Interstate 129 is a short freeway spur off of Interstate 29, protruding west across the Missouri River into South Sioux City, Nebraska. The whole route is under three miles long, and only a fraction of a mile rests in Iowa. In both Iowa and Nebraska, Interstate 129 is entirely concurrent with both US Route 20 and US Route 75. The eastern end of Interstate 129 is on the south side of Sioux City, Iowa, at Interstate 29; however, the freeway continues east as US Routes 20 and 75, to curve northerly around the east side of Sioux City. As for Interstate 129's stint in Iowa, though, it just runs from Interstate 29 west to the Missouri River, covering a paltry distance of about a quarter mile.
I mean, seriously, look at how the Iowa welcome sign is right along the signage for the exit to Interstate 29 south. The start of Interstate 129's eastbound foray into Iowa is practically its eastern endpoint, too.
And here is the least eastbound exit sign for Interstate 129. Note that the freeway continues east, but it's only designated as US Routes 20 and 75 beyond this point. The Interstate 129 designation ends here.