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An extension of Interstate 26 is under construction from Asheville to the Tennessee state line. Once this is done, the entire freeway up to I-40, including part of existing I-181, will be renamed I-26.
I-126 South Carolina
Matt Burwick observes that SCDOT "acts as if they don't really want to acknowledge the existence" of Interstate 126. Signs on I-26 southbound only mention "Downtown Columbia," or the text "Route I-126 Downtown Columbia"; the first I-126 marker you see is on the road itself. The I-126 exit is the mainline at that interchange, while I-26 veers to the right [which might imply that I-126 was constructed first.]
I-126 is four lanes wide and flat, with a Jersey barrier and unnumbered exits.  It appears to be a road for which it is difficult to whip up any sort of sustained enthusiasm.
However, South Carolina once planned to connect I-126 and SC 277 in an "inner loop"; those plans were cancelled some time ago.  By the way, SC 277 looks like it would make a fine I-277, but it has never had that designation.
I-126 (numbered as another interstate) Tennessee
This was Tennessee DOT's 2003 proposal for renumbering part of I-181 after I-26 was extended northward from Asheville, N. C.
I-181 south of I-81 would be part of I-26. But the remainder of 181 leading to Kingsport would become Interstate 126.  Kingsport officials prefer extending the 2-digit I-26 designation to their city.  As it turned out, when I-26 opened on Aug. 5, 2003, the remainder of I-181 was not renumbered at all.
I-326 (numbered as another interstate) South Carolina
I-326 was a temporary designation during the 1980s for a 4.95 mile segment of what is now I-77, between I-26 and SC 48 (Bluff Rd.) near Columbia. The number was added to the federal logs between 1971 and 1978. 
Interstate 326 was never signed as such. Below are the opening dates of for I-326/I-77 and how the road was signed:
However, the 326 number appeared in: road atlases; The State (a Columbia newspaper); and the FHWA interstate route log, which even in 1998 still showed 5.24 miles assigned to I-326.  (In 1999, the route was gone. )
It took about 15 years to complete the 15-mile I-77 bypass. A Columbia reporter commented in 1992: "I have heard it said that a Boy Scout Troop with a wheelbarrow and two shovels could have done it faster and cheaper. It would seem that this is just another chapter in the saga of a highway department that is patently rife with ineptitude." 
I-526 South Carolina
19.26 miles ; the Mark Clark Expressway lets you avoid having to take US 17 through downtown Charleston. It's more of a bypass than a spur; you could call it I-426, but it only touches I-26 once. How about US 417? Formerly SC 31, the 526 started construction in the late 1980s, and finished in 1992. It was extended slightly in 1995 or 1996.
AASHTO approved the I-526 numbering, from I-26 southerly to SC 7, on Oct. 6, 1989. 
The east leg of I-526 includes about 12 miles of elevated roadway over marshland, from I-26 to US 17. This segment has two very high bridges over the Cooper and Wando rivers; both have a climbing lane for trucks on the incline. The view from over the Cooper river looking south is quite impressive with the older bridges (US 17) in the background and large naval vessels in the foreground surrounded by marshland.  Ground was broken on a new 8-lane bridge over the Cooper River on July 2, 2001. 
Ian Beverly writes that although only a couple of exits were numbered in 1997, most of them are numbered now. Also, there used to be a traffic light at SC 7 before I-526's western terminus at US 17; now there's an overpass, but SC 7 traffic must use US 17 for access to I-526. 
Southwestern Extension Pondered
The west end of I-526 has an "interchange to nowhere," with accommodations for extending the highway.  In late 2005, Charleston County requested $420 million from the South Carolina State Infrastructure Bank to extend I-526 from Johns Island to James Island. Unfortunately, at the time, bank funds totaled only $300 million. 
Charleston is not a good city for triskaidekaphobics; I-26, US 52, and US 78 all come into town from the northwest.