The "Mexico travel warning" published by the US government highlights certain places with increased violence due to the combat between cartels and the Mexican armed forces. It provides recommendations and details of recent violent cases affecting American citizens. It also states that millions of American citizens safely visit Mexico each year, and it provides some recommendations for a safe trip.
Here is a series of extracts from the Mexico travel warning that specify the places to avoid:
The US government has "urge US citizens to delay unnecessary travel to parts of Durango, Coahuila and Chihuahua states and advise U.S. citizens residing or traveling in those areas to exercise extreme caution."
"Large firefights have taken place in towns and cities across Mexico, but occur mostly in northern Mexico, including Ciudad Juarez, Tijuana, Chihuahua City, Nogales, Matamoros, Reynosa and Monterrey"
"Recently, the cities of Durango and Gomez Palacio in the state of Durango, and the area known as "La Laguna" in the state of Coahuila, which includes the city of Torreon, experienced sharp increases in violence."
"Robberies, homicides, petty thefts, and carjackings have all increased over the last year across Mexico, with notable spikes in Chihuahua, Sinaloa, and northern Baja California. Ciudad Juarez, Tijuana and Nogales are among the cities that have experienced public shootouts during daylight hours in shopping centers and other public venues."
In Summary it recommends to avoid the US-Mexico border cities and states.
Other locations including beach resorts and inner Mexico colonial cities are perfectly safe.
Mexico City is another special case where extra precautions need to be taken. Here are some recommendations:
Do not walk at night in none tourist areas.
Do not take a taxi on the street. Take one from the hotel and ask the hotel to give you a number to call for picking you up. If you must take a taxi on the street, there are taxis that have a base. In Spanish they are called "sitio" (See-tee-oh). There are hundreds of bases around the city.
Do not carry jewlery on the street
Carry some money but not all of your money, and just one or two credit cards.
In general just by using common sense you will have a wonderful experience.