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NIST NCSTAR 1-7, WTC Investigation 1

EXECUTIVE SUMMARY FOR

OCCUPANT BEHAVIOR, EGRESS, AND EMERGENCY COMMUNICATIONS

E.1 OVERVIEW 

While most attention has properly focused on the nearly three thousand people who lost their lives at the 

World Trade Center (WTC) site on September 11, 2001, five times that many people successfully 

evacuated from the WTC towers due to heroic efforts of occupants, as well as emergency responders. 

Understanding why many, yet not all, survived the WTC attacks was one of the four objectives of the 

federal building and fire safety investigation of the WTC disaster led by the National Institute of 

Standards and Technology (NIST). 

Success in evacuating a building in an emergency can be characterized by two quantities: the time people 

needed to evacuate and the time available for them to do so. To the extent the first time exceeded the 

second, it follows that there will be casualties. When the second time exceeds the first, perhaps by some 

suitable margin, nearly all should be able to evacuate the building. 

For the WTC towers, the times available for escape were cataclysmically established by the collapses of 

the buildings. Those times were not known in advance by the building occupants or the responders. The 

times were also considerably shorter, by a factor of three or four, than the time needed to clear the tenant 

spaces of WTC 1 following the 1993 bombing and an additional factor of two shorter than the time 

needed to clear the last person from the elevators in the building. Further, some occupants would have 

been unable to evacuate the buildings given any amount of time due to injuries, entrapment, and/or toxic 

exposure. 

NIST examined the design of the building, the behavior of the people, and the evacuation process in detail 

to ascertain the factors that factored prominently in the time needed for evacuation. 

In order to accomplish this objective, numerous sources of data were collected and analyzed, including: 

over 1,000 new interviews with survivors; a collection of over 700 published interviews with 

WTC survivors; 9-1-1 emergency calls; transcripts of emergency communication among building 

personnel and emergency responders; historical building design drawings, memoranda, and calculations; 

building modifications and upgrades; formal complaints filed with Occupational Safety and Health 

Administration; and other relevant material. 

There were three forms of interviews with survivors: 803 telephone interviews, over 225 face-to-face 

interviews, and 6 focus groups. The telephone interviewees were randomly selected using independent 

proportionate stratification from a list of occupants who had badges to enter WTC 1 or WTC 2 on 

September 11, 2001. In other words, each occupant of a particular tower had an equal probability of 

being selected. Roughly 400 occupants in each tower were interviewed in order to achieve a high level of 

statistical precision within each tower. Reported percentages from tower-specific survey data (n=400) 

exhibited sampling errors no greater than 2.5 percentage points, and 95 percent confidence intervals of 

percentages are no greater than ± 5 percentage points. This level of precision was more than adequate for 

examining characteristics of occupants and egress attributes. With telephone interview results, primary 
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2 NIST NCSTAR 1-7, WTC Investigation

statistical analyses were in the form of tabulations and linear statistics (e.g., reporting of percentages and 

average/means). The telephone interview results enabled a scientific projection of the population and 

distribution of occupants in WTC 1 and WTC 2, as well as causal modeling and multivariate regression 

analysis to explore fundamental egress issues such as sources of evacuation delay. 

The objective of the face-to-face interviews was to gather first-hand accounts and observations of the 

activities and events inside the buildings on the morning of September 11. This approach identified 

unknown information, aided in the evaluation of technical hypotheses, and explored motivations for 

occupant behaviors, while allowing for comparisons to the telephone interview data. There was no 

recording of the face-to-face interviews, other than random selections, with consent of respondents, for 

quality control purposes. A typical face-to-face interview averaged approximately two hours. The 

methodology for the face-to-face interviews was a synthesis of two established methodologies, designed 

to assist survivors in providing comprehensive and accurate accounts of their evacuation, given the 

latency between experience and interview. Some groups of occupants were specifically sought in order to 

explore targeted unknowns. These included occupants near the floors of impact, witnesses to fireballs, 

mobility-challenged occupants, floor wardens, building personnel with emergency response 

responsibilities, family members who spoke to an occupant after 8:46:30 a.m., and occupants from 

regions of the building not addressed by other groups in order to ensure adequate interview coverage for 

all areas of both towers. 

