The following quotes are part of a report, "National Review of Hurricane Evacuation Plans and Policies," prepared by the Louisiana State University Hurricane Center, which you can download here:
http://www.hurricane.lsu.edu/&EvacuationReview.pdf<snip>
"...The number of people
without access to transportation in New Or-
leans, has been estimated as high as 25 to 30
percent of the population. In addition to people
without vehicles, potential evacuees include the
indigent, elderly, prisoners, the infirm, and
tourists."
<snip>
"...Busing is the most common mode of
transportation for low mobility groups. To
transport people in busses, emergency man-
agement agencies have in the past contracted
with local transit authorities, school districts,
and tour operators, with varying levels of
success. Many heavily populated cities do not
have an adequate supply of busses to move all
low-mobility evacuees. "
<IMPORTANT SNIP>
"For example, about
250,000 residents of New Orleans (not includ-
ing tourists or special needs populations)
Low Mobility Groups and Use of Public Transit
have no means of private transportation..."
<AND HERE IS THE MOST IMPORTANT SNIP>
"...The
total number of busses in all of New Orleans
would provide only a fraction of the capacity
needed to transport all of these people. Thus,
Louisiana emergency management officials
plan to use any available alternative means of
transportation, including National Guard
vehicles. They also plan to open local shelters
and refuges of last resort for those not able to
evacuate."