Sometimes I like to think about the logistical challenges with a switch to metric. The one that always gives me pause is highway signs. Thinking about the monumental task of replacing every speed sign, distance sign, and mile marker across the country in any timely period makes my head hurt.
It could certainly be done, and is probably easier than I think with all the state DoTs working independently on it especially over time. We have a lot of road with a lot of signs.