Origins

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 route. 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SBI Route numbers that were superseded by other routes, US or state routes were often reused. For example, SBI Route 68 was in part the current US-41. IL-68 is now an East-West road thru Cook and Kane Counties. (It actually ends at US-41 in Northbrook). SBI-19 was the current US-14, and IL-19 is now Irving Park Road from Chicago to Elgin.

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"

Other abbreviations seen on bridge plates and IDOT Maps include:

SBI-1 to 46

State Bond Issue of 1918

Could be State, US, Interstate, County or other route

SBI -47 to 185

State Bond Issue of 1924

Could be State, US, Interstate, County or other route

FAI

Federal Aid-Interstate

On marked Interstate Highways

FAP

Federal Aid-Primary

On State Primary and Secondary routes

FAS

Federal Aid-Secondary

On State Primary and Secondary routes

HB

House Bill

Authorized via IL House appropriations

SB

Senate Bill

Authorized by IL Senate

OR

Other Road

Financed by some other means than above

CO

County Road

County road, part of State Primary or secondary network


IDOT Maps

Any road considered a part of the Illinois Primary or Secondary Highway System will be shown on IDOT District Maps. These are amazing references to the state highway system, in that they show the origin of each portion of the states road system. They are of various sizes, depending on the district involved. The District 1 map (covers the Chicago area) is about 5 by 4 feet!

The Illinois Department of Transportation produces an Official Highway Map every other year (in odd years). These maps are also used by the Secretary of State (SOS) and Department of Community Affairs (DCCA), each agency providing a different cover and occasional message inside. For a definitive guide to IL (and other states) official highway maps go to the RMCA website.

I collect Official IL road maps for my research. If you have some you want to trade, send me an e-mail


Mileage Signs and

900 series "Route Numbers":

Illinois posts unique mileage signs on rural routes. These signs are square, with a white background, and a green circle containing a dividing line horizontal within the circle. The name of the county in which the sign is in is in the dividing line, and the number of the route is above the divider. The mileage from the county line is on the lower half of the circle. The mileage starts again at the next county line.

These signs are posted on most IL state and US posted highways in rural areas, at integer (whole number) mile points, and at rural features such as bridges, culverts, and intersections. Not all of these features have these signs, and they do not seem to be posted everywhere. No distinction is made as to the type of highway (State vs. US) except that rural Business routes (Business US-24 for example) will have "BUSN XX) in the Route Number area.

Some highways that have been realigned or replaced by a freeway have these signs also, even though they are no longer part of the numbered highway system. These roads are still maintained by IDOT, but have no route number. They are each assigned a number in the 900 series and this number only appears on these mileage signs. These signs are part of the rural referencing system. They are used for IDOT sign inventories, accident coding purposes, guardrail inventory, preparing road projects, and a few other things. Those signs are somewhat used for navigation. They run from south to north and west to east.

The 900 series numbers I know about include:

910 (The former IL-121 north of Lincoln, replaced by I-155)

911 (A former alignment of US-51 in Union County)

913 (A former alignment of IL-13 between Murphysboro and Carbondale)

  In western IL there have been a new series of numbers showing up on these signs. For example, Business Route 67 at Roseville has these Inventory signs with "8967" as the number.


Letter Appended Routes

Some IL routes had letter appended to them. These routes have been eliminated over the years by changing to regular numbers or removing the number altogether. The last one was IL-116A which became IL-117 in the early 1990's. These are considered Spurs or Alternates of the parent route, and letters used have been A,B,C, N (North), and S (South). These should not be confused with the US appendages or Alternate/City/Business Routes, as different rules were used. A few State routes had Alternate, City, or Business Routes in addition to the regular State route, often these were the old route thru towns when the new alignment was built outside town.

This table shows each letter appended state route in IL that I know of, and the current number if any. Each route is described individually elsewhere.