Six focus groups were conducted in order to elicit accurate group representations of specific events or 

themes and complement the findings of the telephone and face-to-face interviews. The focus groups and 

the corresponding objectives were: 

1. Occupants located near the floors of impact: to explore the extent of the building damage and 

how the damage influenced the evacuation process. 

2. Floor wardens: to explore the implementation of the floor warden procedures and the effect 

those actions had on the evacuation of the occupants on a floor and the evacuation of the floor 

warden. 

3. Mobility-challenged occupants: to explore the effect of a disability on the evacuation of the 

occupant and any other individuals who may have assisted or otherwise been affected by the 

evacuee. 

4. Persons with building responsibilities: to capture the unique perspective of non-traditional 

occupants, including custodians, security, maintenance, or other building staff. 

5. Randomly selected evacuees in WTC 1: to further explore the variables from the causal 

modeling which best explained evacuation delay and normalized stairwell evacuation time, 

including environmental cues, floor, and activities. 

6. Randomly selected evacuees in WTC 2: to further explore variables used in the causal 

modeling that best explained evacuation delay, including environmental cues, floor, risk 

perception, and use of elevators. 
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NIST documented the WTC egress system, including the location of the three primary stairwells, exit 

doors, core hallways, transfer corridors, wall construction, location and layout of the 100+ elevators in 

each tower, and emergency communication devices. The design of the egress system was compared to 

building code requirements of the New York City Building Code, National Fire Protection 

Association 101 (Life Safety Code), and International Building Code. 

NIST documented the emergency procedures, both as they were planned to be carried out, as well as how 

they were actually implemented on September 11, 2001. The procedures included responsibilities for 

tenant safety through the floor warden system; pre-planned content of public address system 

announcements (which varied from public address system announcements made on September 11, 2001); 

responsibilities of the fire safety director, deputy fire safety director, building security, and supervisors of 

various contractors (including mechanical, vertical transportation, and electrical). Additionally, 

interaction among responding agencies such as the Port Authority of New York and New Jersey, the Port 

Authority Police Department, the New York City Police Department (NYPD), the New York City Fire 

Department, and contract security were documented. 

NIST estimates that there were 8,900 ± 750 people in WTC 1 at 8:46:30 a.m. on September 11, 2001. 

Similarly, NIST estimates that there were 8,540 ± 920 people inside WTC 2 at 8:46:30 a.m. New York 

City officially announced 2,749 fatalities at the WTC complex, including emergency responders, airplane 

passengers and crew (but not hijackers), and bystanders. NIST estimated that of the 17,400 ± 1,180 

occupants inside WTC 1 and WTC 2 at 8:46:30 a.m., 2,163 to 2,180 perished. No information could be 

found for 17 persons. More than twice as many occupants were killed in WTC 1 as WTC 2, largely due 

to the fact that occupants in WTC 2 used the 16 minutes between the attacks on WTC 1 and WTC 2 to 

begin evacuating, including the use of elevators by some occupants in WTC 2. 

The demographic characteristics of the evacuees was explored where the characteristics were relevant to 

the evacuation on September 11, 2001. Few differences in the characteristics of WTC 1 or WTC 2 were 

observed. Men outnumbered women roughly two to one. The average age was mid-forties. The mean 

length of employment at the WTC site was almost 6 years, while the median was 2 and 3 years for 

WTC 1 and 2, respectively. Sixteen percent of 2001 WTC evacuees were also present during the 1993 

bombing, although many other occupants were also knowledgeable about the 1993 evacuation. Twothirds of the occupants had participated in at least one fire drill during the 12 months immediately prior to 

September 11, 2001. Eighteen percent did not recall whether they had participated in a fire drill during 

that time period and 18 percent reported that they did not participate in a fire drill during that time period. 

