Interesting how 40% of state capitals aren’t served by a 3-di
Well, some states are very small, and some quite large states have chosen to place their capital in something other than the largest city, or in some cases in places that can hardly even be called a city.
Austin is the second most populous state capital in the country and doesn't have a 3di.
Phoenix is the most populous state capital in the country and also doesn't have a 3di. It does have routes that elsewhere would be 3dis such as Loop 101, Loop 202, Loop 303, and AZ 51 but they're signed as state routes.
Phoenix did once have a signed I-510 that got partly absorbed into an I-10 reroute and otherwise renumbered as SR 51.
I-410 was also absorbed into the same I-10 reroute while the old I-10 became an extended I-17.
I always thought these were proposals and never officially approved or signed in the field.
I thought so too, but the
AARoads Phoenix guide suggests 510 was signed early on.