Illinois State Highways


Route Markers

The original Illinois Route marker was a rectangle with a state outline containing the numbers. They looked something like this:

This made it difficult when marking 3 digit routes. # digit routes usually had the numerals extended past the edges of the state outline which was broken for that portion. Routes with a letter appendage like IL-59A usually had the letter below the number like this:

During the 1960's IDOT changed the markers to the current style, a square or rectangle with the word ILLINOIS on the top and the route number below. This allowed for larger digits to be used. Older signs of this style used 4 inch letters for the state name and 12 inch numerals for the route number. Current editions use 3 inch letters for the state name and 10 inch numerals.

1960's style Route Marker
Current style Route Marker

3 digit routes, such as IL-120 or IL-159 usually get rectangular signs. This allows regulation sized numerals to be used. Sometimes a square blank is used and the numerals will either have reduced kerning (closer together) or reduced size. I have seen square IL-176 signs with both styles, the reduced numeral signs had 8 inch numerals.

Most current signs are 24 inch square, with 10 inch numerals and 3 inch letters. Some signs on major routes are 36 inches with correspondingly larger numerals and letters.


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"

SBI Routes

An organized system of numbering state highways started in 1918 with the first State Bond Issue (SBI) Routes, 1 thru 46. SBI Routes 47 thru 185 were authorized in 1924. Bonds were floated to pay for specific routes. SBI # 1 paid for Route 1, and so on. Remarkably, many of these numbers still exist on the original or nearby alignment. As the highway system grew the numbers were altered to accommodate new roads or extensions of older roads. When the US Highway System of Interstate Roads started to be posted in 1926, the US numbers were just tacked onto the existing IL/SBI number unless the US route was routed along a new road. In the 1930's the IL/SBI numbers were dropped when they were only a second number on a US marked road. Other IL (and US) routes were dropped over the years as the Interstates were built, and traffic patterns changed.

SBI Numbers are still used for several purposes, even when they do not match the posted number. IDOT District maps still refer to SBI numbers on the various roads it maintains, along with other non-posted designations that refer to how the route was authorized. Bridge weight plates refer to SBI numbers instead of posted route numbers as well. If you look at a bridge plate along old US-66, you will see the route referred to as "SBI-4".


US Routes

US Routes appeared on IL roads in 1927 after they were approved by the states in 1926. They first appeared on official IL maps in 1928. The US signs were usually posted on existing state highways, but in some cases new pavement was laid for the US routes. The new roads would certainly have been built as state routes if there were no US routes though. Dates shown on these pages are usually the date the markings appeared on the Official IL Maps of the time. Actual markings may have been posted the year before.

The original US Route Marker in Illinois was shield shaped with a separator line at the top. Above the line said ILLINOIS, below the line said US.

During the 1950's and early 1960's the separator line was dropped as was the state name. Eventually Illinois, along with most of the rest of the country, adopted the current US highway marker, a white shield surrounding a black number enclosed on a square blank.

Interstate Highways

Interstate Highways in Illinois range from the nonexistent (but proposed) to the 358 miles of I-57. Several routes proposed have never been built, and some may eventually be built.

Miles and some endpoints shown on 2 digit Interstates here come from the Zzyzx Interstate Page, an amazing reference to 2 digit Interstate highways.

Interstates are the main routes between cities, and are divided multilane highways. Most are 2 lanes in either direction rural areas, but can swell to 6 or more in large cities. Interstate numbers are on most freeway and tollway miles in Illinois. Interstate numbers are posted with a red-white-blue sign in a shield shaped design. Interstate numbers may exist in multiple states and are occasionally reused.

2 digit Interstate Numbers denote main routes, with even numbers running East-West, and odd numbers running North-South. These are not absolute, as you can tell by looking at I-94 in IL. While I-94 runs N-S in IL, it runs East-West in the US as a whole.

3 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.

Interstates were usually funded by a partnership of the State and Federal governments. Tollways that carry an Interstate number do/did not receive federal monies. In recent years the "90/10" financing of Interstates ended. Reconstruction of existing and building of new roads are financed on an individual basis, usually with some federal money. Although the original Interstate system was built with mostly federal money, the financing of a specific route does not determine the type (Interstate/US/State) number it may get.

The Interstate Route Marker has remained constant over the years, with the only change being to drop the state name on many of the markers to allow for more room for larger numerals.


Read all about your favorite route:

Route overview and markers

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Updated 11-18-2007