In Nogales, Arizona, the southern terminus of I-19 is adjacent to the international port of entry, and southbound travelers can continue into Nogales, Sonora and connect with the northern terminus of Mexico Federal Highway 15.
Nearly the entire route of I-19 follows or is adjacent to the routing of (former) U.S. Highway 89 and the Santa Cruz River, which flows northward from Mexico, through Tucson and eventually disperses into the desert between Tucson and Phoenix. Most of the time, much of the river is dry, but heavy storms can cause it to overflow its banks.
The official "completion" date of the I-19 segment between Tucson (km 100) and Green Valley, Arizona (actually Helmet Peak Road at km 75) was February 12, 1972. A 1978 Project Report for the Arizona Department of Transportation lists entire I-19 project as "completed," which includes segments between Green Valley and Nogales, Arizona.
Interstate 19 is unique amongst US Interstates, because signed distances are given in meters (hundreds or thousands as distance-to-exit indications) or kilometers (as distance-to-destination indications), and not miles. Speed limit signs give speeds in miles-per-hour, however. According to the Arizona Department of Transportation (ADOT), metric signage was originally placed because of the "metric system push" in the United States at the time of the original construction of the highway.
Recently, the Arizona Department of Transportation announced that they would convert metric signs back to United States customary units in stages, replacing signs in specific areas of the freeway during construction projects in those areas. This is due to the high cost of replacing all the metric signs at once. New signs were put into place between Exit 99 (Ajo Way) and Exit 101 (Interstate 10) in 2004 after the completion of the new Interstate 10-Interstate 19 interchange.
A recent reconstruction project at the Interstate's northern terminus with Interstate 10 in Tucson, Arizona (at the interchange commonly called "The Crossing") was begun in 2002 and completed in August 2004.