Mass evacuations, themselves, can cause very significant issues, including loss of life.
It may be that holding off on issuing the evacuation orders is being done for strategic purposes, to help them get resources to assist with the evacuation in place, move more vulnerable populations out first, etc.
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For intelligent people, evacuation orders are superfluous.
Governments are, by my experience, not overly stocked with gray cells. I depend on them for absolutely nothing.
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The governor has already called out the National Guard to help in South Florida. Other counties have ordered evacuations. It’s just Miami/Dade. Tolls have been lifted, and probably shortly they will turn all four lanes of I-95, I-75, and the Sunshine Parkway to one way north.
If they wait too long, there will be traffic jams. These highways have massive traffic jams on a regular basis when there is a wreak. So everyone needs to start early.
Anyone in Miami who has lived there at least five years and/or has half a brain has already boarded up the house and hit the road north. Can the mayor protect against looters? Probably not.