Scientists are completing a major study of the behavior of World Trade Center survivors in a bid to make skyscrapers safer and easier to evacuate.
Key recommendations and suggestions include blaze-proof elevators, sky bridges linking high-rise buildings and curbs on mobile phones during evacuations.
"It would add tragedy to tragedy if we did not learn from 9/11," said the study's leader, Ed Galea of the University of Greenwich, England.
Galea and his team analyzed the written accounts of 250 survivors of Sept. 11, and found they show a startling variation in human behavior. Almost half those working on floors below where the planes struck took more than five minutes to begin leaving the building. Incredibly, five percent were still there more than an hour later.
"They sat at their computers while the building blazed," he said. "It is astonishing. We also found most people had long mobile phone conversations to relatives during their evacuation. Such behavior contrasts with the expectations of high-rise building designers. They assume people will exit in an orderly fashion in seconds of a fire alarm sounding."
The Sept. 11 disaster had three zones: the floors where the two planes struck; the floors above, and the floors below.
"Everyone on the impact floors ... was killed very quickly," Galea said.
Nearly all those in floors below the impact survived, however, while all but a handful of those above died -- because the crashed planes severed all elevators and each tower's three emergency staircases.
The staircases were clustered together and made of plasterboard. Had they been widely spaced and made of concrete, one staircase might have survived and those above the impact could have got out before the towers collapsed."
Using special software called Exodus, Galea also calculated how long it would have taken to evacuate the towers if they had been fully occupied.
"What happened on 9/11 was horrible, but it could have been a lot worse," he said.
"Each tower usually had about 25,000 occupants. However, Sept. 9 was local election day and many people were voting, it was the first day of school term and many parents were late coming in, and it was quite early in the morning. The towers were struck at 8:45am and 9:03am. As a result, there were only about 8,500 people in each tower," he said.
"Had it been later on a normal day, about 13,000 people would have died as opposed to the estimated 2,700 victims who died that day," he said.
In fact, the people on floors below the impact zones only just got out, Galea's calculations revealed. There were only a few minutes between the last occupants reaching the ground and each tower's collapse. Many refused to leave their desks, a situation that could have been improved if senior executives, and other figures of authority, had been recruited as fire marshals.
In addition, as people descended the narrow staircases, firemen were coming up, carrying equipment, causing delay though, according to simulations, not enough to cause loss of life.
The report warns that many high rise buildings are currently not designed to enable the complete evacuation of their occupants.
"We can no longer tolerate that attitude," Galea said. "That is the real lesson of 9/11. We have to find ways to all people out."