Arizona's Interstate 19 stands apart from other freeways

Interstate 19 sign

        Interstate 19 that connects Tucson with Nogales at the border with Mexico isn’t your normal interstate freeway.

For starters, I-19 is the fourth shortest double-digit freeway in the interstate system in the Lower 48 states, running just over 63 miles long.

Secondly, the distances on “mile” posts and freeway signs use the metric system, so they’re in meters and kilometers rather than miles. When the freeway was built, the United States was considering adopting the metric system and signs have just remained that way over the decades. It’s the only interstate in the system with metric signage.

I-19 follows the route of U.S. Highway 89, which once stretched down the United States, connecting the Canadian and Mexican borders.