That's not a terribly remarkable figure when you understand that an LSU study ("Hurricane Pam") done a couple of years before Katrina showed
more than 100,000 people living in the Greater New Orleans metropolitan area did not have independent means of transportation to leave the city in the event of a mandatory evacuation:
Most of us assume that it would be simply a matter of deciding whether or not to heed an evacuation order. For some, the ability to heed that order requires that they get some help -- maybe the public transportation that they've relied upon for most or all of their lives, or perhaps, hoping that they're not too late in calling one of the few friends who has a car and might have a little bit of extra space to squeeze in one more evacuee.
City and State leaders did a very poor job of providing public means to aid in the evacuation of the large number of people they knew would be otherwise unable to leave New Orleans, particularly given the late hour at which evacuations were ordered.
I don't know that a person is an "idiot" if he or she is too poor to own a car. The poverty issues in New Orleans are still another facet of Katrina that warrants exploration in other contexts. Katrina revealed what a lot of people had long known about that city -- that the chasm between wealthy and poor was immense and not closing.