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: