Naples (Italy, not Florida) being in the shadow of Vesuvius with 3.5 million people, has practiced city-wide evacs. In 2000? 2001? the entire city got "out" in three hours...got out to where, I'm not sure. Wish I had a citation for this but I just heard it from a guide.
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(posting this as someone that has unintentionally done a bunch of stupid stuff in the past and will again...)