The only American interstate highway that measures its distances in kilometers, not miles

Countries & Cities · Published 15 October 2026 Swipe more like this →

In short: Interstate 19 in Arizona is a structural anomaly. While most US highways rely on customary units, this 102-kilometer stretch uses the metric system for its signage. Originally intended to align with a national push for metrication in the 1970s, the highway remains a unique, if confusing, relic of that era.

Interstate 19 (I-19) serves as a vital trade artery, connecting Tucson to the Mexican border at Nogales. Spanning just over 102 kilometers, it is the ninth-shortest primary interstate in the contiguous United States. Despite its modest length, the route is a critical component of the CANAMEX Corridor, facilitating freight movement from Mexico through Arizona and toward Canada. The highway follows the historical path of US Route 89 and runs alongside the Santa Cruz River, a waterway that is frequently dry but prone to sudden flooding during heavy storms.

The decision to sign I-19 in metric units originated from a 1980 Arizona Department of Transportation contract, reflecting a broader, albeit short-lived, national interest in adopting the metric system. While speed limit signs were intended to display values in kilometers per hour, they were never fully implemented, leaving the highway with a strange hybrid of metric distance markers and customary speed limits. Even as the state attempted to standardize these signs in 1999, the underlying engineering plans remained dimensioned entirely in feet and inches.

By the early 2000s, the metric experiment began to fade. Motorist confusion prompted the Arizona Department of Transportation to announce a transition back to US customary units. This process has been gradual, occurring only as specific sections of the freeway undergo construction or renovation. Today, the project remains partially stalled due to local opposition from businesses that would face the logistical burden of updating their own directional signage. Consequently, I-19 stands as a persistent, if fading, reminder of a failed attempt to modernize American road infrastructure.

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