US highways, I-55 replaced US-66, I-94 replaced US-41, I-70 replaced US-40, etc. Except for US-66, the US route usually remained on the old road, but most traffic moved to the new facilities.
Two-digit Interstate numbers denote main routes, with even numbers running East-West, and odd numbers running North-South. This rule is not absolute; look at I-94 in Illinois. While I-94 runs N-S in Illinois, it runs East-West in the US as a whole.

Three-digit Interstates are 'loops' (if the first digit is 2,4,6,or 8) or 'spurs' (first digit 1,3,5,7,9) of the main route.

After the Interstate system was nominally complete in the early 1970s, the state cleaned up the route systems and shortened many numbered routes, both US and State. This "Great Purge" removed irrelevant numbers by shrinking the numbered routes or dropping some numbers altogether.

The state route numbers and their history played a big part in the regulated trucking industry. Before deregulation in the 1970s and 80s, companies were given the right to serve specific areas and use specific roads to get there. The route they served would stay the same if the highway's number did, so they had to know what roads existed to maintain compliance. The Central Motor Freight Association held an extensive list of the routes in Illinois to assist their members in abiding by their regulated routes.

Unlike Wisconsin, Illinois did not have a coordinated County road system. While many counties use the "National Association of Counties (NACo)" pentagram marked and numbering system, this is a more recent development over the last 20 years.
In the larger metro areas, county roads usually are named and fairly indistinguishable from local roads. Many county roads are named in rural regions on geographic terms, with numbers such as 200N or 450E. These indicate hundredths of a mile from the county line in the direction noted. CR 200N would be an E-W road about 2 miles north of the county line, and CR 450E would be an N-S road 4.5 miles east of the county line. Obviously, most of these numbers would change from county to county.

The NACo system uses letters and numbers, with the lowest designators being in the far NE corner of the state (for E-W roads) and the SW corner (for N-S roads). Lake County, for example, has route 1A, as could any other county on the far north edge of the state. Some counties that use the blue pentagram marker do not use the NACo numbering system; instead, they use whatever numbers they elect.

Excerpts from Richard Carlson's Illinois Highways Page.
Edited by Dr. Neil Gale, Ph.D.