What is the only U.S. highway with road signs using the metric system?

The United States is one of only three nations not to use the metric system, but in the 1970s that almost changed. That decade saw the last major push to adopt the metric system in the U.S.

The Metric Conversion Act, which declared metric to be the preferred system of measurement in the United States, was passed by Congress in 1975. At that time, the U.S. Metric Board was created to implement the conversion, and America began testing road signs in kilometers under President Jimmy Carter, who supported efforts to go metric.

Interstate 19, which connects Tucson, Arizona, to Mexico, was one of the routes selected, and today it remains the only highway in America with distances posted only in kilometers. When the federal government started testing road signs in kilometers instead of miles, it made sense to include I-19. Although it is relatively short, it serves as an important corridor, linking the cities of Tucson, Arizona and Nogales, Mexico. The idea was to make the interstate more accessible (and understandable) to tourists coming from Mexico, where kilometers are the standard unit used to measure highway distances. For safety’s sake, speed limit signs were posted in both miles and kilometers.

The experiment was short-lived. The Metric Conversion Act and U.S. Metric Board were dismantled just seven years after they were created. However, the metric road signs remain on I-19, despite efforts to replace them. It seems the locals like them.