In February of 2012, the United States State Department updated its travel warning for Mexico. I will post excerpts of the advisory statement text below. The overall message of the advisory is to exercise extreme caution when traveling in northern Mexico especially.
For years, we have received travel advice on Mexico. Much of that advice applies to all areas of Mexico and is common sense advice.

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Travel on highways during the day
This is good advice for many different reasons. Many Mexican vehicles are not properly maintained. When they do break down, they do not always get moved to the shoulder of the road. This is usually our first thought in the United States, but not so in Mexico. There have been many accidents on Mexican roads at night where a vehicle has crashed into a disabled vehicle that doesn’t have its emergency lights flashing and has not been pulled off the road.
Another good reason to travel during the day on highways is because a lot of Mexican highways wind through mountains. If road markers and signs have been previously damaged or removed, it may be difficult to see the curves in the road. If you’re traveling too fast, it’s easy to lose control of your vehicle and maybe even go over a cliff.
Keep yourself and your valuables safe when vacationing
Avoid withdrawing money from an ATM machine after dark. People have been assaulted by thieves who wait in the shadows for someone to walk out of a bank ATM room. If you are in well-lit and well-traveled touristy areas, your chances of assault are less. If you are in a higher traffic tourist area, you may not need to worry as much about assault when visiting an ATM, but you do need to concern yourself with pickpockets.

When you are staying at a hotel, it’s a great idea to use the in-room safe that is commonly found in most Mexican hotels. These safes are usually free to use and bolted into the room’s closet. They are usually digital and you can program the code when you begin your stay at the hotel.
No matter where you are, it’s best to walk in a group after dark. Mexico is a very safe place to visit and enjoy. You really have no reason to feel paranoid or unsafe. However, that being said, you never know when something unexpected might happen. You can increase the odds of something not happening by being with a group of people.