The "number-direction parity" concept in US highway route numbers

US highway numbering conventions stipulate that 2-digit ("main") US routes running generally north/south are to be assigned odd numbers, while even numbers go to east/west routes. This concept has been implemented with very few exceptions.

Apparently there has never been a similar guideline for numbering 3-digit ("branch") US routes - that is, a stipulation that north/south branch routes should be assigned odd numbers (so as to be considered branches of north/south main routes), and that even numbers should be given to east/west branch routes. Perhaps the reason is that in some cases there is no appropriate number available. For example, there are some east/west branch routes that connect only with north/south main routes. So they can't be considered branches of east/west routes, and an even number wouldn't make sense.

Despite the fact that there is no "official" guideline for numbering 3-digit branch routes, my question is: wherever possible, has the number-direction parity concept also been followed among 3dus routes?

I submit that, for the most part, it has - whether intentionally or by accident. I believe the maps below illustrate how most north/south 3dus routes have been assigned odd numbers, and in most cases even numbers have been given to east/west 3dus routes. You can decide for yourself - the maps below show:

current 2dus in purple
current 3dus (and some alternate routes) in light blue
decommissioned routes in red

If you notice any errors or omissions, please let me know


This map shows all odd-numbered 2dus and 3dus:

The map below shows all even-numbered 2dus and 3dus: