I-129 begins along US 20 on the western edge of South Sioux City, Nebraska, just west of exit 1, a cloverleaf interchange with US 75 and US 77.

One mile (1.6 km) later, I-129/US 20/US 75 intersect Dakota Avenue at a partial cloverleaf interchange.

For the rest of I-129's length, the two directions of I-129/US 20/US 75 traffic are separated by a Jersey barrier instead of a grassy median.

The three routes cross the Missouri River and immediately intersect I-29 at an interchange.

Iowa received the smallest allocation, 0.5 miles (0.80 km), for the southern bypass of Sioux City.