Old Number
Current Number
 
Old Number
Current Number
IL-4A
IL-171
 
IL-94B
IL-135
IL-7A
IL-71
 
IL-95A
IL-94
IL-22A
IL-68
 
IL-97A
None
IL-42A
IL-43
 
IL-113N
IL-102
IL-59A
IL-60
 
IL-113S
IL-113
IL-70A
IL-170
 
IL-116A
IL-117
IL-83A
None
 
IL-119A
US-136
IL-88A
IL-17
 
IL-127A
IL-143
IL-89A
IL-71
 
IL-128A
None
IL-89B
IL-18
 
IL-130A
None
IL-89C
IL-18
 
IL-142A
IL-242
IL-94A
IL-164
     


"Banner" Illinois Routes:

There were not many of these routes in IL, each that I know of are shown below. Each is described in detail in the narrative section.

Banner IL-xxx Routes

Current Number

IL-394
Truck
Bypass
IL-3
None
None
None
Business
US-51
Truck
Bypass
IL-64
None
Business
US-51
(Proposed)
Alternate
IL-159

3 Digit IL routes

There are many 3 digit IL route numbers. Most do not relate to a "Parent" route, but some do:

Original Number

Current Number

Notes

IL-138

IL-138

Became a spur off of the original IL-38 from Mt. Olive

IL-140

IL-140

Was used as a spur off of US-40, and was later extended on part of an old alignment of US-40.

IL-190

I-88

Was a spur off of I-90. (The original part of the East-West Tollway to Aurora)

IL-192

IL-192

Is an old alignment of IL-92

IL-194

I-90

Was a spur off of I-94. (This was the Kennedy Expressway west of the I-94/Edens Split)

IL-203

IL-203

Is an old alignment of IL-3

IL-242

IL-242

Is the current number on a road that had once been IL-142A.

IL-250

IL-250

Is an old alignment of US-50.

IL-251

IL-251

Is an old alignment of US-51.

IL-255

IL-255

Is an extension of I-255.

IL-267

IL-267

Is an old alignment of US-67.

IL-316

None

Was an old alignment of IL-16.

IL-336

IL-336, I-172

Is a spur of US-36 (now I-72). A former section of IL-336 is now I-172.

IL-351

IL-351

Is an old alignment of US-51.

IL-394

IL-394

Is a spur of I-94.

IL-594

I-190

Was a spur off of I-94.


Number Patterns

IL Numbered routes do not follow any other known pattern. The "Even/Odd" configurations of the Interstate and US systems do not apply for IL routes. Some routes in IL are multidirectional, in that they do not have ANY posted direction. Sometimes numbers are assigned because they sound good, or the route is related to another route, or because the number had once been assigned to a portion of the route.

There was a pattern of "Bunching" when SBI routes were numbered, and then later when numbers were reassigned. The area south of St. Louis has a bunch of highways numbered in the 140's, 150's and 160's dating from SBI days. Lake County has IL-131, 132, 134, and 137, which all were reassigned after the original routes bearing these numbers were renumbered.


IL Route Number Oddities

Illinois has some strange number assignments, here are a few of the stranger ones:

IL-137 in Lake County runs N-S from the WI line thru Waukegan and North Chicago, then heads due West to the Round Lake area. None of the IL-137 signs have a directional indicator on them anymore. This had been an E-W Route until it was extended north in the 1990's to cover a small freeway in Waukegan. Portions of the N-S leg was originally IL-42, and parts of the E-W leg were once IL-21. As far as I know, this is the only "J" shaped route in Illinois.

IL-172 today is weird enough in that it is an "L" shaped route from south of Rock Falls to Tampico and IL-92. In the 1940's this route was "T" shaped and extended west to Prophetstown. In 1946 IL-172 was used here in place of IL-226. Other "T" routes existed in SBI days, but few were used afterwards.

IL-38 was numbered that since it had a good sound! The route was the original US-30, also known as the Lincoln Highway west of Geneva. It became US-30-A (or Alternate US-30) after US-30 was rerouted south of the original road. After the Great Purge, the portion of US-30-A from Dixon to Bellwood became IL-38. Sound this out...

