maybe we should look at the simple process of "evacuation" and be a little bit humble. Many Americans have become "creatures of the automobile". A lot of people hardly spend any time walking any significant distance at all.
I have a feeling that a lot of folks who tried to walk out of a major emergency zone ... might get into serious discomfort pretty quickly. At which point they would just kinda "shut down" and become stranded. Hence the modern day evacuation might turn into a very short exodus to the closest bedroom suburb (or town) beside the city that was affected. I pity the residents of those small towns and suburbs.
it seems to me that I vaguely recall a conversation a few years ago with someone who lived in a town just north of San Francisco. They said pretty much what I was hinting at above. The locals had some kind of plan to close off all the exits from the freeway, and compel refugees to keep moving ON - by force if necessary.
Pete2