Disney has an excellent transportation system for guests to use. It's included in the cost of your stay and it is possible to spend an entire vacation at Disney without every needing a car. But sometimes there are good arguments for using a car.
Some of the best restaurants in Walt Disney World are housed inside the hotels. If you are planning to have meals at hotels other than your own, having a car is your best bet. The bus system does not travel between hotels, so you would have to take a bus to one of the parks or down town Disney, get off the bus and get in line for the bus for the hotel you want to visit and eat at. This takes a great deal of time and in this case, a car would make life easier.
Disney is one of the most handicapped accessible vacation destinations you'll ever find. People who have health issues that stop them from being able to walk around all day can still have a great time at the park using a motorized scooter--they can even get on the buses! This a great thing about Disney but the scooters take some time to load unto the buses securely and safely. If you are inpatient, renting a car is a better choice for you.
When you are done at the park, you know your car is waiting for you in the parking lot. Sure, it's a much longer walk than waiting on the buses but you are guaranteed that when you get to the car, you get to sit down--no more waiting in lines after a long day of waiting in lines. If you take the bus, you could have to wait a few minutes in line for a bus to arrive, and if too many people are in front of you, you may have to wait for a second bus.
The Disney transportation system will take you anywhere you want to go--inside The World. If you want to check out other spots in the Orlando area, you will need to rent a car.
When you ride the bus, the time of arrival is out of your hands. If you have dinner reservations at 7:00 at a restaurant on the opposite side of Disney's property from where you are staying, you will need to give yourself an hour and a half or so to get to where you need to be--especially if you have to transfer buses. With a car, you won't need to give yourself more than half an hour to get there on time.
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