I just remembered a story my dad told me years ago, but I can no longer confirm. The officials have a bunker somewhere high and either on the mainland or a high point in the Keys. However, if the bridges go, stragglers are on their own. As a preteen, in the 70s, I asked about that. I plan to call my family in the area tonight. Perhaps I can confirm it for you, and for my own curiosity.
I’m not privy to information like that about the area. Could be, I just don’t know.
Good luck with the family though. I hope they’re on the move shortly.