I admit I'm not too up to date on katrina but...
didn't the news say, leave town there's a hurricane coming? Like 3 days before it hit... Levy or nature, there was still a warning that a dangerous condition would exist, regardless of crowding, there was still a reasonable ability to leave if one really wanted to... right?
on 9/11 the first hint they got that they needed to get out was "boom" which, for most, was wayy too late.
I'm sure someone will chime in here with some facts to squash my argument... but, this is my general take for a 3rd party onlooker's standpoint.