Who doesn't like a math pop quiz when driving down to the Mexican border? If you turn onto Interstate 19, which runs 63 miles (101 kilometers) from Tucson, Arizona, down to Mexico, the road signs suddenly change to metric units. Interstate 19 is a relic from a more optimistic era, when we believed all of the U.S. could and would convert to the metric system.


The great metric conversion obviously never happened, and it's now I-19 that stands out as anomaly. This is the only stretch of continuous highway in the U.S. where road signs, markers, and exit numbers are all counted off in kilometers.

That made sense in 1980, when I-19's signs first went up and when U.S. was near the peak of its flirtation with the metric system. Five years earlier, President Ford had signed the Metric Conversion Act, declaring the metric system "the preferred system of weights and measures for United States trade and commerce" and establishing United States Metric Board to guide the conversion. Schoolchildren dutifully learned their kilograms and centimeters.


But the Metric Conversion Act was only voluntary, and there was far too much inertia to change every single label in the country voluntarily. Reagan disbanded the Metric Board in 1982. Instead of leading the charge into brave new metric system, Arizona's highway is a reminder of a failed experiment.

To get a taste of how difficult it was to switch units, just consider the battle to switch I-19 back to miles. Arizona is currently trying again, after a failed attempt in 2010. Along I-19, 400 road signs would need to be replaced, which is no inconsiderable expense. And then there's hotels and restaurants that advertise their exit numbers (based on kilometers, remember), who have vociferously opposed the switch back to miles.

That's just for a single 63-mile stretch of highway—imagine that times 1,000 for the entire interstate highway system, times a bajillion for everything in the entire country. No wonder attempts to upend the unit status quo in the U.S. have failed. [AP, New York Times]

Top image: AP Photo/Astrid Galvan