It's next to impossible to live in Georgia without a vehicle. It just wasn't built that way. It's getting better, as Jeanette mentioned, there are other places inside the city that offer this.
That being said, once they added the marta train it made it infinitely easier to get to downtown if planned right. Most people COULD get around in Atlanta Proper without a car, but it would be difficult. In Alpharetta, it just wouldn't be plausible.