Length in Iowa: 313 miles/504 kilometers
Western terminus: South Dakota state line
(Big Sioux River) east of Canton, SD
Eastern terminus: Wisconsin
state line (Mississippi River) at Marquette
Counties: Lyon, Sioux, O'Brien, Clay, Palo Alto, Kossuth, Hancock,
Cerro Gordo, Floyd, Chickasaw, Fayette, Allamakee, Clayton
Cities along route: Inwood, Rock Valley, Hull, Boyden, Sheldon,
Sanborn, Hartley, Spencer, Dickens, Ruthven, Emmetsburg, Cylinder, Algona,
Wesley, Britt, Garner, Ventura, Clear Lake, Mason City, Nora Springs, Rudd,
Floyd, Charles City, Bassett, New Hampton, Fredericksburg, West Union,
Clermont, Postville, Luana, Monona, Marquette
NHS: From IA 60 in Sheldon to the Wisconsin state line
20 miles, between the I-35/IA 122 interchange at Clear Lake
and the Cerro Gordo/Floyd county line southwest of Nora Springs, including
3½ miles co-signed with I-35 and IA 27.
6 miles, between Floyd County Roads B35 and T64 west and south of Charles City (co-signed with US 218 and IA 27).
4 miles, bypass to the west of New Hampton (with US 63).
About two of the 4 miles it shares with US 71 through Spencer.
17 miles, between the Cerro Gordo/Floyd county line and the interchange with Business
US 218/Floyd County Road B35 near Charles City (partially co-signed with US 218 and IA 27).
4 miles, between the end of the freeway segment south of New Hampton and the
junction with IA 346 (co-signed with US 63)
Segment between I-35 and US 218
(it is in reverse order, so scroll upward to read the eastbound exits)
New Hampton bypass
1 mile with US 75 in northern Sioux
County, north of Perkins
1 mile with US 59 east of Sanborn
4 miles with US 71 through Spencer
5 miles with IA 4 west of Emmetsburg
1 mile with IA 15 in western Kossuth County north
1 mile with US 69 west of Garner
43 miles with IA 27, between the I-35/IA 122 interchange at Clear Lake and the interchange with County Road T64 south of Charles City; this includes a 3½-mile triplex with I-35 between exits #190 and #194, and an 8-mile triplex with US 218
between Floyd and T64.
7 miles with US 63 west and south of New Hampton
16 miles with US 52 between Postville and Froelich
Designated: October 16, 1926, replacing IA 19 (I) east of US 75 at Perkins. This was based on the Bureau of Public Roads' original 1925 plan; as late as October 4, the Highway Commission declined to join South Dakota in requesting that US 18 be extended westward. It wasn't until the American Association of State Highway Officials finalized the US highway numbering plan on November 11, 1926, that US 18 replaced the rest of IA 19 (I) west of US 75.
Paving history: At the time of designation, the segments from Perkins (US 75) to Hull, through O'Brien County, and from Algona to Charles City were paved.
1928: Paved from Emmetsburg to Whittemore and from New
Hampton to the split with IA 59 (now US 63)
1929: Paved from Whittemore to Algona (on a new alignment approved December 21, 1928) and from Postville to McGregor
1930: Paved from the junction with IA 102 (I) west of West Union to Postville
1931: Paved from the O'Brien/Clay county line to Spencer, Dickens to Ruthven, from Charles City to the Floyd/Chickasaw county line, and from McGregor to Marquette
1933: Paved from Spencer to a point south of Dickens (on a new alignment that was approved on February 1, creating IA 257 to serve Dickens), and from Ruthven to Emmetsburg
1934: Paved from US 75 to Hull and from the split with US 63 south of New Hampton to Fredericksburg
1935: Paved from the junction with IA 330 (I) north of Ionia to New Hampton
1936: Paved from Hull to Sheldon and from the Floyd/Chickasaw county line to IA 330
1937: Paved from the South Dakota state line to Inwood and from County Road C (now V56) east of Fredericksburg to the Chickasaw/Fayette county line
1938: Paved from Inwood to the Lyon/Sioux county line, from Fredericksburg to County Road C, and from the Chickasaw/Fayette county line to IA 102
1939: Paved from Rock Valley to US 75
1949: Last segment, from the Lyon/Sioux county line to Rock Valley, was paved
Major alignment changes:
April 1930 (approved March 19): Realigned along a gravel road (at the time) between Inwood and US 75, replacing part of IA 26 (I). It was also realigned between Everly and Spencer that year, one year before paving; the old route followed present-day County Roads M27 and B24. This realignment created IA 170 to serve Everly.
