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.
OK. Thanks for the tip about Napoli.
I found an article about it.
About 1/2 million people in the danger zone.
They didn't try a full scale evac and the results were just a wee bit dissapointing.
http://www.spiegel.de/international/spiegel/0,1518,445941,00.html
A more recent evac drill by the US marines there only involved 150 people.
So far it does not look good for any planned evacuation of large numbers of people.