The metric system and a popular southern Arizona interstate

Here’s an aerial photo of what could be nearly any highway in Arizona in the 1970s.

But if you engage with our weekly Flashback Trivia questions on social media (Facebook and Twitter/X), you may know what highway we’re looking at and the exact location: This is a photo of Interstate-19 between Nogales and Tucson.

There’s something distinct about I-19. It’s the only interstate in the country to fully measure distance by kilometers and not miles.

In 1975, Congress passed the Metric Conversion Act, declaring the metric system as the preferred method of measurement in the United States. The act itself was merely a strong suggestion, however, and didn’t contain any compulsory provisions. That same year, President Gerald Ford established the U.S. Metric Board in hopes of making the act’s recommendations a reality.

Which brings us back to the photo above from the 1970s. I-19 was nearing completion in the 1970s and was used as a pilot for using kilometers. ADOT proposed converting Interstate 19 to mileposts around 2010, but this idea was dropped because of opposition from communities and businesses along the route.

So, if you want to get a metric system fix, take a trip along I-19, which measures 102 kilometers.