I've been wondering how there are THAT many zombies around in the first place. When humans die in this universe, they're generally either killed outright or straight-up devoured - either way there isn't anything that can reanimate. Likewise, when you die, it can take minutes to many hours to reanimate, during which time your corpse is a ready-made buffet for the walkers. Where are the undead coming from?
I'm guessing that during the initial outbreak there would have been enough people getting bitten and not treated because no one knew how to deal with it. When things got really bad, lots of people would have been bitten, run off, and then died; there wouldn't yet be hordes of the undead wandering around to eat you up.
In the firmly post-apocalypse world, there would be far ewer "new" zombies reanimating after an attack. When a settlement or group is over-run, it's generally because there were far too many walkers to deal with; the victims would thus be brain food.
As for the Red Zone, we know that there are herds of walkers roaming around. Perhaps if they're mostly originating from the cities, I figure that topography would be a factor. Inasmuch as water will flow according to gravity, maybe the walkers' aimless wanderings will force them into valleys or clearings by simple physics of motion and tendancy to move in less obstructed areas, unless stimulated by some external force (food, etc.). Thus, after months of stumbling about, the walkers would generally end up in certain areas more than others. There would certainly be outliers (such as the group Rick and company ran into this week, a mere mile from Woodsbury), but overall there may be more dangerous areas simply because the geography of the area makes walkers stumble into one place more than others.
That, or whoever came up with the Red Zone was spouting pure BS in an attempt to apply some logic to the world and/or make the population of survivors feel better. Certainly not out of line with what we've seen so far.