A Review of Building Evacuation Models - NIST Virtual Library

When occupants are deciding which route to take or which exit to use, they choose the path with
the lowest score. If multiple paths have the same score, the occupants randomly chose between
them. STEPS uses an algorithm to score each Target for each individual, and this algorithm is
divided into 8 stages:
• Time needed to reach the target.
• Time needed to queue at the target.
• Adjustment of the walking time to take into account the time that is not actually walked to
reach the end of the queue.
• Calculation of the real time needed to reach the end of the queue.
• Adjustment of the queuing time to take into account the people that will get out while the
person is walking.
• Calculation of the real time to queue.
• Incorporate patience levels.
• Calculation of the final score
To calculate the time needed to reach the target, T walk , the distance to the target (D, obtained
from the potential table described above) is divided by the person’s walking speed (W, entered
by the user). This is shown in Equation (A.1).
T walk = D/W
(A.1)
The time needed to queue at the target (T queue ) divides the number of people that will reach the
target before the current person (N, by comparing the “time needed to reach the target” of the
current person with all others in the same plane) by the flow rate of the target (F, also specified
by the user in p/s). This is shown in Equation (A.2).
T queue = N/F
(A.2)
All occupants with a lower T walk are considered to be in front of the current person. Since T walk
gives the total time to walk to the target if there was no queuing, the additional of T walk and T queue
would give a larger evacuation time than needed for the occupant to reach the exit. The program
makes adjustments to these values, naming them “real time to walk” and “real time to queue.”
The “real time to walk” is found by subtracting off the time to walk through the area where the
queue has formed, resulting in the time to walk until reaching the end of the queue for that
current person. The queue time also has to be adjusted because while the person is walking to
the queue, others are leaving through the exit, reducing the queue. The “real time to queue” is
calculated by subtracting the time it takes for those occupants to leave through the exit before the
current person joins the queue. Patience coefficients are also factored into the score and
influence how long the occupant will wait in the queue. There are also walking and queuing
coefficients that are not quite explained in the users manual that also play a role in the score for
route choice.
The user specifies (or maintains the default values for) a number of attributes for the people,
such as body width, depth, and height, patience, walking speed, and their people type/group.
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