U.S. Highway 12 (US 12) is an east–west United States Numbered Highway in the state of Montana. It extends approximately 597.576 miles (961.705 km) from the Idaho state line east to the North Dakota state line, the greatest distance that US 12 traverses through any state.

Quick Facts Route information, Length ...

U.S. Highway 12
Lewis and Clark Highway
US 12 highlighted in red
Route information
Maintained by MDT
Length	597.576 mi[1] (961.705 km)
Existed	1926–present
Tourist routes	Lewis and Clark Trail
Major junctions
West end	US 12 at the Idaho state line at Lolo Pass
Major intersections	US 93 from Lolo to Missoula I-90 from Missoula to Garrison I-15 in Helena US 287 from Helena to Townsend US 89 near White Sulphur Springs US 191 in Harlowton US 87 in Roundup I-94 from Forsyth to Miles City
East end	US 12 at the North Dakota state line east of Baker
Location
Country	United States
State	Montana
Counties	Missoula, Granite, Powell, Lewis and Clark, Broadwater, Meagher, Wheatland, Golden Valley, Musselshell, Rosebud, Custer, Fallon
Highway system
United States Numbered Highway System List Special Divided Montana Highway System Interstate US State Secondary
← MT 7 → MT 13

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US 12 in Montana has been defined as the Lewis and Clark Highway^[2] despite not being the route followed by Lewis and Clark across the state.

US 12 enters Montana at Lolo Pass, seven miles (11 km) southwest of Lolo Hot Springs in Lolo National Forest. After passing Lolo Peak to the south and traveling east for 33 miles (53 km), it meets with US 93 at Lolo and continues running concurrently northeast for 7.5 miles (12.1 km), where US 93 heads due north on Reserve Street, toward Kalispell and Glacier National Park. US 12 continues northeasterly through Missoula's downtown, eventually meeting Interstate 90 (I-90). It then overlaps I-90 for 69 miles (111 km), until Garrison, where it heads eastward toward Helena for 48.8 miles (78.5 km). This two-lane section of the trip passes through Avon and Elliston winding through Helena National Forest, over the Continental Divide at MacDonald Pass, and then through Montana's capital city, Helena.

On the east end of Helena, US 12 passes over I-15 at which point it joins US 287 south. US 12 overlaps US 287 and heads southeasterly, toward Townsend for 33.4 miles (53.8 km), where it splits from US 287, which heads south toward the intersection of I-90 near the town of Three Forks. US 12 heads eastward toward White Sulphur Springs for 42.2 miles (67.9 km); the route joins US 89 (which heads south toward Livingston and Yellowstone National Park) for 8.4 miles (13.5 km) before entering White Sulphur Springs and for another three miles (4.8 km) east of town. US 12 continues east for 54.4 miles (87.5 km) to Harlowton where it joins US 191 and share a 1.4-mile (2.3 km) concurrency through town. On the east end of Harlowton, the routes meet with Montana Highway 3 (MT 3), where US 191 and MT 3 share a common alignment and continue north to Lewistown and MT 3 joins US 12. US 12 and MT 3 continue east for 45.2 miles (72.7 km) before MT 3 heads south near Lavina and continues to Billings. US 12 continues east 21.4 miles (34.4 km) until the junction of US 87 and the two routes continue as a 2.1-mile (3.4 km) concurrency through Roundup. US 87 splits north and US 12 continues east on its own for 101 miles (163 km), until the junction with I-94 at Forsyth as a concurrency northeast for 45.8 miles (73.7 km) to Miles City. At the east exit for Miles City, US 12 splits again from I-94 and heads almost directly east to the North Dakota border at a distance of 92.4 miles (148.7 km).^[1][3]

This section needs expansion. You can help by adding to it. (June 2023)

US 12 had a proposal to be routed southwestward from Miles City through Forsyth, Hysham, Billings, Laurel, Rockvale, Red Lodge, and Cooke City, partially on the path of US 212. This path would have also taken the highway into Wyoming, with a second piece in Montana.^[4]

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