In WTC 1, all three stairwells and the elevators were destroyed in the impact region, extending as low as 

floor 92. No occupant evacuated from above the 91st floor, although some survived until the building 

collapsed after 102 minutes. Helicopter rescue from the roof was considered by an NYPD aviation unit, 

but deemed not possible due to the heat and smoke from the building fire. Occupants of both towers 

delayed initiating their evacuation after WTC 1 was hit. In WTC 1, the median time to initiate evacuation 

was 3 minutes for occupants from the ground floor to floor 76, and 5 minutes for occupants near the 

impact region (floors 77 to 91). Occupants observed various types of impact indicators throughout the 

building, including wall, partition, and ceiling damage and fire and smoke conditions. The most severe 

damage was observed near the impact region, fatally trapping some occupants. Announcements in 

WTC 1 were not heard by the occupants, despite repeated attempts from the lobby fire command station 

to order an evacuation. Damage to critical communications hardware prevented announcement 
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4 NIST NCSTAR 1-7, WTC Investigation

transmission. Evacuation rates reached a peak, steady-state in approximately 5 minutes, and remained 

roughly constant until the collapse of WTC 2, when the rate in WTC 1 slowed to about one-fifth of the 

peak, steady-state. WTC 1 collapsed at 10:28:22 a.m., resulting in approximately 1,500 occupant deaths, 

107 of which were estimated to be below the 92nd floor. 

The evacuation of WTC 2 was markedly different from the evacuation of WTC 1. There was a 16 minute 

period after WTC 1 was attacked, but before WTC 2 was attacked. During this time period, occupants 

were forced to decide whether to remain inside WTC 2, and if they decided to leave, they had to choose 

between using one of the three stairwells or using an elevator. Further complicating this decision process 

were multiple, conflicting announcements around 9:00 a.m., first instructing occupants to return to their 

offices, and then within one minute of impact, instructing them to begin an evacuation if conditions on 

their floor warranted that decision. Over 90 percent of WTC 2 survivors started to evacuate the building 

prior to its being attacked. Sixteen percent of the survivors used elevators to evacuate. Approximately 

75 percent of the occupants who were above the 78th floor (the lowest floor of impact) descended to at 

least below the impact region prior to the attack on WTC 2. Over 40 percent of the survivors had left 

WTC 2 prior to 9:02:59 a.m. After WTC 2 was attacked, at least 18 individuals used Stairwell A, located 

in the northwest corner and furthest from the impact damage, to descend below the 78th floor to evacuate 

the building. Additional public address announcements were made after the airplane strike on WTC 2, 

although occupants who survived generally did not hear those announcements. After the initial peak in 

evacuation rate, the rate reached a steady-state similar to the rate observed in WTC 1 until approximately 

20 minutes prior to collapse of WTC 2. The evacuation rate during the final 20 minutes dropped 

significantly, likely due to a decreased number of occupants remaining in the egress system below the 

78th floor. NIST analysis indicated only 11 occupants initially below the 78th floor were killed when 

WTC 2 collapsed at 9:58:59 a.m. Overall, NIST estimated that 630 occupants of WTC 2 perished. 

Using the statistical power of the telephone interview results, causal models were constructed to explain 

both evacuation initiation delay and average stairwell travel time per floor. The factors that best predicted 

evacuation initiation delay in WTC 1 were (1) which floor the respondent was on when WTC 1 was 

attacked, (2) whether occupants encountered environmental cues, and (3) seeking additional information 

(or milling) about the nature of the event. In WTC 2, the same process occurred as in WTC 1, except that 

perceived risk (sense of immediate danger) was a predictor of seeking additional information (along with 

floor and environmental cues). Analyses explored factors that affected time spent in the stairwells in 

WTC 1 exiting the building. The floor an occupant was on when WTC 1 was attacked (distance to safety) 

increased the probability of encountering an environmental cue (smoke, damage, fire, etc). Additionally, 

being on a higher floor predicted greater evacuation initiation delay times and encountering 

environmental cues, which predicted higher normalized stairwell travel time. Independently, interrupting 

evacuation for any reason increased the normalized stairwell travel time. 

