Introducing South of Tucson
From Tucson, the I-19 is a straight 60-mile shot south through the Santa Cruz River Valley to Nogales on the Mexican border. A historical trading route since pre-Hispanic times, the highway is unique in the US because distances are posted in kilometers – when it was built there was a strong push to go metric. Speed limits, however, are posted in miles!
Though not terribly scenic, I-19 is a ribbon of superb cultural sights with a bit of shopping thrown in the mix. It makes an excellent day trip from Tucson. If you don't want to backtrack, follow the prettier Hwys 82 and 83 to the I-10 for a 150-mile loop.