Length in Iowa: 215 miles/346 kilometers
Northern terminus: Minnesota state line
south of Worthington, MN
Southern terminus: Missouri state line
south of Shenandoah
Counties: Osceola, O'Brien, Cherokee, Ida, Crawford, Shelby,
Pottawattamie, Mills, Fremont, Page
Cities along route: Sanborn, Primghar, Larrabee, Cherokee, Holstein,
Ida Grove, Schleswig, Denison, Harlan, Avoca, Hancock, Oakland, Emerson,
Freeway segments: None
Expressway segments: The multiplexed segment with US 20 (see below)
1 mile with US 18 east of Sanborn
5 miles with IA 10 in southern O'Brien County
1½ miles with US 20 east of Holstein
3½ miles with IA 175, in Ida County through Ida Grove
10 miles with IA 141 in Crawford County through Denison; this
includes a half-mile triplex with US 30 through Denison.
2½ miles with US 6 through Oakland
Designated: February 15, 1935 (approved November 20, 1934), to the former IA 73 (II), a short-lived highway created from former segments of IA 21 (I)
(north of Denison) and IA 4 (I) (to the south) earlier that year. This was after plans to redesignate the former IA 59 into Minnesota (which became US 63 instead) as US 59 and designate US 59's eventual route as US 73 (which ended up in Nebraska instead) failed.
Paving history: At the time of designation, the segments from Sanborn to the split with IA 10 northeast of Calumet and from the split with IA 141 northwest of Denison to Avoca were paved.
1937: Paved from the Minnesota state line to the Osceola/O'Brien county line, the O'Brien/Cherokee county line to Cherokee, and from the Cherokee/Ida county line to Ida Grove (on a new alignment north of US 20 that took effect September 15, superseding part of IA 131 (I))
1939: Paved from the city of Cherokee to the Cherokee/Ida county line (the segment between Cherokee and IA 31 was realigned in 1937)
1941: Paved from the O'Brien/Osceola county line to Sanborn
1946: Paved from the split with IA 10 to the O'Brien/Cherokee county line (on a new alignment; IA 388 was then created to serve Calumet)
1948: Paved from Ida Grove to the Ida/Crawford county line (on a new alignment; the old one followed present-day County Roads M15 and D54)
1949: Paved from the Ida/Crawford county line to IA 141 and from IA 244 east of Henderson to US 34 at Emerson
1952: Segment from Avoca to Oakland upgraded from bituminous to paved
1954: Paved from Emerson to IA 184 east of Imogene
1956: Paved from Oakland to the junction with IA 244
1959: Segment from IA 2 to the Missouri state line upgraded from a bituminous surface to a paved surface
1960: Last segment, from IA 184 to IA 2, upgraded from a bituminous surface to a paved surface
Major alignment changes:
September 1938: Straightened between Oakland and Emerson on a new gravel alignment. IA 244 (I) to Henderson was realigned and IA 362 (I) was created to serve Macedonia after the realignment. The former route in eastern Mills County followed present-day County Roads H12 and M16.
October 17, 1939: Realigned between US 34 at Emerson and a point east of Imogene on a new gravel alignment; original route followed present-day County Road M21 (gravel) and part of IA 184.
November 1970: Placed on a new 2-mile stretch of road south of IA 44 in Harlan
July 12, 1971: Placed on a new road between Defiance and IA 44, one mile west of its original alignment.
August 25, 1964: Expressway segment near Holstein that is co-signed with US 20 opened