In a measured episode of Half As Interesting, the frenzied narrator Sam from Wendover explains why a single stretch of Highway 19 moving south across the Arizona desert is marked in kilometers rather than miles. As a means of providing context, Sam recounted the history of the metric system, the rush to do away with the imperial system after British decolonization, the big errors made in converting imperial to metric (and vice-versa), the big push in the 1970s and 1980s for the US to move to the metric system, only to be opposed by then Congressman now Senator Chuck Grassley, who stated “forcing the American people to convert to the metric system goes against our democratic principle”. This sudden turnaround essentially forced the United States to go back to the imperial system, all except for Highway 19 from Tucson to Nogales.
I-19 is the only highway in the country to use exclusively metric units. They’ve thought about changing it since the great metric revolution never came, but all the exits are numbered off of how many kilometers they are from the border so they’d have to all change and then businesses would have to change what exit they say to get off on in their ads.