In 1989 Arkansas and Louisiana jointly requested the specific designation
US 425 for a route through the two states. AASHTO approved during their October meeting. So the roadgeek community's long-running questions have been:
- How did those states arrive at the number "425"?
- Why did AASHTO approve the number, despite the route having no relationship to US 25?
Prompted by Mapmikey's comment
in this thread (https://www.aaroads.com/forum/index.php?topic=27710.msg2602107#msg2602107), I attempted to investigate this from the viewpoint that the number 425 may have been the lowest number that was not in use as a highway designation in either state. Here is what I've found so far:
- Arkansas had a reason to propose a 4xx number (for details, please see the OP in the US 412 thread (https://www.aaroads.com/forum/index.php?topic=30983.0)).
- Louisiana had already gone way beyond the 4xx range of numbers.
- However, LA state highway 425 had been decommissioned in 1961, and was probably the lowest available 4xx at the time.
So I suspect what happened was something along these lines: AR approached LA and said, "We want a 4xx number for this route, so that we won't have to renumber any of our existing state highways." LA said, "We have 425 available; let's propose that."
So then, why did AASHTO approve it... especially in light of the fact that they had questioned the proposal for US 412 nine years earlier? Well, we don't yet know why AASHTO approved US 412 either. Possibly in both cases, Arkansas pointed out that they didn't want to renumber any existing routes, and convinced AASHTO that there was "no harm" in using either of those numbers (other than not fitting within the framework of previously-established numbering conventions).
A few years later (1994) Arkansas and Louisiana jointly requested another route, this time with the designation
US 427. In April AASHTO replied that the number 427 was "under investigation". In June they approved the corridor as a US route, but changed the designation to
US 371.
As far as where the number 427 came from: I suspect Arkansas chose it simply because it was the next odd number up from 425. It's unclear why Louisiana was ok with the number 427, considering that since 1955 they already had a highway designated LA 427... unless they weren't as concerned as Arkansas was about renumbering their routes. Both states did renumber their SH 371s later that year, for obvious reasons.
Even by 1989 the US numbering scheme was such a disaster it would be hardly an imposition to randomly dump 425 into it even without any relation to the actual US 25. So proposing something that does not cause any in-state confusion seems like a reasonable ask on the state's part, and I can't see it being rejected by an organization that has given up on correct numbering either.