In 1975, the U.S. decided to align with the vast majority of other countries throughout the world and adopt the metric system. The Metric Conversion Act of 1975 created the U.S. Metric Board that was set to ease the public into metric usage, but after only seven years—thanks to serious public opposition—the board dissolved and the initiative fizzled out.
During those seven years the board made little progress towards conversion. The exception: signage along a stretch of Interstate 19 in Arizona that went up in 1980. Covering 100 kilometers (that’s 62 miles) from Tucson to Nogales, the signage marks distance in kilometers and meters. The speed limit signage was and is still marked in miles per hour, rather than kilometers per hour, at least keeping confusion to a minimum. (Easing into things and all that.)
The Arizona Department of Transportation has received pushback in returning the signage to miles, as the businesses along the route claim they would be more difficult to find. The affecting communities have the prerogative, and there is significant business that travels across the border from Mexico to spend money in these areas, so familiar metric signage makes it easier for those visitors to travel and locate destinations.
So while most of the U.S. is sticking to the imperial system, one stretch of highway has no intention of reverting from the metric system anytime soon. Like the old saying goes, give someone a centimeter and they take a kilometer.