"New Orleans Times-Picayune five-parter from 2002, "Washing Away," which reported that the city's 100,000 residents without private transportation were likely to be stranded by a big storm. In other words, what's happening is what was expected to happen: The poor didn't get out in time. "
Taken from
http://www.slate.com/id/2124688/nav/tap2/
The report cited in the article
http://www.nola.com/printer/printer.ssf?/washingaway/
I don't know if that accounts for most, but probably some people left in NO.