From the U.S. State Department:
Millions of U.S. citizens safely visit Mexico each year for study, tourism, and business, including more than 150,000 who cross the border every day. The Mexican government makes a considerable effort to protect U.S. citizens and other visitors to major tourist destinations, and there is no evidence that Transnational Criminal Organizations (TCOs) have targeted U.S. visitors and residents based on their nationality. Resort areas and tourist destinations in Mexico generally do not see the levels of drug-related violence and crime reported in the border region and in areas along major trafficking routes.
Nevertheless, U.S. travelers should be aware that the Mexican government has been engaged in an extensive effort to counter TCOs which engage in narcotics trafficking and other unlawful activities throughout Mexico. The TCOs themselves are engaged in a violent struggle to control drug trafficking routes and other criminal activity. As a result, crime and violence are serious problems throughout the country and can occur anywhere. U.S. citizens have fallen victim to TCO activity, including homicide, gun battles, kidnapping, carjacking and highway robbery.
San Luis Potosi: You should defer non-essential travel to the state of San Luis Potosi, except the city of San Luis Potosi, where you should exercise caution. The entire stretch of highway 57D in San Luis Potosi and portions of the state east of highway 57D towards Tamaulipas are particularly dangerous. In February 2011, one U.S. government employee was killed and another wounded when they were attacked in their U.S. government vehicle on Highway 57 near Santa Maria del Rio. Cartel violence and highway lawlessness are a continuing security concern. USG personnel may not frequent casinos, sport books, or other gambling establishments and adult entertainment establishments.
Click the State Department link below for more information.
U.S. State Department Warning