Over the course of a few group trips to the Yucatan, our members have rented half a dozen vehicles, logged hundreds of miles on the road, paid bribes to local policemen, had to ask for roadside assistance, and experienced a few instances of theft. But, driving remains the preferred transportation due to the flexibility it offers, and we'll continue to rent cars on future trips. Knowing what to expect should make the experience much more pleasant.
When renting in the US, I typically shop around through online aggregators (Orbitz, Expedia) and directly with rental agency websites, trying different promotions to find the best price. Because in Mexico a large portion of your total cost lies in hidden fees, online quotes are nearly useless. The main undisclosed fee is the cost of mandatory liability insurance, which I've seen in the range of $10-$25/day. I found that the lower the quote you saw online, the more they will charge you for liability to get you to the price they'd really like to charge you. Unfortunately, liability coverage for rental vehicles is not provided as a credit card perk, isn't provided by your own auto policy, and there doesn't seem to be a good way to purchase it from a 3rd party.
If you're traveling during a busy season, it might make sense to make reservations at a number of different agencies so you have them available to you, but wait until you get to the CUN airport to make a decision (as there's no penalty for cancellation). Once at the airport, the booths are manned with salespersons from the agencies that will be able to give you the 'real' price. Then make a choice based on these quotes, not what you saw online.
You might save some money by declining collision (CDW) insurance and buying only liability, but some agencies combine the two, and charge the same fee whether you have liability only, or liability + CDW. If you have collision coverage through your credit card, it's usually in your interest to decline. Credit card insurance usually has no deductible, while I've been quoted $1500 deductibles for the rental agency CDWs. They'll usually try to discourage you from declining by forcing a larger authorization hold on your credit card, but I just accept this.
To be safe, document everything. Take pictures of the inside and outside of the car, spare tire, and all documents you sign. Perhaps also the engine compartment if you are paranoid. Try all the vehicle functions before you leave. Tip well, the guy who does the initial walkaround of the vehicle, and get him to liberally document any existing damage.
When in the US, it's a bad idea to talk, text, or otherwise use a phone while driving. It's an even worse idea in Mexico, as the roads are less well engineered for safety. Roads there leave a smaller margin of error, and hazards have little, poor, or no notice. Remember to keep an eye out for the speed bumps ('topes') that are throughout, including the highways. Driving at night is naturally more dangerous for all of these reasons combined with poor markings and lighting, but not an untenable risk as long as caution is exercised.
Even as a safe driver, it's not difficult to misinterpret signals and signs in a foreign country, accidentally break a traffic law, and possibly initiate an encounter with the police. This has happened several times to our group, and the pattern in each case was identical. Your license is requested, you are told of the infraction, and then a cash payment is demanded on the spot. The key is that the bribe is negotiable. The officer might name a fine in the neighborhood of $100-$300, but a reasonable amount for most infractions is in the range of $40-$80. One driver got away with paying $20 for a traffic light violation. What might help is to keep a very small amount of money in your wallet, and then another wallet in the trunk with the "rest of your money", which contains the remainder of what you hope to pay. Then offer it as all you have. Like a good salesman, the cop will look at your appearance to get an idea of what you're able to cough up, so don't dress too fancy out there. Bribes are an accepted custom, and there's no use in getting indignant about it, as this will only make the situation worse. Be contrite and make it appear you're willing to pay for your alleged misdeeds.
Note - the rental agency bumper stickers seemed like giant advertisements for "please pull me over and ask for a bribe". Not sure what to do about that, though. Maybe get some generic magnetic decals to put over them, next time.
The emergency number for the police is "066". The State Department website cautions that the police don't always answer, but it's good to know anyways.
"E" is the symbol for parking, so look out for the "No E" symbol to avoid parking tickets. Don't park where the curb is yellow, don't block driveways or hydrants, and in general follow the same rules as you would in the US. If you should get a ticket, you'll receive a slip on your windshield, and the police will take one of the car's license plates. To satisfy the ticket and retrieve the plate, visit the station in person. The penalties are way cheaper than in American cities - it was 300 pesos in one case, but the cost can easily double if the rental agency is forced to pay it and retrieve the plates on your behalf.
Like anywhere else, don't let belongings in plain sight. But in particular, *hubcaps* seem to be strangely popular targets. Although they should be covered by credit card damage policies, it might make sense to store them in the trunk while using the car. Who needs em, anyway?
Hope this helps those thinking of renting in Mexico. Public transportation is great as well (ADO, collectivo, cab), but there's nothing that beats the spontaneity of having a car available at moment's notice.Edited: 17 January 2014, 16:39