To discourage police from stopping tourists and demading a bribe, the Cancun government passed Article 152—a law that allows tourists two minor traffic violations with no fine.
When you rent a car, ask for the "Tarjeton Turistico" (Tourist Card—not to be confused with the tourist card you get from immigration). At the bottom of this card are two white boxes on a red background. If you are pulled over for a minor traffic offense, hand this card to the officer and he will mark one of the boxes, and you still get yet another mark before the officer will take your card. If you return your rental car without the card you will be charged (500 pesos or about $45us). This is your fine for not driving very well.
This is a fantastic program, but not all rental companies seem to be participating all of the time. Government agencies are working on the problem.