Here's a preliminary idea of where my thinking is going on this, using the U.S. as an example. Note that I'm leaving out usaif because I hate it and think we should delete it and usansf because it will go away.
Replace the numbered tiers with:
- USInterstates
- USBanneredInterstates
- USHighways
- USBanneredHighways
- USFreeways
- USPrimaryRegional (most state sets)
- USSecondaryRegional (some state sets like usamts, usatxf)
- USOthers (usanp,usaush)
Then separately define a hierarchy of these for ordering in tables and on maps.
Named tiers could have a default color, so they wouldn't all need to be in systems.csv. Users could override colors by named tier or individual system.
We wouldn't need to worry about what does "Tier 2" mean in Europe or Australia to be consistent with what "Tier 2" means in the US. Continents or countries or whatever is appropriate can define their own.
I especially like to have the ability to specify "all state level primary systems" in a meaningful way that wouldn't wind up bringing in or excluding, depending what I'm trying to do, some other systems that were arbitrarily assigned the same tier number.