The ancient Pompeii is much closer to Vesuvius than it is easy to gather from photos... 
The eruption took time, when the pyroclastic flow finally came down and hit the city, the city got buried quite quickly.
However, before the pyroclastic flow came down, many people had plenty of time to escape, and many others died because of respiratory problems, or hit by debris.<p>Key thing to bear in mind is that, back then, very few people would have had an idea of what was happening and how dangerous it was, even less so about the fact that there was an actual need to escape and what was a safe distance...
(Consider this: there is a lot of stuff falling from the sky and low visibility because of the dust in the air, would you run outside or seek cover insider?)<p>Here there is a more accurate and comprehensive description of events:
<a href="http:&#x2F;&#x2F;www.bbc.co.uk&#x2F;history&#x2F;ancient&#x2F;romans&#x2F;pompeii_portents_01.shtml" rel="nofollow">http:&#x2F;&#x2F;www.bbc.co.uk&#x2F;history&#x2F;ancient&#x2F;romans&#x2F;pompeii_portents...</a>