Constraints or aids to the evacuation progress were documented. Building announcements were cited by 

many in WTC 2 as a constraint to their evacuation, principally due to the 9:00 a.m. announcement 

instructing occupants to return to their work spaces. Crowdedness in the stairwells, firefighter 

counterflow, lack of instructions and information, as well as injured or disabled evacuees in the stairwells 

were the most frequently reported obstacles to evacuation. The most commonly mentioned forms of aid 

were assistance from coworkers and emergency responders and the photoluminescent markings in 

stairwells. Six percent of survivors in WTC 1 and WTC 2 reported a mobility challenge which slowed 

their evacuation. Sometimes the evacuation speed of others in the immediate area slowed down occupant 
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NIST NCSTAR 1-7, WTC Investigation 5

evacuation speed. Recent pre-existing injuries, medications, or medical treatments were the most 

commonly reported mobility challenges, while a small number used wheelchairs, were pregnant, or were 

elderly. A rest station for mobility-challenged occupants was established in WTC 1 somewhere between 

floors 12 and 20. Less than 10 minutes prior to the collapse of WTC 1, the occupants and helpers on the 

floor were ordered to evacuate, although it remains unclear whether all rest station residents survived. 

Minutes prior to the collapse of WTC 2, an NYPD Emergency Services Unit (ESU) officer radioed from a 

floor in the 20s to the outside that he was having trouble ascending the stairwell due to the large number 

of occupants descending (Interview 24 NYPD [NIST 2004]). As only seven occupants who started 

evacuating below the impact region were known not to have survived, among several possibilities, a large 

group of occupants from above the impact floors may have identified the passable stairwell (Stairwell A) 

and may have been making their way out of the building as it collapsed. 

Multiple evacuation models were used to simulate different WTC tower evacuations, subject to a number 

of assumptions. The goal of the modeling was to frame an understanding of actual evacuation findings on 

September 11, 2001. Simulations demonstrated that a phased evacuation (also known as defend-in-place, 

whereupon occupants on the fire floor and the immediately surrounding floors descend to three floors 

below the fire floor) would have taken between 4 minutes to complete (without delays in evacuation 

initiation) and 11 minutes to complete (with evacuation initiation delays between 0 and 10 minutes). 

Total evacuation of a tower assuming a full occupant load without visitors (19,800) would have required 

as few as 92 minutes to 112 minutes. With visitors (total population 25,500 people) total evacuation 

would have required as little as 114 minutes to 142 minutes. The ranges reflect two different model 

outputs, each assuming two different delay times (no delay and a 10 minute distribution of delay times). 

An evacuation simulation for 8,800 people (approximately the number present in each tower on 

September 11, 2001) in the absence of any damage to the building, would have required at least 

52 minutes to 71 minutes, depending on the model or the delay times. Finally, the EXODUS model was 

‘calibrated’ to approximate the gross evacuation rates observed in WTC 1 and WTC 2 on 

September 11, 2001. Once the model input necessary to approximate the observables was determined, 

additional occupants were added in order to estimate how many occupants might have been unable to 

evacuate on September 11, 2001 (given the damage to the building and observed delay times) if the 

buildings had had larger occupant loads. NIST estimated that approximately 14,000 occupants would 

have been unable to evacuate from WTC 1 and WTC 2 on September 11, 2001, had the starting building 

population been 19,800 in each building. 

E.2 REFERENCE 

NIST (National Institute of Standards and Technology). 2004. NIST WTC Emergency Responder 

Interview Data Set. Gaithersburg, MD.