That is a very valid point. I have often thought how I would get out of Phoenix, which is tough to get out of on a weekend. If there were an emergency requiring an evacuation, the roads would be almost impossible. Then again, it could be impractical to stay if things like power were out for very long, It sometimes gets up to 118 degrees and without air conditioning, survival is tough.
Walking is not a very good option either. Personally, my plan is to move out of the city to a small town where it is seldom over 90 degrees and there are plenty of places to camp, stay or survive.
I guess the good news on Phoenix is that we have not had hurricanes, tornados, major floods and there is not much to burn for very long. My reason for wanting to leave has more to do with being tired of the heat after 40 years and wanting to go someplace more interesting. I am retiring in a few years:)