US 75 between the north side of Sioux City and Merrill is the oldest four-lane road in the state system, in use for 70 years. But, as a road from 1949, it was not built to anything resembling expressway standards of today.
That is going to change with an upcoming project between Hinton and Merrill. The road is going to be leveled out, removing the large differences in roadbed height between the northbound (oldest) and southbound lanes. The only places in Iowa with similar situations, where you can easily tell which was the old road and which is the newer, are US 30 between Clinton and Z24 and US 63 between the north end of the Denver bypass and IA 3.
According to the plans on the DOT’s website, this will also remove traces of the “Old Highway 75” four-lane on the south side of Merrill, turning the intersection into a regular T. The road into Merrill is billed as secret IA 470 on the DOT’s city map but its existence is a mystery considering it was bypassed in 1957 and this designation did not appear until around 2011.
The four-lane 75 between Merrill and the south end of the Le Mars bypass was leveled out in the late 2010s. The Le Mars Sentinel pegs a construction date of 2023.