There are some forty frontier posts along the US–Mexico border. Many of them are only open during the day, and are more or less inaccessible without your own transport. For a full list, see
wapps.cbp.gov/bwt. The main ones, open 24 hours a day, seven days a week, are, from west to east:
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San Diego, California (San Ysidro)–Tijuana, Baja California.
Calexico, California–Mexicali, Baja California.
Nogales, Arizona–Nogales, Sonora.
Douglas, Arizona–Agua Prieta, Sonora.
El Paso, Texas–Ciudad Juárez, Chihuahua.
Laredo, Texas–Nuevo Laredo, Tamaulipas.
Brownsville, Texas–Matamoros, Tamaulipas.
Crossing the border, especially on foot, it’s easy to go straight past the immigration and customs checks. There’s a free zone south of the frontier, and you can cross at will and stay for up to three days. If, as likely, you’re heading further south, however, be sure to stop at the Mexican immigration office, pay the M$294 entry fee (derecho de no inmigrante), pick up a tourist card (FMM), and get it stamped and your bags checked. Otherwise, you’ll be stopped after some 20km and sent back to complete the formalities. See “Entry requirements”, for more information.
The area bordering the US, particularly Ciudad Juárez, and also Tijuana, is the biggest hotspot in Mexico’s drugs war. Extra caution should be exercised when crossing this area, especially by car.