http://www.uwec.edu/Geography/Ivogeler/w188/no.htm
This raises another aspect of the evacuation issue: if those who failed to evacuate did so because they were unable to, could not the city government have targeted those parts of the city which were disproportionately poor (who cares whether they are black, white, or purple with pink polka dots) where it might reasonably be assumed those who lacked the resources to remove themselves from the city would be clustered?
This strategy might have reduced the area the city needed to worry about, evacuation-wise, in the 24-48 hours before the storm hit. For all we know, between the now-imfamously unused school buses and the public transport buses all of those supposedly unable to evacuate themselves (and I have my doubts about that, see my earlier posts) could have been removed from the city in 4-5 round-trips.