
Adaptive Decision-Making in Response to Crowd Formations
in buildingEXODUS
Given the importance of occupant behaviour on evacuation efficiency,
a new behavioural feature is under investigation for implementation into
the buildingEXODUS
evacuation
model. This feature concerns the response of occupants to exit selection
and re-direction. This behaviour is not simply pre-determined by the user
as part of the initialisation process, but involves the simulated occupant
taking decisions based on their previous experiences and the information
available to them. This information concerns the occupants prior knowledge
of the enclosure and line-of-sight information concerning queues at neighbouring
exits.
The manner in which occupants behave in response to crowd formations
is of fundamental importance to the success of an evacuation. Their
ability to ascertain the likelihood of extensive delays and possibly alter
their exit route accordingly is essential to the navigation process.
Occupants determine their choice of exit through examining a number of
factors. Initially, the occupant must be aware of the existence of an exit.
Obviously, inherent in this knowledge is the geometric layout of the enclosure
surrounding that exit. Occupant familiarity has long been seen as
of fundamental importance to the progress of an evacuation. Instead of
occupant’s heading towards the nearest exit – of which they may have no
prior knowledge - as would be assumed by the majority of present building
regulations, they are more likely to move towards other more distant exits
with which they have had previous experience and with which they feel more
confident.
The impact of familiarity upon the behaviour of the occupant is not
limited to exit usage. Horiuchi recorded the increased levels of confidence
which familiarity bred in occupants, allowing them to perform actions not
directly linked with speedy evacuation. Familiarity with the enclosure
may generate a level of confidence that allows the occupant to attempt
activities such as fire-fighting, delaying their response or attempting
to follow alternative, less direct routes. Although in the short term these
routes may not be considered optimal, they will have been adopted through
calculation on the occupant’s part to guarantee safe egress and to minimise
the imminent risk and the evacuation time. It is unlikely that occupants
make decisions concerning redirection in isolation, but instead weigh up
the data available to arrive at a final decision. Influential factors
which are likely to affect this decision include the length of the queue
at any exit, the existence and severity of smoke and the distance to the
exit. The movement of the queue will in itself be dependent upon the geometry
(terrain, size of exit, etc.) as well as the make-up of the crowd. As a
survivor of the Beverly Hills Supper Club incident recalls,
“my dad told us that since the exit was so crowded to turn around
and climb over the railing and go out the entrance doors”
In this incident, before the occupant commits to another course of action,
he is considering information concerning the exit configuration and the
crowd conditions around each of the exits. Except for the occupant’s previous
familiarity with the enclosure, all of the factors require the occupant
to be in visual contact with the necessary environmental cues to make these
determinations. Therefore, the decision to redirect egress movement is
not solely based on factors determined prior to the evacuation, but is
likely to be influenced by dynamic factors such as population size and
environmental considerations.
The visibility of the exit determines the level of information that
the occupant may use in any calculation of the tenability of any future
use. For a thorough appraisal to take place, the occupant has to be in
visual contact with the exit, to examine the surrounding population, environmental
conditions, etc. If the exit is not within visual range, the occupant has
to rely solely on his recollection of exit details from memory, such as
position and distance, or possibly from information communicated to them
from the surrounding population or from a procedural influence such as
an intelligent alarm systems.
Finally, through examining these factors and their own experience, the
occupant must come to a decision on a course of action. This might involve
a crude determination of which route would enable the most ‘efficient’
and safest path of egress. As highlighted previously, this calculation
can only be made in respect to the information available to the occupant
and any previous experience that he might have. This represents the occupant
as being capable of information processing as described in recent psychology
literature.
The familiarity concept is introduced into EXODUS through the introduction
of the DOOR VECTOR. This is a list of exits known to the occupant.
Occupants are also provided with ‘line-of-sight’ information concerning
neighbouring exits. A complete ‘line-of-sight’ system would be computationally
expensive and complex to implement. An alternative method has been developed
to represent the ability of occupants to examine exits within their line-of-sight
and reflect upon the information gleaned. In this simplified system, the
user supplies the visibility status of exits. This is achieved through
grouping the exits according to which of them can be seen simultaneously.
The proposed adaptive queuing behaviour is reliant upon the introduction
of the door vector and the exit line-of-sight feature. Initially, the occupants’
situation is examined to determine whether he desires (i.e. estimated exit
time is reduced) and whether it is possible for him to alter his target.
This involves the examination of a number of factors including:
- the distance between the occupant and the exit,
- the extent of time occupant has spent waiting,
- the occupants patience level,
- whether occupant is completely surrounded by other occupants,
- the estimated time of arrival at the alternative exit,
- the estimated time of arrival at the current exit, etc.
As an example of how these factors influence an occupants decisions,
consider the following case. An evacuating occupant has waited in
an exit queue for a period of time greater than his patience level. He
is also sufficiently distant from the exit not to be committed to using
the exit. He is therefore now willing to consider redirection. The occupant
is situated on the periphery of the crowd and is therefore able to contemplate
redirection. Examining the exits available to him, he determines
that he will arrive at another exit more quickly. The occupant therefore,
moves off towards his new target. This decision making process contains
a stochastic element and will therefore alter between repeated simulation
runs.
There is of course no guarantee that recommitting to another exit will
produce a better outcome for the individual (i.e. decrease personal evacuation
time). Taking this course of action may result in a sub-optimal outcome
for the individual concerned. Furthermore, by allowing the occupant
to move to an unseen exit, the chances of the occupant delaying his evacuation
are greatly increased. This is due to the fact that the unseen exit
may in-fact not be viable due to the extent of crowding around the exit.
As the exit is unseen, the occupant is deprived of this information and
essentially takes a chance.
In the prototype implementation of this behaviour, the simulation is
shown to provide a more complex and arguably more realistic representation
of human behaviour than that provided by the existing model. Furthermore,
the implementation demonstrates the significance to both the evacuation
as a whole and the occupant as individuals of the inclusion of such behaviour.
If the occupant is able to utilise his ability to determine a more effective
route through the analysis of exit crowding, then the optimality of the
evacuation is increased. However, the capability of the occupant to switch
between available exits does not guarantee the reduction of individual
and total evacuation times.
For more information about evacuation modelling and the EXODUS software
visit the EXODUS Web Pages.
For a complete listing of EXODUS and evacuation publications visit the FSEG
Publications pages.
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