Interstate 19, Interstate highway in Arizona, United States

Interstate 19 is a federal highway that stretches approximately 63 miles from Tucson to the Mexican border at Nogales. The route passes through both Pima County and Santa Cruz County, crossing relatively flat terrain with scattered development.

I-19 largely follows the former route of US 89 along the Santa Cruz River. In 1980, the Arizona Department of Transportation awarded a single contract to install metric signs, overlay customary-unit expressions with metric ones, and set kilometer posts, reflecting the push toward the metric system at the time of construction.

I-19's signs give distances in meters and kilometers, a legacy of the 1980 metric signing contract awarded by the Arizona Department of Transportation during the US push toward the metric system. Speed limits, however, remain posted in miles per hour, so drivers see both systems on the same route.

I-19 runs about 102 km from Nogales to Tucson, with two lanes in each direction except at the I-10 interchange, where there are four. Advance signs give distances in meters and kilometers, but speed limits stay in miles per hour. Southbound traffic continues into Heroica Nogales via state surface roads; the SR 189 interchange at exit 4 bypasses Nogales for freight heading to the Mariposa port of entry.

I-19 is the only US Interstate whose distance signs use metres and kilometres, a scheme ADOT installed in 1980 during the national push toward metrication. Exit numbers are still mile-based, so the sign for Valencia Road reads "EXIT 95" above "500 m". Speed limits remain in miles per hour.