Apparently, they do plan for the unthinkable, Clint--From what I have read and heard about the design process--there was concern from the beginning that a plane crash would occur one day, and the design was intended to withstand a collision--

Kim-There are emergency escape plans, and it appears that, although many were lost, a very large number of people who were in the buildings did get out--even people who had been on floors above the point of impact-