He had at least 650 local buses which could have been used in an evacuation. Provisions for this would have to been made in adavance and in the city's evacuation plan, of course. The point is, these resources were left unused.
I estimate the total number of school buses in Orleans county to be at least 325 and the number of city buses to be about the same. The buses could not only have been used to remove thousands from the city before the storm, but they obviously could have already been used to make several subsequent trips to and from the downtown core for more evacuations.
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http://www.ersys.com/usa/22/2255000/ptranbus.htm
333 public transit buses
1996-1998 data
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http://www.doe.state.la.us/lde/scs/661.html
324 Orleans Parish school buses
02-03 data
Not to mention the fact that had it been asked I am 100% positive that other counties/states would have provided busses to help transport people out. Probably still would too.
650 buses at 48 people per bus is 31,200 people that could have been taken out per trip. If they had started Saturday afternoon, and had places for everybody (that's the other thing; once you actually start taking people away as opposed to "merely" ordering people to get out, you become responsible for them), that would be 2-3 trips.
And I'm not even counting the city buses that were used to bring people to the Superdome.
The evacuation plan seems to have been that those in a position of responsibility hoped it would turn out fine, and if not, they'd find someone to blame.