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Bounding defaults in egress models



Egress model developers are in a difficult position. It is in their interest to develop models that are simplified representations of reality and at the same time reduce inadvertent misuse. While default values enable immediate (i.e. out-of-the-box) use of models without in-depth familiarization with modelling assumptions, defaults often represent optimistic and/or even unrealistic evacuee behaviours. In this paper, the term ‘default’ relates to a preset, fixed setting or value (or distribution) for a parameter or algorithm. Most egress models provide default values for five core behavioural elements: pre-evacuation time, travel speeds, route usage, route availability and flow conditions. These behavioural elements typically need to be represented in order for the model to function. The authors suggest that bounding default settings, rather than optimistic values, should be provided for each behavioural element. Here, a bounding default setting is a value derived from relevant empirical data that prolongs the overall evacuation time produced for a particular design. If a user wishes to decrease the conservative nature of a setting, he or she would be required to justify the modification of the bounding default value(s). This approach allows immediate model use but forces the user to modify the settings to obtain credible design scenarios. Copyright © 2013 John Wiley & Sons, Ltd.