US 421. When the Indiana State Highway Commission, in 1926, performed the “Great Renumbering,” most the route of the original Michigan Road became known as SR 29. This was especially true in southeastern Indiana. In 1951, most of SR 29 was recommissioned as US 421, again especially in southeastern Indiana. Less than a decade of this renumbering, the ISHC started working on a plan to build an interstate highway from Indianapolis to Cincinnati. This highway would be Interstate 74. The route for I-74 would roughly follow the old Michigan Road, and hence US 421, as far as Greensburg in Decatur County.
The first section of I-74 opened in 1960. East of Acton Road, one mile west of the Marion-Shelby County line, the new interstate had been built right on top of the old US 421 to two miles east of that county line, with the exception of the section through the town of Pleasant View. As a result of this replacement of the old road, the US 421 designation was logically moved to the new interstate. However, when the old road reappeared on the north side of I-74, the US 421 designation didn’t move back to the old road. It remained multiplexed with the interstate to the end of the road at SR 9. US 421 then followed SR 9 south into Shelbyville, then SR 44 east out of the city, where it followed Michigan Road southeast out of town. The western end of I-74, from Acton west to the end of the road west of what is now I-465, wasn’t multiplexed. US 421 remained on the old Michigan Road, called Southeastern Avenue from Acton all the way to downtown Indianapolis.
In 1961, I-74 was completed to SR 244 in east central Shelby County. US 421 still maintained its route through Shelbyville. The section on Southeastern Avenue in Marion County was removed, placing the US 421 designation on the interstate from then end of I-74 to SR 9.
The next change in US 421 came in 1963 when the ISHC moved the US 421 designation to I-74 between SR 9 and SR 244. SR 244 was reeled back to the JCT I-74, with US 421 replacing the less than 1/2 mile on the west end of SR 244. Thus, US 421 continued on its way along the old Michigan Road from that point. I-74, at the time, had been completed to an exit that was built east of Middletown on the Shelby-Decatur County line.
1964 saw the completion of I-74 to Cincinnati, with the exception of the section between US 52 and SR 1. This would also be the year that the eastern end of US 52 in Indiana would be multiplexed with I-74.
Another pullback of US 421 occurred in 1965 with the road being rerouted to I-74 at the exit east of Middletown mentioned above. US 421 followed the old Michigan Road, or a close facsimile thereof, to the Shelby-Decatur County line, then went due north to the interchange with the interstate.
The last part of US 421 to be rerouted would happen in 1968. The current end of the separate US 421 northwest of Greensburg was officially put in place. This was even though the interchange was built in 1964. The interchange between I-74 and US 421 at this location would be rebuilt and expanded, moving “Michigan Road” to allow the larger connection.
Today, the old US 421 is still (mostly) travelable (there are slight deviations at locations of interchanges with I-74, since the old road is literally right along side the interstate in most places) from northwest of Greensburg to London Road, and from Acton Road to downtown Indianapolis. It is also marked as the Michigan Road Historic Byway to guide that travel.