It is a well known secret that more than 10 percent of the victims of 9/11 did not die in the explosion - they voluntarily jumped. With the pictures still in memory, many office workers and high-rise residents in cities, such as New York, are reasonably worried that they may not get out in case of an incident. Other places, notably Tokyo, require public buildings, such as schools, to provide descenders or other means for emergency escape.
