An Arizona highway has used the metric system since the 80s

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.