There are several border crossings, but most people cross at Nogales. Interstate 19 in Arizona travels south from Tucson and becomes Highway 15 beginning in Nogales, continues south through Hermosillo, and reaches the Mar de Cortés at Guaymas, 418 km (260 mi) from the Arizona border.
Owing to concerns about gang-related violence, some people are avoiding Nogales and instead taking Highway 85 from Phoenix or Highway 86 to Highway 85 from Tucson and crossing at Lukeville to the Mexican town of Sonoyta. From here Highway 8 runs to the coastal town of Puerto Peñasco; Highway 2 runs to Highway 15 and on to Hermosillo.
You no longer need a car permit (though you do need Mexico car insurance) unless you are traveling south of Ciudad Obregón. Permits cost $31 if you plan to travel elsewhere in Mexico. Make sure you check in with Mexican immigration to get your six-month tourist visa, which will cost $27.