And the other thing to keep in mind is not whether New Orleans should order an evacuation whenever a hurricane is in the Gulf - but instead, why was there NO PLAN to get the people without cars and the marooned tourists OUT? You can't activate what isn't there.
It's the big question for everybody now. You need something that's a) already built; and b) available without high storage overhead.
It strikes me that the use of freight trains would answer the bill. All major cities have railyards. You can carry thousands of people on a single train, and you can also carry along pre-positioned containers of food, equipment, and temporary shelter.
Still gotta deal with getting the people actually to and onto the trains, though.