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"This guy [Kurumi] may know nothing much about St. Louis but recently uploaded a new page with loads of corrections." - Kim Harvey, circa 1997
You heard it here first.
32.13 miles ; north from I-55 near Lincoln to I-74 near Peoria.
Before Interstate 155 was built, the 2-lane state route 121 was the most direct route from Peoria to Springfield and St. Louis. A freeway upgrade IL 121 was first proposed in 1969. In the mid-1970s, a short freeway section near IL 122 was built when a bridge over the Mackinaw River needed replacement anyway.
Illinois' first choice for an interstate designation was Interstate 37. On Dec. 7, 1990, AASHTO deferred approval on this number until the FHWA approved. AASHTO also recommended choosing a 3-digit interstate number (more befitting the route's auxiliary role).  On June 9, 1991, AASHTO approved the designation Interstate 155, subject to completion of the route. 
By 1993 or 1994, I-155 was completed.
I-155 Missouri; Tennessee
Interstate 155 was added to the Interstate system on August 28, 1964.  I-155 broke ground in the early 1970s, finishing around 1980.
I-255 Illinois; Missouri
The first segment of I-255, between I-55 and Koch Road at the river, opened in 1967. (US 50 continued across the Jefferson Barracks ("JB") bridge to Illinois.) In 1976, this stretch, along with the old I-244, was absorbed into I-270.
In 1983, construction began from the JB bridge to I-64 in Illinois. All indications were that the road would be an extension of I-270. When the road opened in 1985, however, it was signed I-255, a sleight-of-hand worthy of that magician dude who's dating that model.
In November 1986, I-255 was extended to I-55/70 near Collinsville. On July 26, 1988 at 1:30 pm, the final segment north to I-270 opened, after about six years of construction. Kim Harvey writes: "The [opening] ceremony was on 270 westbound between IL 111 and 157 and featured an 'antique car parade' which jammed up traffic for miles. The blue baseball caps worn at the ceremony by Gov. Thompson, a few mayors, and the contractors were very ironic. They said 'Last interstate mile in Illinois'. At that date, I-39 wasn't completed, I-72 was only 79 miles long, and I-155, I-172, and I-355 didn't even exist."
Kim Harvey adds:
"IH 255 is basically a roadgeek heaven. Anything that you don't see in St. Louis is usually seen on IH 255. 7 foot light poles in Cahokia (not kidding), a section known for nasty accidents (where you got a left exit, a level junction (sort-of), and a split in 4 miles), some very inconsistent signage, some wacko overpasses, wacko gantries, and everything is brown. There is a green guide sign near Cahokia that is in ALL CAPS, another near Collinsville with the WRONG exit number on it, another set in Collinsville that is very inconsistent, control cities that are ironic in nature, a section where IL 3 takes prioity over US 50 as a multiplexed route, and my downright favorite -- the northbound control city selection: Chicago, Indianapolis, or I-270! :)"
I-255 (numbered as another interstate) Tennessee
See also: Memphis Freeways (Michael Adams; maps of historical and proposed freeways)
20.01 miles . Also called the North-South Tollway, Interstate 355 connects the Chicago suburbs of Bolingbrook (I-55) and Itasca (I-290). It opened on December 24, 1989, and for the first two days was toll-free. (Hey, it was Christmas!)  
The 1987-88 Illinois Tollway map marked the corridor as I-455 -- either a typo or an earlier designation for the tollway. .
An extension south to I-80 is planned, but has been held up since 1996 by environmental and funding issues.
Southern extension planned
A 12.5-mile extension south to I-80 in New Lenox has been planned for some time. In 1999, the project cost was estimated at $700 million. . In late 2004 the first contract was approved by the toll authority.
Even during construction of the original 20-mile section, provisions were made for a future extension southward:
Southern extension: recent events
Land Value in the extension's ROW
Aaron Nathan wrote in 1999: "The land along this path is practically worthless. I have heard some amazing stories about people renting buildings for around $10 a month to people because (1) everyone knows these buildings are going to be destroyed and (2) the owners don't want insurance liability involved with an abandoned building." 
I-555 (future) Arkansas
Interstate 555 will be the official designation for 44 miles of of US 63, between I-55 at Lake David and US 49 in Jonesboro, once it has been completely upgraded to interstate standards. Until then, "Future I-555" signs will mark the highway. 26 miles of US 63, from Jonesboro southeastward, are already fully upgraded; the remaining 18 miles are scheduled for future improvement. 
Arkansas requested the I-555 designation in 1999 but was turned down: AASHTO's Route Numbering Committee said the route "does not meet Interstate standards and will not for some time."  In 2000, the state tried again, and AASHTO approved a Future I-555 designation. The Future can be removed when FHWA adds the route to the Interstate highway system. 
The purpose of the designation is to help economically link the Jonesboro and Memphis areas. A redesignation for similar reasons is North Carolina/Virginia's I-785.