Part 1; the history of the US Route System over Targhee Pass
Targhee Pass is a 7,072 foot high mountain pass located in the Henry Lake Mountains (part of the Rocky Mountains) of Targhee National Forest. Targhee Pass is part of the Continental Divide and marks the boundary of the Idaho/Montana State Line. Historically Targhee Pass is most well known for the Nez Perce War during which Chief Joseph utilized the pass circa 1877 to evade the U.S. Cavalry.
The Henry Lake Mountains became relevant as a modern transportation corridor when West Yellowstone was plotted by the Oregon Short Line Railroad as it's eastern terminus during November of 1907. The Oregon Short Line Railroad subsequently completed it's line to West Yellowstone by June of 1908. The rail terminus of the Oregon Short Line Railroad in West Yellowstone was the head of a stage road via the Madison River into Yellowstone National Park. The Oregon Short Line Railroad bypassed Targhee Pass for the more favorable grades of nearby Reas Pass located southeast of modern US Route 20 ("US 20"). The Oregon Short Line Railroad can be seen terminating in West Yellowstone on the 1912 Rand McNally Map of Montana.
During the emergence of the automobile the Oregon Short Line Railroad was supplemented by a highway over Targhee Pass. The 1924 Rand McNally Map of Montana, Wyoming, Colorado, Idaho, Utah and Nevada shows the Banff-Grand Canyon National Highway and Salt Lake-Yellowstone Highway aligned over Targhee Pass.
The US Route System was created on November 11th, 1926 as a replacement for the preexisting Auto Trails. US 191 was selected to be aligned from the western entrance of Yellowstone National Park towards Idaho Falls via Targhee Pass. The 1927 National Company Map displays US 191 utilizing Targhee Pass and terminating at West Yellowstone via Idaho State Route 29.
US 191was extended north from West Yellowstone to Bozeman circa 1934. The extension of US 191 towards Bozeman left no US Route reaching the western entrance of Yellowstone National Park. US 191 can be seen on the 1937 Gousha Map of Idaho extending north of West Yellowstone through the Gallatin Mountains.
On October 30th, 1940 Idaho Department of Public Works acknowledged in a letter to the AASHO that US 20 had been extended through West Yellowstone via multiplex of US 191 to Sugar City. The AASHO description of US 20 had it jog west via Idaho State Route 28 to Sage Junction to reach US 91. The Idaho Department of Public Works requested US 20 be clarified to multiplex US 191 to Blackfoot (through Idaho Falls) to reach US 91 due to Idaho State Route 28 not being constructed to Sage Junction.
1944 State Farm Map of Idaho shows US 20/US 191 multiplexing from West Yellowstone to US 91 in Idaho Falls.