March 7, 1932: Bridge between Marquette and Prairie du Chien, WI, opens; US 18 traffic used a ferry from McGregor before that. (The Highway Commission did not formally reroute US 18 until June 7.) This was replaced by the current Marquette-Jolliet Bridge on November 14, 1974.
October 25, 1952: Straightened between Garner and Clear Lake. IA 35 (II), later IA 409, was created two years later
to serve Garner.
November 1961: Realigned to the north of Wesley and Britt, which created
IA 424 (I) to serve Britt
November 16, 1989: Rerouted to the north of Marquette and McGregor; the old route became Business US 18 and was designated as an extension of IA 76.
May 1, 2006: Rerouted around Charles City, following the US 218/IA 27 bypass to the Grand Avenue interchange and replacing the unsigned IA 162. It previously followed 5th, Main, Gilbert, and 2nd Streets downtown (sharing part of IA 14 after the bypass opened). IA 14 was extended for about four blocks along US 18's old alignment to end at US 18.
December 6, 1999: Rerouted to the south of Mason City along a new 24-mile freeway/expressway
segment; the old route between Clear Lake and Rudd became IA 122.
August 1, 2000: 3-mile freeway segment opens west of Charles City as part of its
bypass; it was then re-routed as a second number over three miles of IA 14 (5th and Main Streets) between the bypass and the existing
US 18 in Charles City.
September 2000: 4-mile expressway segment between Rudd and Floyd opens
October 20, 2000: 3-mile expressway segment between Floyd and the northwest Charles City
bypass interchange opens, completing the four-lane link between I-35 and US 218.
October 5, 2002: New Hampton bypass segment between the split with US 63 and 225th Street
opens; IA 24 replaced part of the old segment through New
Hampton. The segment between 225th Street and IA 346 opened in early November.
A west-side bypass of Spencer had been approved in December 1997 for completion
in 2005, but it was taken off the five-year plan in November 2001 due to budget cuts. (The city of Spencer ultimately completed a "West Beltway" in 2013 but neither US 18 nor US 71 have been rerouted.)
The DOT also added US 18 from Postville eastward into Wisconsin to its list of
roads that are to be upgraded to "Super-2"
highways, which are two-lane roads with wider shoulders and more controlled
access. The "Super-2" system is part of the state's 25-year transportation
plan, but short-term plans to upgrade this segment were delayed in September 2001.
Business US 18
Mason City: Part of the existing US 18 received a business route
designation once the Avenue of the Saints bypass was completed; it is co-routed
along IA 122 and US 65.
This route was not very well signed at first, according to
Mark Hasty in an April 2000 e-mail: "The only place I can recall seeing signs for it is at the US-65
junction. Every former US-18 sign in Mason City has been changed to IA-122."
Signage of Business US 18 has improved since then, with more reassurance markers along
the route as of September 2000, but IA 122 and US 65 remain the primary designations.
Marquette/McGregor: Designated in 1989 along the old alignment of US 18 through these historic river towns, but it was not approved by AASHTO until October 4, 1996. It is co-signed with IA 76, although IA 76 was originally unsigned along this segment.
City US 18
Mason City: Jeff Morrison found this on a 1959 US Geological Survey topographical map in August 2003. "From 18, it went NE on Crescent Drive to 1st St. S (SW/SE), and then split into one-ways with State at Adams Ave. Both ways rejoined at Connecticut Ave. The route continued east on State to Maple Drive, intersecting 18 at Louisiana Ave. It was never more than 4 blocks north of 18." US 18 followed these streets until about 1958, and this route appeared on the state highway maps' Mason City insets as a green line (labeled as "Business Route") from 1964 to 1975. Exactly how long this designation lasted is unknown. (This is not to be confused with the current Business US 18.)