AOL has an article on Safety in Mexico fear factor 1 to 5.... 5 being safe.
CANCUN, THE RIVIERA MAYA AND COZUMEL
Fear Factor: 5
Cancun is one of Mexico's most popular beach resorts, which average around four million American visitors per year. Last year a retired Mexican general investigating corruption was assassinated by drug traffickers, but that's been an isolated event. Over-consumption of alcohol by younger tourists is a problem, and there have been rapes. But on the whole, Cancun is extremely safe. "The leading cause of foreign tourist deaths in Cancun is heart attacks, car accidents and accidental drowning," says Canadian writer Marlo-Renay Heresco, a Cancun resident who blogs about her life in Mexico on her website, atravelartist.com. "The key to success when traveling or living abroad is exercising common sense." The Riviera Maya (the Yucatan coast stretching south from Cancun) has little to fear beyond sunburn. The island of Cozumel off the Riviera Maya is a popular, very safe destination for cruise ships, where problems are the occasional purse-snatching or picked pocket.
MERIDA AND THE MAYAN RUINS IN YUCATAN
Fear Factor: 5
Although many people visit Chichen Itza on day-trips from Cancun, Merida is the gateway to comprehensive exploration of Uxmal and other significant Mayan ruins scattered across the state of Yucatan. Merida is a quiet, charming city, and the main ruins have well-organized tours and visitor's centers, as well as guards. In addition to hotels in Merida, the Yucatan has a number of colonial-era haciendas that have been converted into small resorts. Mayan villagers are welcoming. Here again, it's not a good idea to drive on unlighted roads at night, but central Merida's busy colonial-era streets are safe to stroll at night. "Mexico is a large country... deciding not to travel "to Mexico" because of violence is like saying you won't go to New York because of a murder in Denver," says Merida resident Ellen Fields. " Yes, there are places in Mexico where violence is on the rise. Where I live, Merida on the Yucatan Peninsula, and the nearby Mayan Riviera, has not seen this violence and is a very safe place to visit or to live."
Fear Factor: 4.75
No one thinks twice about visiting our nation's capital, and the same should go for Mexico's capital. In 2008, Mexico City had a homicide rate of nine for every 100,000 people, while Washington D.C. had a rate of more than 30 per 100,000 -- over three times higher. Visitors to Mexico City should exercise the same precautions taken in any of the world's big cities; sticking to busy, central areas and remaining aware of one's surroundings. It's very important to take only radio-issued taxis or taxis from official stands, never the "libre" (independent) or Volkswagen cabs, as there have been many instances of robbery and kidnapping. Don't walk at night except short distances on busy streets. This is an exciting city full of museums, art galleries and fabulous restaurants, not to be missed. Most people include a visit nearby to the majestic ruins of Teotihuacan, which are well patrolled and perfectly safe, with a visitor's center and organized guides.