Starting from the southern terminus at kilometer post 0 (not mile post 0), I-19 initially heads briefly south then west on surface streets, navigating its way through the town of Nogales for before becoming an Interstate–grade freeway and making the turn to head north toward Tucson. It has interchanges with two other state highways near the southern end of the route, State Route 189 (SR 189) at exit 4 and SR 289 at exit 12. The interchange with SR 189 at exit 4 both serves to funnel traffic so as to bypass around Nogales, Arizona, and Nogales, Sonora, for travelers bound to or from Hermosillo or Mexico City, and provides for the continuous flow of freight and truck traffic through the larger Mariposa Port of Entry to Highway 15, which has its northern terminus at the US–Mexico border with SR 189 and its southern terminus away in Mexico City. After exiting Nogales to the north, I-19 passes near and around a series of sparsely-populated towns and retirement communities along the banks of the Santa Cruz River, including Rio Rico, Tubac, Amado, Green Valley, and Sahuarita. For several miles near Amado and Green Valley, the eastward view from I-19 provides scenic views of Madera Canyon and the Santa Rita Mountains in the Coronado National Forest.