U.S. Route 12 in Montana

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

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

Route description
US 12 enters Montana at Lolo Pass, 7 mi southwest of Lolo Hot Springs in Lolo National Forest. After passing Lolo Peak to the south and traveling east for 33 mi, it meets with US 93 at Lolo and continues running concurrently northeast for 7.5 mi, 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 mi, until Garrison, where it heads eastward toward Helena for 48.8 mi. 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 mi, 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 mi; the route joins US 89 (which heads south toward Livingston and Yellowstone National Park) for 8.4 mi before entering White Sulphur Springs and for another 3 mi east of town. US 12 continues east for 54.4 mi to Harlowton where it joins US 191 and share a 1.4 mi 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 mi before MT 3 heads south near Lavina and continues to Billings. US 12 continues east 21.4 mi until the junction of US 87 and the two routes continue as a 2.1 mi concurrency through Roundup. US 87 splits north and US 12 continues east on its own for 101 mi, until the junction with I-94 at Forsyth as a concurrency northeast for 45.8 mi 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 mi.

History
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.