What would you say is the number one threat to lead to an end-of-the-world-like scenario? A terrorist attack? An EMP strike? A natural disaster? An economic collapse?

All of these are possibilities, but in each one, a thick population density will make it far worse. There's no denying that people panic when a crisis occurs, and that panic is only multiplied when more people are living closely to one another.

More people will be killed in a shorter period of time in the major cities, the roads will be clogged as people and families try to escape, and furthermore, just look at the other threats that we listed first. Many of them are directly connected to population density.

If an economic collapse were to occur, then urbanized cities would be simply unable to rebuild their economies as fast as more rural areas (with coal mining, logging, farmer's markets, etc.) could.

There are other factors that make certain areas in America unsafe and unsuitable for outlasting an apocalypse: