For some bizarre reason, this intrastate Interstate is given metric measurements.
After writing that, Richard C. Moeur sent me the following email:
It's not bizarre at all that I-19 is signed in metric. One, when I-19 was signed in the late 70s, it seemed the whole USA was on the verge of going metric, and I-19 was just the start. Things happen, though. Two, I-19 travels (almost) directly to a country which is signed entirely in metric, Mexico. Therefore, signing I-19 in metric makes more sense that signing, oh I-238 in metric. ;)