Driving in Mexico can be a convenient way for U.S. residents to see the country, and many visitors to Mexico cross the border in their own vehicles. But you will find the driving etiquette to be significantly different from that of "El Norte".
It's a good idea to watch the behavior of the vehicles around you carefully - but don't necessarily follow their lead. If the vehicle ahead speeds through a stop sign without slowing - go ahead and stop anyway.
It's a good idea to drive at or below the speed limit, which will be shown (in most places) in the metric system. An easy conversion is that 100 km/h is approximately equal to 60 mph, so if you see 100 km/h posted, drive at around 55 mph (90 km/h). In the US it's common to "push" the speed limits; in Mexico it's advisable to hold back a little. This will mean driving through some small towns at 15 mph (25 km/h).
Have a reliable road map with you. GuiaRoji is a good option, but there are also others that are just as good if not better. Your first stop in Mexico should be to pick up a map. When planning your route in Mexico, remember that a straight line is not always the best way from point A to point B. cochera andina [dead link] provides information about road conditions, travel times for more than 240 routes in Mexico. If you haven't driven in Mexico before, and especially if you are not very fluent in Spanish, stick to the toll roads.
Much of Mexico is covered by modern "Cuota" toll roads, most of these are privately owned. They are generally much faster than the free "Libre" roads, if the latter run parallel, as the 'libre' roads will slow considerably as they pass through small towns and villages.
Using toll roads in Mexico is quite expensive: the tolls range from about 25 to 150 Mexican pesos (approximately 2.50-15 American dollars) for passenger cars, depending on the section of highway. If you are planning on making a long drive on toll roads, make sure you have plenty of Mexican pesos with you. U.S. dollars and credit cards are NOT accepted on many toll roads, though they may be accepted in some heavily-touristed areas (such as Highway 180 in Yucatan). The price goes up only if you're in an RV or towing something. Also, there can be several tollbooths between cities.
Even with the prices of using Mexican toll roads, some are not in conditions one would expect for the price you pay to use them. Some are in need of resurfacing, and will abuse your car if you travel the speed limit (which is normally 110 km/h or 65mph). It is a good idea to travel these sections of road under the speed limit to make sure your car makes it back up north without any suspension or other issues. If the conditions of the toll road cause damage to your vehicle (including blow-outs), insurance is included in the price of the toll (make sure to keep your toll payment receipt). This does not include an accident with another vehicle. (Your Mexican car insurance covers that.) Damage caused to your car has to be reported immediately at the next tollbooth you come to. It would be a good idea to take note of the "kilometer" marker where the incident happened, and have all of your papers ready (including toll receipt). If your only loss is a tire, and you're in a hurry, you may not want to bother making a claim since it can take some time. Having someone very fluent Spanish is also a strong recommendation, since most employees working the toll roads may not speak enough English to process a claim.
A plus for toll roads is that there are clean bathrooms and snack shops at most toll booths. It's a good opportunity to stretch your legs a bit, have a bite to eat, or visit the restroom before continuing on your way.
Once you get off the main highways, it's common to find potholes, dropoffs, dirt roads, and other hazards. The Mexicans brave these in standard passenger cars, but you'll probably be more comfortable in a higher-clearance vehicle such as an SUV, especially if you plan to visit hot springs, beaches, camping areas, or other off-the-beaten-path locations.
The Mexican Federal Police (Policia Federal or "Federales") patrol the roads. Unfortunately, the culture of "mordida" (the term for bribe in the local slang) still prevails, and as often as not the Federales may be willing to let you off with a warning in exchange for some folding money. However, do not by any means assume that the officer is expecting a bribe--the Police are well aware that it is illegal and rarely will ask directly, occasionally an officer might be offended or may even arrest you for offering. As in most places, courtesy and respect are most important. Knowing Spanish is also very useful in these situations, since the police often do not know English.
Insurance in Mexico is not mandatory but highly advisable, so don't leave home without it: it can be purchased online, or in offices near the border. Insurance should cost less than $10 per day. If you are in an accident without insurance, you could have trouble with police.
When buying Mexican auto or RV insurance it is important to compare. Good companies today include legal aid and some bundle medical air evac, which is superfluous, since medical care in Mexico is excellent. While the above quote of $10 is a general guideline, it is cheaper with any carrier to purchase a 6 month or annual policy if your trip covers more than 16 days.
