If an actual zombie outbreak were to happen in New York City, everyone would be ‘undead’ within 24 hours. However, researchers who estimated the zombie infection rate in America using the same method epidemiologists would for real diseases also found that people who live farther from big cities would have a whole month to prepare, with the safest place being the northern Rockies. Source Source 2 Source 3

This is a simulation of the rate/spread pattern: