U.S. Highway 789 (Proposed/Never Commissioned)
U.S. 789 was a number proposed for the Canada to Mexico Highway. Boosters wanted to route this new highway from Nogales, Arizona, north to Sweetgrass, Montana. Since the highway was to be routed along existing U.S. highways for the majority of its journey, an application for this route to be signed as a U.S. highway was denied by AASHTO. The remnant of U.S. 789 in Wyoming remains to this day as Wyoming 789.
Wyoming 789 was commissioned in 1954 as part of a multi-state route that traveled through Arizona, New Mexico, Colorado, Wyoming, and Montana. Since it was not to be U.S. 789, the individual departments of transportation assigned 789 as a state route in each state along the Canada to Mexico Highway. The route began at the International Border in Nogales, Arizona, then traveled north along U.S. 89 through Tucson to Florence. State Route 789 then turned east along U.S. 60-70 to Globe, then followed U.S. 60 northeast to Arizona 61 and U.S. 666. U.S. 666 and State Route 789 turned due north to meet U.S. 66 (now Interstate 40), which carried both routes to Gallup, New Mexico. State Route 789 continued north with U.S. 666 through Farmington into Cortez, Colorado. U.S. 160 and State Route 789 turned east to serve Durango.
At Durango, State Route 789 turned north again, this time via U.S. 550. At Montrose, State Route 789 followed U.S. 50 northwest to Grand Junction, then turned east again, this time via U.S. 6-24 (now Interstate 70). At Rifle, State Route 789 turned north along Colo. 13, which took State Route 789 to its present Wyoming routing at Baggs. State Route 789 is still designated through Wyoming today; see the routing section above. North of Frannie, State Route 789 continued into Montana via U.S. 310 to Laurel. State Route 789 turned east via U.S. 10-212 (now Interstate 90 and U.S. 212) into Billings. U.S. 87 and State Route 789 merged from Billings all the way to Great Falls, which brought SR-789 westward again. Then State Route 789 turned due north along U.S. 91 (now I-15) to its end at Sweetgrass, Montana.
Matt Salek researched State Route 789 to determined that State Route 789 was designated in 1954 as the Canada to Mexico Highway. Matt found a book written in 1976 entitled The High Road at the Colorado State Library. Here is an excerpt from that publication:
There was also a route from Dallas to Canada via Denver called the D.C.D. This followed the Great North and South Highway through Colorado, but reports indicate interest in this road lagged somewhat by the time it reached Montana. Years later, interest in a continuous north and south route was rekindled. A concerted drive resulted in signing U.S. 789 [now Colorado 789] which runs through far Western Colorado. This route was originally promoted as a Canada-to-Mexico Highway, running from Nogales, Arizona, to Sweet Grass, Montana.
Matt also indicates that on both the 1972 and 1980 Colorado Department of Highways (predecessor of Colorado DOT) maps, Route 789 is given special mention in the map legend. There is the symbol that is used on the map (circle with 789 in it), then next to it it says "Mexico-Canada Highway (through Colorado)". Also, the 789 designation is not exclusively used on any stretch of highway; it's carried concurrently with whatever the normal designation of the highway is (Colorado 13, U.S. 550, etc.). So Colorado didn't do what Wyoming did, renumbering the route to only be 789.
However, 789 is not shown on the 1955 Colorado map, but it appears on the 1954 Wyoming map. Matt and I think either Wyoming was premature or Colorado might have been a little late in adding Route 789 to their state maps.
For more, visit Matt's Colorado Highways page.