Something else to consider is will the company fix your car in the United States as well as Mexico. Do they offer any trip advice? A company that was a few years ago may not be so good now. Also, if your trip is only for the Baja or Sonora, region-specific policies are a better deal. Beware of any company selling the cheapest policy. You get what you pay for.
The Mexican government operates a roadside assistance program called the "Angeles Verdes" or "Green Angels". The green trucks and their operators have the wherewithal to fix many debilitating automobile conditions. In fact, many travellers who have benefited from their services consider them miracle workers: did you know you can fix a leaking radiator with pepper or an egg? Services and information are free; parts or gasoline if necessary must be paid for.
You can call the Green Angels for assistance by dialling 078.
Topes and Vados
In Mexico, they're serious about speed control. "Topes" or speed bumps often consist of a large steel pipe with small asphalt ramps.
When driving cross-country, you will often encounter "vados" or dips. These are generally places where a stream or other feature crosses the road, and often they are severe - slow down and keep your eyes open. Cattle tend to congregate in vados.
When on steep, narrow or single lane roads, the downhill traffic has the right of way over uphill traffic. If you are going uphill and you see oncoming traffic, pull over to the side of the road, well out of the way, and either slow down or stop. If you are going downhill, use caution as the driver may not have read this.
Interesting point: from a practical standpoint, it is wise to keep an eye out for pullouts. The closest one may be behind you. This tip applies only to some really out-of-the-way roads, as there are few of these old roads left.
A practical tip for mountain driving is to keep your cool. If you get behind a big truck on a narrow 2 lane road, do not try to pass on blind curves. Be patient. Eventually, there will be a pullout or slightly wider section where it will be safe to pass. For this and other tips about driving in general see here. 
One situation that you will probably never see is if you see a sign with arrows pointing to the opposite side of the road (as well as arrows painted on the road), they are telling you to move to the OPPOSITE lane, that is, drive on the left. The idea is the car going uphill should move to the outside lane. It is scary, but if you don't, it could be disastrous. That said, the likelihood of most people to be on a road like this is pretty slim.
Expect checkpoints along most major and some minor roads manned by the Mexican military: mostly with automatic weapons. You will be asked for driver's licence and insurance information. Your vehicle will be searched, with varying degrees depending upon your attitude, your load, and how bored they are. They are looking for drugs or weapons, which you should not have with you.
They do not expect (or accept) bribes of any sort, but cold non-alcoholic beverages are often appreciated.
At military checkpoints, you will never be asked for identification, the army has no right to do so.
Driving at night
Many guidebooks and travellers advise against driving at night. This is excellent advice. When driving the "libre" roads at night you will likely encounter pedestrian traffic (people on the way to church every night), animals, slow moving traffic, road hazards and the occasional person who has fallen asleep on the warm road or is simply just sitting there. Driving at night, outside of a municipality, tourist area, or on a toll road is risky and dangerous. There are some Mexicans who (perhaps to save fuel) drive without lights; drunk driving is very common at night.
Driving on a toll road, or cuota, is considerably safer than on "libre" roads but can still be hazardous. These roads may be unlit and may sometimes be used by pedestrians and/or bicyclists. Use them with caution.
- If you're driving behind someone on a two-lane highway, and they signal a left turn without slowing down, they're indicating that it's safe for you to pass. The same signal from someone behind you may indicate they wish to pass.
- If someone driving toward you flashes their lights, it means there may be hazards ahead.
- If you are in a line of traffic on a two-lane federal highway, and the driver ahead of you turns on his hazards, SLOW DOWN quickly. Then you will want to leave some room between you and the car ahead of you.* Flashing hazards while driving means that there is some sort of danger ahead, or a fully-loaded semi is going extremely slow. You can also show this courtesy if you are the first car in line and you spot a situation that warrants slowing way down (i.e. cattle, debris, or other hazards in the road). Hit your brakes and reach for the hazards at the same time.
- Mexican semi-trucks ("camiones") drive fast and furiously. They will take as much of the road as they are able; give them as much room as you can.
- AAA and other travel magazines advise you not to stop for broken-down motorists because this is a common trap set by bandits who are out for your cash.
- If you are in an accident and someone is injured or killed, you may end up in jail, no matter whose "fault" it is. Have insurance. More often than not, Mexican liability theory operates from the starting assumption that one who hits pays.