U.S. 12 (Decommissioned)
In 1926, U.S. 12 was commissioned to run from Miles City, Montana, east to Detroit, Michigan. As more roads were built in the West, U.S. 12 was extended several times before it finally reached U.S. 101 in Aberdeen, Washington.
With the completion of the Beartooth Highway in 1936, U.S. 12 was extended from Miles City via U.S. 10 to Billings, then headed southwest along what is now U.S. 212 to end at the Silver Gate to Yellowstone National Park. U.S. 12 maintained this designation, which had it entering Wyoming, for over twenty years.
In 1960, U.S. 12 was rerouted via old U.S. 10N across central Montana. When this happened, what was once U.S. 12 via the Beartooth Highway was recommissioned as U.S. 312. U.S. 312 lasted as the designation of the Beartooth Highway for about two years; in 1962, U.S. 312 was redefined to run from Billings to Broadley via U.S. 10 and Montana 59.
It was that same year that the Beartooth Highway was recommissioned U.S. 212, a highway that previously ended near Custer Battlefield National Monument.
To this day, U.S. 12 does not enter Wyoming, and U.S. 312 has since ceased to exist.