U.S. Route 12, also called U.S. Highway 12 (US 12), is a highway that runs east-west across the United States. It starts in Aberdeen, Washington, and ends in Detroit, Michigan, covering almost 2,500 miles (4,000 km). Although most of this highway has been replaced by Interstate 90 (I-90) and Interstate 94 (I-94), US 12 is still used as an important route for local and regional travel. The highway begins in Aberdeen at an intersection with US 101. In Detroit, it ends near the corner of Michigan and Cass avenues, close to Campus Martius Park.

Route description

The western starting point of US 12 is in Aberdeen. The highway partially follows the eastbound path of the Lewis and Clark Expedition between Wallula and Clarkston, which is why it is marked as part of the Lewis and Clark National Historic Trail. The eastern end of the highway in Washington is at Clarkston, where it crosses the Snake River into Idaho at Lewiston.

In Washington, the section of US 12, except where it runs with I-5, is defined in Washington Revised Code § 47.17.055.

US 12 enters Idaho from Clarkston, crossing the Snake River. It climbs the Clearwater River and runs with US 95 for seven miles (11 km). It becomes a two-lane road with signs that say "winding road next 99 miles [159 km]" and continues to Orofino. It follows the middle fork of the Clearwater River to Lowell, where it meets the Lochsa and Selway rivers. It travels up the Lochsa River to Lolo Pass at the Montana border. This part of the highway is also part of the Lewis and Clark National Historic Trail. Most of US 12 in Idaho is within Clearwater National Forest. The section from Lowell to Lolo Pass, which is remote and mountainous, was built in the early 1960s. This makes it the last U.S. Highway constructed. There are no services available between Lowell and Powell, about 70 miles (110 km) east.

In Montana, US 12 is called the Lewis and Clark Highway, even though it was not the route taken by Lewis and Clark. US 12 travels 598 miles (962 km) through Montana’s mountains and plains, which is the longest distance of any state. It enters Montana at Lolo Pass, seven miles (11 km) southwest of Lolo Hot Springs in Lolo National Forest. It passes Lolo Peak, travels east for 33 miles (53 km), and meets US 93 at Lolo. It runs with US 93 for 7.5 miles (12.1 km) before continuing northeast. US 12 passes through Missoula’s downtown and meets I-90. It overlaps with I-90 for 69 miles (111 km) until Garrison, where it heads east toward Helena for 48.8 miles (78.5 km). This two-lane section goes through Avon and Elliston, winds through Helena National Forest, crosses the Continental Divide at MacDonald Pass, and passes through Helena. US 12 crosses I-15 and joins US 287 south. It runs with US 287 for 33.4 miles (53.8 km) until Townsend, where it splits from US 287. US 12 continues east to White Sulphur Springs for 42.2 miles (67.9 km). It runs with US 89 for 8.4 miles (13.5 km) before entering White Sulphur Springs and for another three miles (4.8 km) east of town. US 89 turns north, and US 12 continues east for 233 miles (375 km) until meeting I-94 at Forsyth. It runs with I-94 for 45.8 miles (73.7 km) to Miles City. At the exit for Miles City, US 12 splits from I-94 and heads east to the North Dakota border, 92.4 miles (148.7 km) away.

US 12 is a two-lane road that runs 87.47 miles (140.77 km) through Adams, Bowman, and Slope counties in southwest North Dakota. The speed limit is 65 mph (105 km/h) on rural parts, with slower speeds in cities like Marmarth, Rhame, Bowman, Scranton, and Hettinger. US 12 meets US 85 in Bowman, and the two highways run together for a short time through the city.

US 12 enters South Dakota from North Dakota as a two-lane rural road about 10 miles (16 km) west-northwest of Lemmon before reaching the Standing Rock Indian Reservation. It runs parallel to North Dakota’s border for about 70 miles (110 km). At Walker, it turns southeast for 37 miles (60 km), crossing the Missouri River at Mobridge and leaving the reservation. It continues east for 18 miles (29 km) until meeting US 83 near Selby. It runs with US 83 for seven miles (11 km). After leaving US 83, it turns east and travels 80 miles (130 km) as a rural two-lane road. Near Aberdeen, it becomes an at-grade expressway. After meeting US 281, it returns to two lanes through Aberdeen and past Aberdeen Regional Airport before becoming a four-lane expressway again until two miles (3.2 km) before Waubay. East of Waubay, it becomes an at-grade expressway again until meeting I-29 near Summit. The speed limit from Aberdeen to I-29 is 70 mph (110 km/h), except in Groton, Webster, and Waubay. From there, it travels 22 miles (35 km) southeast to Milbank. At Milbank, it continues east for 12 miles (19 km) until crossing into Minnesota at Big Stone City, just south of Big Stone Lake. The South Dakota section of US 12 is legally defined in South Dakota Codified Laws § 31 April 132.

