Cancun Car Rental agencies see nothing but $$$ when you walk in to pick up your vehicle. After all, Americans are mooches and just love to throw money away.
Make sure you understand the contract and inspect the vehicle carefully, making note of every scratch and dent, missing windshield wipers, etc. Cancun car rental companies will try and charge for them when you return the car.
Avoid driving at night. Few roads are lit and many cars will be driving with their lights off. Often there is livestock on the roads and you may have to manoeuver around cows, horses, sleeping college students, etc.
If you come to a military checkpoint on the highway, be polite and you will soon be on your way. They are just maintaining a presence and checking for drugs, rocket launchers, beaver, and sometimes fresh foods (if you’re crossing state lines).
Cancun couple extra careful when driving in Cancun, the Riviera Maya, and Mexico in general. The police are looking for you (the tourist), and will stop you for no reason on occasion, hoping for a bribe. Refuse the intimidation unless you were indeed wrong, in which case paying off the cops is easier and probably less expensive than going through the system.
If you violate parking, the police will remove the license plate and you will have to go to the station to get it back (and pay a stiff fine). The car rental agency will, of course, charge you for any plateless vehicle you return.