Right, but if I'm not mistaken, Katrina wasn't declared a Cat 3 storm until 5 AM on Saturday. That meant they only had maybe 42 hours at most to get everyone out, according to the plan. And the plan said to let the lower parishes out first, so the city of New Orleans really didn't have much time at all.
I'm not sure a pre-hurricane bus evacuation was at all feasible. They MAY have been able to get ONE round of buses out, IF there was somewhere for them to go. So I can't really criticize Nagin for not mounting a pre-hurricane evacuation.
But some thought should have been given to keeping the buses and drivers in safe shelter, so they could get going as soon as the roads were clear.
Using properly implemented contraflow, commandeered trains, and all available buses, greater New Orleans could have been down to about 30K or less by the time Katrina arrived. There was enough time for the buses to make two trips each to Houston at *least*!