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I-129 Iowa; Nebraska
3.48 miles ; from I-29 / US 20 interchange south of Sioux City, Iowa, crossing the Missouri River into South Sioux City, Neb. Interstate 129 was added to the interstate system in 1968,  and the Iowa segment opened on Nov. 22, 1976.  The Nebraska segment is known to have completely opened in 1977. 
The freeway continues east of I-29 as US 20, then curves north to an interchange at old US 20 (Gordon Drive). For many years, it ended there, with cuts and fills at the interchange indicating plans for continuing the freeway north and providing a full cloverleaf. On Nov. 19, 2001, a 7-mile extension north to US 75 opened. 
I-129 decommissioned? It only seemed that way
In 1997, it appeared I-129 had been depublicized, if not decommissioned. The freeway signs at I-29 had dark spots where the interstate shields used to be; and the road to Nebraska was signed only as US 20/75.
In 1998, a fellow roadgeek noticed that IADOT had placed "I-129" in plain text where the interstate markers used to be; another posited that the original markers had merely fallen off.
In fall 1999, I was in the area and able to travel I-129. The Iowa signs are as noted in 1998. In Nebraska, the road is marked I-129, but seemingly as an afterthought; the US 20 / US 75 signs are more prominent. Iowa does offer an "END I-129" sign, with the interstate marker, to eastbound drivers having crossed the Missouri.
Regardless of signage, I-129 is still in the federal route log as of 2002, and does not appear to be on the block for deletion. In 1999, both Iowa and Nebraska referred to I-129 in their five-year plans .
A loop around Sioux City... but interstate number in doubt
In 1999, Iowa and its neighbors planned to complete a loop from I-29 in North Sioux City, S.D. to US 20/IA 12 near Lawton, Iowa, connecting with the US 20 freeway and existing I-129. This would have taken about 10 years.
On Nov. 19, 2001, the state opened a 7.2 mile bypass from the end of the US 20 freeway east of Sioux City to connect with US 75 to the north.  In mid-2001, Iowa set up exit numbers for the US 20 and US 75 bypasses.  However, the I-129 designation has not been extended.
In July 2000, however, South Dakota Gov. Bill Janklow threw some cold water on the I-429 proposal: "I don't see any real prospect for South Dakota working on a 429 corridor." The reason: lack of funds. Iowa Gov. Tom Vilsack expressed his dismay at Janklow's remarks. 
Fellow roadgeeks may express surprise (as I did) that the loop won't get the 429 numbering instead of 129, as odd numbers are generally for spurs. But there's plenty of time for things to change.
I-229 South Dakota
11.33 miles ; bypasses Sioux Falls to the southeast. It's been around since at least 1965.
Since I don't have much to say about I-229, we'll quote the Sioux Falls MPO's 2000-2025 Long Range Transportation Plan:
"Interstate 229 functions mainly as a high-speed corridor, which serves local residents. Interstates 29 and 90 serve cross-country truck and passenger vehicles as well as local residents.
Short westward extension contemplated
The Sioux Falls LRTP recommended extending I-229 west of I-29 (with a full interchange there) to connect with Sertoma Ave. or Ellis St., tying into 69th St. The $30 million project, under study in 2001, would be completed within 6 to 15 years. 
14.97 miles ; serves St. Joseph. Interstate 229 started construction in the 1970s; finished around 1986.
There's a neat double-decked section between downtown and the Missouri River; see Eric Stuve's site for photos.