The tool, created by Alexander A. Alemi and Matthew Bierbaum (based on paper they wrote with Christopher R. Myers and James P. Sethna) allows you set a “Kill to Bite Ratio” and the time it would take a zombie to shamble one mile. You can then set a starting point and see a graphical view of how fast the zombie virus might spread.
Currently the tool seems rather simplistic, showing only a regular geographic progression. Variables like highways and other transportation arteries aren’t represented and would likely, we think, speed the infection even more between urban centers. However, taken as a best-case scenario of the worst case scenario the tool is still pretty damn mesmerizing.