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*!
I doubt there would have been time to fill up the buses with passengers twice. Travel time, yeah it could be done. But actually convincing people to get on the bus and go? I think one trip out was all that was feasible. The bigger issue to me is the fact those buses were there after the storm and could have almost immediately been put into use getting people out.