At least six special routes of U.S. Route 18 have existed.
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Hot Springs bypass
U.S. Route 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. It skirts 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 ends at US 18/US 385 in the southeastern corner of the city.
Mason City business loop
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.
Marquette–McGregor business loop
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.