Interstate 129 (I-129) is an auxiliary Interstate Highway which connects South Sioux City to Interstate 29 in Sioux City, Iowa. Opened in 1976, I-129 is a 3.48-mile-long (5.60 km) route, running 3.21 miles (5.17 km) in Nebraska. At 0.27 miles (0.43 km), Interstate 129 is the shortest highway in the state of Iowa. A rare characteristic of I-129 is that it is one of only four Interstates to enter a state that its parent does not enter (I-29, in this case, does not enter Nebraska).