That is true, and I believe especially true in New Orleans. Trips to the other gulf areas are more prone to automobile travel than is travel into and out of New Orleans. I do know that flights were cancelled long before the mayor decided that people should consider evacuation. And rental cars ran out quickly, leaving many travellers stranded.
Had the levees held on, New Orleans would not be in nearly the situation that it is now in. It would have, in the premature words of Shepard Smith, "dodged a bullet".
I've never been to NO but I imagine that by the time their officials said it was time to leave it was too late to try. The incompetence is breathtaking.