I am wondering if one reason that Nagin did not use the buses was because Blanko refused to allow state resources to be used. She also put a kink in the mandarory evacuation by not opening up enough shelters to house these people. I do remember Nagin mentioning that with a mandatory evacuation there had to be a place to put them.
That's where city/state cooperation and planning was crucial -- there needed to be inland shelters in schools and public buildings -- but if Gov. Blankhead ever thought of doing that she must have balked at the idea of trying to empty NOLA all over the state..... even though she had to KNOW that without a realistic evac plan the results would be just the sort of catastrophe we have seen. In fact, horrible as it was, this fell far short of the real potential catastrophe: Katrina could have remained a strong Category 5 and could have hit NOLA directly up the Mississippi and crushed the entire city immediately with a 25-30 foot storm surge. NOLA actually got 'only' a glancing blow compared to what could have happened, and 100,000 could have been dead due to the dereliction of the LOCAL and STATE officials.