AMC's The Walking Dead is back for its sixth season this weekend. If you're like me, this means you'll be spending your Sunday evenings gaming out intricate zombie survival scenarios in your own life: Would my home withstand a zombie invasion? How quickly would my town survive? Which of my dopey neighbors would be the first to get bit?
As a public service to others similarly zombie-obsessed, I've created a county-level index of survivability that answers the following question: How likely am I to survive a zombie apocalypse based on where I live? The index assumes that if you want to survive a zombie onslaught, you should live in a place that has the following characteristics:
- Low population density (less people = less zombies)
- Access to guns (self-explanatory, to be honest)
- People with military experience (veterans!)
- Terrain that's difficult for zombies to traverse (when's the last time you saw one scale a cliff?)
- Access to bodies of water (zombies can't swim, and you need water to survive)
Throwing these five variables into the Official Wonkblog Data Blender and hitting "frappé" gives us the following map:
Now, since I know that every. single. one of you will have a different opinion on the relative merits of each of these five variables, I've added sliders to let you set the importance yourselves! Think gun access is the thing you need most to survive? Crank it up! Don't really understand how living in the mountains will make you safer? Turn it down!
I'll discuss each variable in detail below, and at the end give I'll give you my own preferred specification -- feel free to disagree with me in the comments.
It's hard to overstate the importance of this variable. More people = more potential zombies, plus more bands of ragtag marauders fighting over precious resources following the initial onslaught. There's a reason why zombie movies are typically set in densely-populated urban areas and not in, say, the Yukon-Koyukuk census area of Alaska, where there's roughly one human every 26 miles or so.
Access to Guns
If you need to kill a zombie, nothing beats a trusty firearm. Unfortunately we don't have reliable county-level gun ownership rates. So as a proxy, I'm using firearm dealer licenses from the Bureau of Alcohol, Tobacco and Firearms. These include manufacturers, importers and dealers of guns and "other destructive devices." There are roughly 78,000 of these establishments in the U.S. A nearby gun store means a near-infinite supply of ammo when the zombie outbreak happens -- especially if you're in a low-population area.
It's not enough to just have guns, though -- you need to know how to use them. And for that, who better than a veteran? They have military training and perhaps combat experience as well, which will be invaluable when it's time to defend your town. Somebody with firearms experience can teach you how to use a gun and not, say, go shooting people willy-nilly in a panic.
You know what zombies can't do very well? Climb a mountain. In the event of an onslaught head to the high ground, build a bunker, and hunker down for the long haul. These numbers come from the USDA's Natural Amenities Index, which includes a scale of average county-level topographic variation ranging from flat lands to steep mountains.
Access to Water
Zombies can't swim! Although it's a bit of an open question what happens to a zombie if it wanders into the water. Does it sink? Float? Regardless, water is a type of terrain that you can traverse (especially with a boat) but which zombies can't. Build a fortress on a lakeshore and you only have to protect against undead sieges coming from one direction.
With all that said, here's how I'd set my priorities for a zombie invasion:
Population density is insanely important. I'd put access to guns and water secondarily. It would be great to have some vets around to teach me how not to shoot myself in the foot, and terrain, well, eh.
One thing you'll notice is that the overall contours of the map don't change much in response to small tweaks. And this makes sense: look, if you live in Manhattan (population density: 72,000 people per square mile) and the zombies come, you better have your will written. On the other hand, rural areas out west, where there are few people and lots of guns, seem to always rank highly no matter how you set your weights.
Believe it or not, a group of Cornell University researchers modeled the national spread of a zombie outbreak earlier this year. And their final assessment of zombie bite risk, also shows the lowest rates of infection in areas out west.
In all seriousness, of course, the only thing you need to know about a zombie outbreak is that zombies aren't actually real -- sleep tight, Manhattan! But they sure are fun to think about -- and according to the CDC, thinking through a zombie outbreak can make you better-prepared for a real natural disaster too.
Notes on Methodology
The specific data I used for the index is as follows:
Population numbers are 2014 Census estimates. Density was calculated as a function of county land area numbers maintained by the Census. For the final index, I log-transformed the population density figures to lessen the impact of a few extreme outliers on the overall curve.
Gun access is measured as number of firearm dealer licenses per 1,000 county residents, using August 2015 numbers from the Bureau of Alcohol, Tobacco and Firearms.
Veteran population is measured as the percent of county residents who have armed forces veteran status, according to the U.S. Census.
Landscape features are taken from the the USDA's Natural Amenities Scale. Values range from 1 to 21, where 1 represents flat plains and 21 represents high mountains.
Water access is measured as the percent of total county area that's covered by water, and comes from the Census.
For each indicator, I calculated each county's z-score within the range of overall values. This essentially measures how far above or below the average value each county ranks on that indicator. The range of z-scores for each indicator is multiplied by the weights set on the map sliders, and finally for each county all five adjusted z-scores are added to arrive at the final survivability value.