On April 29, 1947, the Iowa General Assembly approved an act designating US 6 as the Grand Army of the Republic Highway, a distinction the route shares in other states. Governor Robert D. Blue dedicated the Grand Army of the Republic Highway at the Iowa Old Capitol Building on September 28, 1947. In attendance were the last two surviving Iowa veterans of the U.S. Civil War. In the 1950s, the Iowa State Highway Commission began to straighten the route. A section of the highway between Grinnell and Ladora was straightened, which resulted in Brooklyn and Victor being bypassed. Between Dexter and West Des Moines, US 6 swapped alignments with Iowa 90 in 1958. In 1961, US 6 was routed onto the new I-80 from the Iowa 90 interchange to the Baxter exit, currently exit 159. Iowa 90 was extended onto the old US 6 alignment. However, in 1967, those changes were reversed and US 6 was taken off I-80 and put back on the road which had been Iowa 90. Iowa 90 was assigned the section of US 6 between what's now exit 106 along I-80 and exit 69 along I-35.
