Special routes of U.S. Route 18

U.S. Highway 18 Business (US 18 Business) begins at a diamond interchange with Interstate 35 (I-35) on the eastern edge of Clear Lake. The route, overlapped by Iowa Highway 122 (Iowa 122), begins as a four-lane divided highway, serving the Mason City Municipal Airport before entering Mason City proper. Between Eisenhower and Jefferson Avenues in Mason City, the highway is undivided. This road is the primary east–west artery through Mason City. At Jefferson Avenue, US 18 Business / Iowa 122 separate into a pair of one-way streets, remaining in this configuration through the downtown area where the Park Inn Hotel, one of Frank Lloyd Wright's few commercial buildings, resides. They intersect US 65 at Federal Street. US 18 Business turns to the south to follow US 65. The business loop rejoins its parent route south of the city limits.[2]

The entire route is in Cerro Gordo County.

Location	mi[1]	km	Destinations	Notes
Clear Lake	0.000	0.000	I-35 / US 18 / Iowa 27 – Des Moines, Minneapolis Iowa 122 begins	Western end of Iowa 122 overlap
Mason City	7.862	12.653	US 65 north (S. Federal Avenue) / Iowa 122 east (6th Street SE)	Eastern end of Iowa 122 overlap; western end of US 65 overlap
	10.752	17.304	US 18 / Iowa 27 / US 65 south – Rockwell, Clear Lake, Hampton, Charles City	Eastern end of US 65 overlap
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

U.S. Highway 18 Business (US 18 Business) begins at an intersection with US 18 west of McGregor. The route descends into the Mississippi River valley along the northern border of Pikes Peak State Park and then turns into McGregor. Through McGregor, US 18 Business heads to the northeast towards the Mississippi River. Along the river, the highway is parallel to the Dakota, Minnesota and Eastern Railroad (DM&E Railroad). For one mile (1.6 km), US 18 Business travels a narrow strip of land between the river and the bluffs where it passes a riverboat casino. At Marquette, US 18 Business ends at the foot of the Marquette–Joliet Bridge, which carries US 18 to Prairie du Chien, Wisconsin.[3]

The entire route is in Clayton County.

Location	mi[1]	km	Destinations	Notes
Mendon Township	0.000	0.000	US 18 – Monona, Marquette Iowa 76 begins	Southern end of Iowa 76 overlap
Marquette	4.149	6.677	To US 18 (Marquette–Joliet Bridge) – Prairie Du Chien Iowa 76 north – Effigy Mounds	Interchange; northern end of Iowa 76 overlap
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

U.S. Highway 18 Business (US 18 Business) is a former segment of US 18/151 between Exits 65 and 69 within Mount Horeb. The route is also shared by US 151 Business, as well as parts of Wisconsin Highway 78 and Dane County Road ID.

The entire route is in Dane County.

Location	mi[5]	km	Destinations	Notes
​	0.0	0.0	US 18 / US 151 / WIS 78 south Bus. US 151 begins	Begin US-Bus 151 and WIS 78 overlap
​	0.1	0.16	CTH-ID west	Begin CTH ID overlap
Mount Horeb	1.7	2.7	WIS 78 north	End WIS 78 overlap
	3.4	5.5	US 18 / US 151 Bus. US 151 ends / CTH-ID ends	End US-Bus 151 and CTH ID overlap
1.000 mi = 1.609 km; 1.000 km = 0.621 mi

U.S. Highway 18 Business (US 18 Business) is a former segment of US 18/151 within Verona. The route is also shared by US 151 Business and Dane County Road MV. US 18 Business/151 Business/CR MV begins at a pair of flyover interchanges with US 18/151 on Exit 76 west of Verona. Eastbound Exit 76 runs directly into the business routes on West Verona Avenue, while westbound Exit 76 connects to the business routes by way of Epic Lane. From there, the business routes run east-northeast as a four-lane divided highway until the intersection with Legion Street, where it becomes a local two-lane undivided road. At the intersection with North and South Main Streets (Dane County Road M), West Verona Avenue becomes East Verona Avenue, making a brief reverse curve to the left before returning to its previous trajectory at School Street and becoming a four-lane divided highway east of there. US BUS 18/151/CR MV ends east of Verona at Exit 81 on US 18/151, but only with an eastbound on-ramp and westbound off-ramp.

The entire route is in Verona, Dane County.

mi[6]	km	Destinations	Notes
0.0	0.0	US 18 / US 151 Bus. US 151 begins	Begin US-Bus 151 overlap
1.3	2.1	CTH-M (Main Street)
2.4	3.9	US 18 / US 151 Bus. US 151 ends	End US-Bus 151 overlap
1.000 mi = 1.609 km; 1.000 km = 0.621 mi

Alternate U.S. Highway 18 (Alt. US 18) is a permanently signed detour route that begins in Verona. The route follows CTH-PB, CTH-M, and CTH-MM to connect with US 14 north of Oregon, then uses US 14 to reconnect with the main route in Madison. The route is concurrent with Alt. US 151.[7]

The entire route is in Dane County.

Location	mi[8]	km	Exit	Destinations	Notes
Verona	0.0	0.0		US 18 / US 151 – Madison, Mount Horeb Alt. US 151 begins	West end of Alt. US 151 overlap
	0.2	0.32		CTH-M west	West end of CTH-M overlap
Fitchburg	7.3	11.7		CTH-M ends / CTH-MM south	East end of CTH-M overlap; west end of CTH-MM overlap
	7.6	12.2		US 14 east / CTH-MM north – Evansville	East end of CTH-MM overlap; west end of US 14 overlap
	11.3	18.2	134	To CTH-MM / Lacy Road	To CTH-MM only signed eastbound
	12.6	20.3	133	To CTH-MM / Mc Coy Road	Westbound exit eastbound entrance
Madison	13.4	21.6	132	US 12 / US 18 / US 151 / US 14 west Alt. US 151 ends	East end of US 14 and Alt. US 151 overlap; access to SSM Health Saint Mary's Hospital - Madison
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus       Incomplete access

U.S. Route 18 Bypass (US 18 Byp.) was a bypass around the Hot Springs, South Dakota, city center. It began on the west side of Hot Springs at an intersection with US 18 and travels in a southeasterly direction. The highway skirted the western and southern edges of the city and meets South Dakota Highway 71 (SD 71) close to the point where the route turns east. It ended at US 18/US 385 in the southeastern corner of the city.[11]

In 2021, SDDOT applied to have the route deleted, which was approved by AASHTO. The mainline of US 18 replaced the old bypass.[10]

U.S. Highway 18 Alternate (US 18 Alt) is a former segment of US 18 within Milwaukee.[4]