At least eight special routes of U.S. Route 18 (US 18) have existed.
U.S. Highway 18 Bypass (US 18 Byp.) is a bypass around the Hot Springs, South Dakota, city center. It begins on the west side of Hot Springs at an intersection with US 18 and travels in a southeasterly direction. The highway skirts the western and southern edges of the city and meets South Dakota Highway 71 close to the point where the route turns east. It ends at US 18/US 385 in the southeastern corner of the city.
U.S. Highway 18 Business (US 18 Bus.) begins at a diamond interchange with Interstate 35 (I-35) on the eastern edge of Clear Lake, Iowa. 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 eastâÂÂwest artery through Mason City. At Jefferson Avenue, US 18 Bus./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 Bus. turns to the south to follow US 65. The business loop rejoins its parent route south of the city limits.
U.S. Highway 18 Business (US 18 Bus.) began at an intersection with US 18 west of McGregor, Iowa. The route descended into the Mississippi River valley along the northern border of Pikes Peak State Park and then turned into McGregor. Through McGregor, US 18 Bus. headed to the northeast toward the Mississippi River. Along the river, the highway was parallel to the Dakota, Minnesota and Eastern Railroad. For , US 18 Bus. traveled a narrow strip of land between the river and the bluffs where it passed a riverboat casino. At Marquette, US 18 Bus. ended at the foot of the MarquetteâÂÂJoliet Bridge, which carries US 18 to Prairie du Chien, Wisconsin.
Business U.S. Highway 18 (Bus. US 18) is a former segment of US 18/US 151 between exits 65 and 69 within Mount Horeb, Wisconsin. The route is also shared by Bus. US 151, as well as parts of State Trunk Highway 78 (WIS 78) and County Trunk Road ID (CTH-ID).
Business U.S. Highway 18 (Bus. US 18) is a former segment of US 18/US 151 within Verona, Wisconsin. The route is also shared by Bus. US 151 and CTH-MV. Bus. US 18/Bus. US 151/CTH-MV begins at a pair of flyover interchanges with US 18/US 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 (CTH-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. Bus. US 18/Bus. US 151/CTH-MV ends east of Verona at exit 81 on US 18/US 151, but only with an eastbound onramp and westbound offramp.
Alternate U.S. Highway 18 (Alt. US 18) is a permanently signed detour route that begins in Verona, Wisconsin. 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.
Alternate U.S. Highway 18 (Alt. US 18) is a former segment of US 18 within Milwaukee, Wisconsin.