When the US route system was conceived in the mid-1920s, one of the guidelines was that the longer, more important routes would be assigned 1- and 2-digit numbers. Given that, US 46 really seems like an outlier. At only 75 miles in length, it is the shortest of all 1- and 2-digit US routes. Not only that, but US 46 is entirely in one state (New Jersey). Why would AASHO consider a short, single-state highway to be worthy of a 2-digit US route designation? Historic documents in their route numbering archive reveal at least some of the answers. As it turns out, when US 46 was first proposed in the mid-1930s, it was envisioned as a route running between New York City and Cleveland, along the corridor shown on this map:
That map is based on the control points in a US 46 route log that AASHO sent to the affected states in August 1934. That highway would have been a much more respectable 500 miles in length, running through three states. A significant portion of the planned US 46 was to replace most of US 322 in western Pennsylvania and northeast Ohio. The following excerpt from an AASHO letter sent to NJDoT that same month suggests that, at the time, they essentially considered the US 46 proposal to be a done deal:
However, it would soon be revealed that not all parties were on board with that idea. Just a few days later, AASHO received correspondence from PennDoT which contained the following paragraph:
(Incidentally, the town of "Mauch Chunk" would be renamed to "Jim Thorpe" about 20 years later.) AASHO responded to PennDoT's objection by trying to persuade them to think in terms of interstate travel:
The three other states were (obviously) New Jersey and Ohio, and (presumably) New York (even though the US 46 designation would never actually enter New York, terminating instead at the state line in the middle of the George Washington Bridge). PennDoT's September response to AASHO contained the following paragraphs:
After that letter, evidently AASHO tabled the idea. In October 1934 an updated route list was sent out to each of the state DOTs. Upon review, NJDoT questioned why US 46 was not among the routes listed therein. Following was AASHO's response:
Similarly, Ohio DOT questioned why the route they expected to be redesignated as US 46 was still listed as US 322. Following was AASHO's December 1934 response:
Shortly thereafter (in 1935 or 1936) the US 46 designation was commissioned in New Jersey (along with a very short segment in Pennsylvania). I have not seen any correspondence related to that, but presumably NJDoT saw no reason why its portion of the route should not be established. And perhaps AASHO approved it with the hope that PennDoT would eventually reconsider extending it through their state. But as it turned out the US 46 designation was never extended any farther west than Portland PA, and in fact by 1953 it was truncated such that it no longer enters Pennsylvania at all. All that to explain how a number intended for a long, multi-state US route ended up being applied to a short, intra-state US route.