I-19 is the only interstate highway in the United States marked in kilometers. In 1979, I-19 was marked in kilometers because the United States was considering changing from the English (miles) to the metric (kilometers) system.
In 2010, the Arizona Department of Transportation (ADOT) was planning to change the signs to miles. There is an ongoing debate on whether or not to keep the kiometer signs. Read here what the Arizona Department of Transportation says, opens a new window about the highway.
Sources:
Kilometers in Arizona? You bet!, opens a new window | Arizona Department of Transportation. September 29, 2020.
"I-19 set for shift to mileage signs." Arizona Daily Star. January 17, 2010. Page B1.
"Metric Interstate Divides Arizonans, opens a new window." New York Times web site. September 14, 2010.
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