Is it me, or are there a hell of a lot of pretty useless Interstsates in Illinois?
This interstate is also multiplexed with the brief bit of I-73 (not yet added) in North Carolina. For more details on the I-74 extension see Andy Field's Interstate 73/74 page.
Thought that I would comment on your opinion of Illinois Interstates.
Geography dictates the rather large number of E-W Interstates in Illinois. Also, Illinois has been more aggresive than other states in taking advantage of federal legislation allowing more Interstates.
As you know, the Great Lakes block direct land travel from the NE U.S. to the NW U.S. Chicago is the great railroad hub of the nation as a result. Illinois (and Indiana) have so many major EW Interstates for the same reason, including overlapping routes such as I-80 & I-90, I-90 and I-94, etc. Several US hwys do the same thing.
As for the federal legislation, since sometime in the 1980's, states have been allowed to add routes to the Interstate system after these routes are brought up to Interstate standards. Illinois is just doing what some other states could also be doing. Other examples of these added Interstates include routes in California such as I-880, the infamous I-238, I-110 and I-710. Thus, Illinois has added I-39, extended I-72, etc. I agree that I-88 should be numbered something different, perhaps I-82. I see the US 20 corridor from Rockford, IL west to Sioux City, IA via Dubuque, Waterloo-Cedar Falls, and Fort Dodge being upgraded to Interstate standards within the next decade or two. Much of this corridor already meets Interstate standards. I think that this route would more appropriately be numbered I-88.