In the 80s the US made an effort to convert to the metric system. Road sign were changed to show both miles and kilometers, bank temperature displays had both F and C, and radio and tv stations gave the temperature in both F and C. I doubt the road signs are 25 to 30 years old but it must be some kind of holdover from then. Maybe they had signs made and stored and rather than buying new repalcements they put out new old stock.
Anyway, signs with miles and kilometers were quite common at one time. The metric system never caught on in the US. Too simple for us I guess, or maybe the fact that most of the rest of the world uses it, we insist on being different. You probably still see bank electronic time and temperature signs with both F and C. I'm with clsutton21, it's a much simpler system and I'd be all for using it.