Chris Hardwick on Talking Dead said "The quarry of trapped Walkers answers what I've wondered, how Alexandria has survived so long"
Yet, Deanna alluded to the fact that area was mostly evacuated early on...less people = less undead. It was already answered.
I think it's more than that. Remember when Rick was talking to Morgan how these people have just been living since the Zombie Apocalypse started and that haven't had to survive. Unlike Woobdury, there hasn't been a lot of Walkers showing up at Alexandria...mainly because they've been getting trapped at the quarry. Guiding a herd of zombies is what they are doing in the comics, so it made sense to me for them to start doing it in the show at some point.