Some of the hotels closest to Cancun will arrange airport transfers. Expect to pay about $50 each way for Playa del Carmen and $125 for Tulum. But it's far more convenient to rent your own wheels, especially if you plan to explore nearby towns and ruins. There's plenty of free on-street parking in Playa del Carmen; everywhere else you'll be able to park at the hotels themselves.
When booking a car, be sure to choose one that can handle the punishing dirt roads. Highway 307 is the main artery that connects the towns of the Riviera Maya and Costa Maya. It runs south along the coast from Cancun to Tulum, swings inland to skirt the Sian Ka'an Biosphere, and continues all the way to the border of Belize. Often one-lane each way without a shoulder, the road leaves little margin for error. Allow more time than the distances suggest, because construction, accidents, or slow vehicles may lengthen the trip. So could the string of potholes passing for dirt roads that connect some hotels to the highway. A 10-mile drive on the worst of these can take 45 minutes.
Be on the lookout for speed bumps, called topes, which are found frequently in towns. Get gas whenever there's an opportunity--stations are few and far between. Finally, avoid driving at night. Road conditions, pedestrians, animals, and lack of lighting make it hazardous.
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