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I-174 (proposed) South Carolina
This is a proposed numbering for the proposed North Myrtle Beach Connector, leading from proposed Interstate 74 to Main Street in North Myrtle Beach. 
Both Interstate 73 (originally heading to Charleston) and I-74 were thought at the time to extend to the Myrtle Beach area. In February 2002, Horry County state house members introduced a resolution to name the following highways as interstates: 
... but what if I-74 doesn't go to Myrtle Beach?
The eastern/southern terminus of I-74 has been under discussion in the past. During the same year I-174 was proposed (2002), officials in Wilmington, N.C. expressed confidence that I-74 would end in their city, effectively orphaning South Carolina's I-174. (See the I-140 entry.)
As of early 2005, the chances of that appear small. On Feb. 12, 2005, a summit of legislators from both Carolinas worked out a cooperative plan to connect their segments of I-73 and I-74 across the state line.  (Another pet project of NC, extending I-20 to Wilmington, gets a chillier reception from SC. ).
I-174 (proposed number rejected) Illinois; Iowa
Interstate 174 was part of a 1991 proposal by the state of Illinois to reroute I-80 and I-74, and delete I-280 in the Quad Cities area. I-174 would have been the new designation for the north-south segment of I-74 crossing the Missouri River (and the Illinois-Iowa border).
Iowa did not favor this change, and AASHTO disapproved Illinois' proposal on June 10, 1991.  Illinois also tried to push this through in Congress, but was unsuccessful; see the I-280 page for details on the conflict.
I-274 (numbered as another interstate) Illinois; Iowa
This was a preliminary numbering for what is now I-80 in the Quad Cities area, from I-74 in Illinois to I-74 in Iowa. I-80 would have overlapped I-74 through Moline and Bettendorf. On Nov. 10, 1958, I-80 was moved to its current location, separate from I-74, and I-274 was no longer needed. 
See also: Interstate system route numbering (Stephen Summers; FHWA and AASHTO info)
I-274 (proposed) North Carolina
Interstate 274 is the proposed numbering for the western portion of the proposed Winston-Salem Northern Beltway. (The eastern half would be part of I-74.)
Construction has not started on future I-274 as of late 2005, but could do so within a few years. The I-274 number appears on NCDOT's Strategic Highway Corridors map, but has not yet been submitted to AASHTO's Route Numbering Committee. This would be one of the preliminary steps toward making the designation official.
14.88 miles ; southern bypass of I-74 in Peoria. Freeway continues to the north as IL 6. Opened in the late 1970s. Rebuilt in 1997.
Extension, loop completion once considered
Two proposals have surfaced in recent years involving Interstate 474. One is the "Ring Road" that would close the I-474 loop on the north side; this controversial plan as of 2005 is dormant. 
The other plan was the Peoria to Chicago freeway study, which is now shelved.  A proposed new road would have extended from I-474 to meet I-180, accommodating Chicago-bound traffic that doesn't want to use I-74 and I-55.