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19.69 miles ; north from the (temporary) end of I-72 to US 24, east of Quincy. Formerly IL 336, Interstate 172 got the upgrade when US 36 east of Springfield was renumbered as part of an extended I-72.
Illinois' first idea for I-172 is a bit of a surprise: in 1991 it proposed the number for the freeway upgrade of US 36 from I-55 to E. Hannibal, near the Mississippi River; a distance of about 93 miles. In other words, this is today's I-72 extension. This was approved by AASHTO on June 9, 1991, subject to completion of the route. 
It became apparent that upgraded US 36 would make more sense as an extension of I-72 itself. On April 23, 1995, AASHTO approved this, extending I-72 westward from I-55 to US 61 in Hannibal, Mo., including a Mississippi River crossing. Interstate 172 became the northward spur along IL 336, leading from I-72 to US 24. 
The 336 freeway has been around, in part, since the late 1970s.
The justification for the 72/172 upgrade is familiar. From the Associated Press:
"'This interstate designation puts the CIE in the big leagues and gives the economy a shot in the arm,' U.S. Rep. Richard J. Durbin, D-Springfield, said at a Capitol news conference.
Don't be too outraged, however. I-172's 19 miles are all "non-chargeable," meaning there was no federal responsibility for funding its construction.