IL-40 now runs from Mt. Carroll to Peoria. This had been IL-88 for many years. When IL-5, the East-West Tollway, was changed to I-88, IL-88 had to change since it crossed I-88 near Rock Falls. In the 1930's, IL 40 existed from Sterling to Mt. Carroll. IL-40 disappeared when IL-88 was extended to Mt. Carroll in 1938. When IL-88 needed to be changed, a number used on a portion of it before was recycled. Former IL-80 is now IL-84, it too had to change because of an Interstate crossing it. When I-80 was built in the 1960's it crossed IL-80 near Rock Island, so they changed IL-80 to IL-84.

IL-54 had originally been where part of IL-83 is now. In the 1940's they extended US-54 from Pittsfield to Chicago as an alternate to US-66 via Springfield. After US-54 was removed from Pittsfield to Chicago during the Great Purge, IL-54 was applied from Springfield to Onarga.

IL-22 was originally a loop around the Chicago area that started in Waukegan, went south to Highland Park, west to Fox River Grove, northwest to Crystal Lake, south to Joliet, and to the Indiana line. It has since been cut back to Highland Park-Fox River Grove.


The Great Purge

From 1968 to 1974 many US and IL routes lost their numbers, were shortened (by removing the numbers, not ripping up pavement...) or changed from US routes to IL routes. I call this the Great Purge. The Great Purge occurred due to the substantial completion of some of the original Interstate Highways.

At one time Downtown Chicago had dozens of numbered US and State routes that ended or traversed it. Now there is only 1: US-41, although IL-19 and IL-64 still go thru to Lake Shore Drive, but north of the Loop. Routes that at one time or another came into the Downtown area of Chicago include:

US-12, Business-US-12, City-US-12, US- 14, US- 20, Business US- 20, City-US-20,

US- 30, Business-US-30, City-US-30, Alt-US-30,

US- 32, US- 34, US- 54, US- 66

IL-1, Alt-1, IL-4, IL-4A, IL-5, IL-6, IL-7, IL-21, IL-42, IL-55


Unused Numbers

Illinois has used every number under 186 for a state route. Over 186, most route numbers were chosen as a relationship to another route of some sort. (See "3 Digit State Routes)

The following numbers are not currently used on State routes in Illinois (Those shown in gray would not conflict with current US or Interstate routes:

Number

Current Number

IL-11

US-40

IL-12

US-50

IL-20

IL-120

IL-24

IL-29

IL-27

US-52

IL-28

US-34

IL-30

US-150, IL-91 and others

IL-36

US-36 and others

IL-39

US-150

IL-42

IL-137 in part

IL-44

US-52

IL-45

unnumbered (was IL-179 for a while)

IL-46

unnumbered

IL-51

US-45

IL-52

IL-83

IL-55

IL-56

IL-63

IL-68, IL-21 and IL-137

IL-65

US-34

IL-66

IL-126

IL-67

IL-176

IL-69

US-52

IL-74

IL-26

IL-77

IL-64

IL-79

IL-35

IL-79 (2)

US-36

IL-80

IL-84

IL-85

US-67

IL-86

IL-5, IL-92

IL-87

IL-26

IL-88

IL-40

IL-93

US-34, IL-91

IL-110

IL-16, IL-159, IL-15

IL-112

IL-159

IL-118

IL-23

IL-139

IL-14

IL-144

IL-149

IL-168

IL-97

IL-174

unnumbered

IL-175

unnumbered

IL-179

LaSalle Co. 44

IL-181

IL-33

IL-182

IL-161

IL-183

IL-154

IL-186

IL-170

IL-187

Never used

IL-188

Never used

IL-189

Never used

IL-190

I-88

IL-191

Never used

IL-193

Never used

IL-194

I-90

IL-195

Never used

IL-196

Never used

IL-197

Never used

IL-198

Never used

IL-199

IL-92

IL-200

Never used

 

 

 

 

 

 

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