. US12 didn't make it all time to the Pacific Coast. At US 10 the west end was there in Miles City MT. This looks west on US12/Business I-94. Regardless of signage, both designations persist to the left on Valley Drive which in turn used to be US10, hence US 12 likely must've ended at this particular junction.
. US 12 designation was extended to Silver Gate MT in the year 1939 at the north east entrance to Yellowstone National Park. This was co-signed with US 10 (roughly along today's I-90), through Billings from Forsyth MT.
. US 12 split off to follow the route which currently is designated US 212 at Laurel via Red Lodge, over Beartooth Pass, via Silver Gate, into Yellowstone.
. This was followed till 1959. And in that particular year US 10 broke off from US 10 at Forsyth to follow its current route via Helena and it ended up in Missoula at US 93, which came in from the south on Higgins Avenue and left town through the Broadway.
. These days westbound Business 12 / Business I-90, and the two routes split up here. Historically northbound US 93 were to the lead and the southbound was also to the left on Higgins and hence US 12 came west to this point and ended up here.
. Later in 1963 US 12 was extended west again and this time it was over Idaho which is roughly along with Lewis and Clark's historic trail to Lewiston.
. But this condition lasted just for four years and US 410 was decommissioned in the year 1967 and US 12 designation was extended west for the final time mostly along 410's previous route to Aberdeen, excluding for the segment amid Naches and Elma WA.
