Actually, if you're coming from Sanford Airport, I'd recommend taking the 417 (Central Florida Greenway) down to WDW. There should be signs out of the airport as this is a major highway. You should also be able to get directions from the airport to the 417 from the representative at the car rental desk. You'll pay more in tolls, but will hit considerably less traffic. Depending on what time you arrive, it could be a real b*tch getting through downtown on I-4. And with all you're paying for everything else on vacation, a couple more dollars for the 417 tolls will be well worth it.