From the South Dakota–Minnesota state line at Ortonville to Wayzata, US 12 is mostly a two-lane rural road with a 60 mph (97 km/h) speed limit, slower speeds in towns, and a four-lane road in Willmar. From western Wayzata to I-394 in Minnetonka, US 12 is a six-lane freeway. East of I-494, US 12 runs invisibly with I-394 and I-94 through Minneapolis and Saint Paul to the Minnesota–Wisconsin state line at Hudson.

The Minnesota section of US 12 is defined as Routes 149, 26, and 10 in Minnesota Statutes §§ 161.115(80) and 161.114(2).

US 12 crosses into Wisconsin from Minnesota, running with I-94. It continues through Wisconsin and into Illinois, where it is an arterial road from Richmond to Des Plaines. It turns south, passing through the Chicago area, and joins US 45. In Stone Park, US 20 joins US 12/US 45. In Hickory Hills, US 45 continues south, while US 12/US 20 runs

History

Since the highway was created in 1926, the eastern end has always stayed near Cadillac Square in Downtown Detroit, Michigan.

• 1926: The highway ended at Cadillac Square, where US 10, US 16, US 25, and US 112 met. US 12 originally ran along Grand River and ended in Miles City, Montana.
• 1939: The American Association of State Highway Officials approved extending US 12 to Yellowstone National Park.
• 1956: US 12 was rerouted along the Lodge Freeway (M-10), ending on Jefferson Avenue. The end point moved four blocks southeast, to the corner of Woodward Avenue (M-1/US 10) and Jefferson Avenue.
• 1959: US 12 was extended to Missoula, Montana.
• 1962: After I-94 was completed across Michigan, US 12 no longer shared the same route as I-94. The US 12 designation was moved to the former route of US 112, which was no longer used. US 12 now runs along Michigan Avenue and ends at Cadillac Square. It was extended to Lewiston, Idaho.
• 1967: US 12 was extended to Aberdeen, Washington, where it now ends at US 101.
• 1970: US 10 was rerouted from Woodward Avenue to the Lodge Freeway and Jefferson Avenue. At this time, US 12 was extended along Woodward Avenue, ending again at Woodward and Jefferson avenues, though the route numbers were swapped from their 1956 arrangement.
• 2001: The City of Detroit and MDOT changed who managed the highway, moving the end point back four blocks to Cadillac Square.
• 2005: Another change moved the US 12 end point four blocks further, ending at the Patrick V. McNamara Federal Building on the corner of Michigan and Cass avenues.

The western end of the highway was gradually extended westward until it reached the Pacific Ocean.

In 1925, US 12 in Michigan was first planned to run from Detroit to Ludington, crossing Lake Michigan via a ferry to Manitowoc, Wisconsin, and continuing along what later became US 10 in those states. A 1974 reprint of the 1926 Rand McNally Road Atlas showed US 12 following the route that later became US 10 through Michigan, passing through Flint, Saginaw, Midland, and Clare on the way to Ludington.

US 12 was planned to enter Wyoming on the current route of US 212 from Mammoth Hot Springs to a point northwest of Clark before its route in Montana was changed. It was also planned to enter Oregon on the current route of US 730 from east-southeast of Boardman to a point northeast of Cold Springs, but this plan was canceled.

In the 1960s, part of US 12 in Western Washington was moved north to the town of Morton when the Mossyrock Dam was built on the Cowlitz River in Lewis County. In the 1980s, much of the old two-lane US 12 in the Yakima Valley was replaced by I-82 and I-182 between Yakima and the Tri-Cities, though the highways still share the US 12 designation. The old two-lane highway now is called Wine Country Road.