An Evacuation Trigger Point Is a Spread Model Run Backwards From Warning Time
A trigger point is a line drawn on the ground such that when the fire reaches it, the time left before the fire arrives at the community equals the time the community needs to leave. Building one runs the argument backwards. First estimate the time needed: warning and notification, people gathering and preparing, then travel out along the roads that exist, timed for the slowest group rather than the average one. Then run a spread model from the fire toward the community and ask where the front was that many hours earlier; that location is the trigger. Two disciplines make it honest. Use a fast plausible run rather than the middle one, because being early costs an unnecessary evacuation while being late costs lives, and widen the buffer where the inputs are least certain. And recompute it when the weather changes, because a trigger point is not a distance — it is a time converted into a distance under one weather scenario, and a wind shift can put the fire past a line that was generous an hour ago. After this Concept you can construct and defend a trigger buffer from a spread run and an evacuation time estimate.
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A trigger point is a line drawn on the ground such that when the fire reaches it, the time left before the fire arrives at the community equals the time the community needs to leave. Building one runs the argument backwards. First estimate the time needed: warning and notification, people gathering and preparing, then travel out along the roads that exist, timed for the slowest group rather than the average one. Then run a spread model from the fire toward the community and ask where the front was that many hours earlier; that location is the trigger. Two disciplines make it honest. Use a fast plausible run rather than the middle one, because being early costs an unnecessary evacuation while being late costs lives, and widen the buffer where the inputs are least certain. And recompute it when the weather changes, because a trigger point is not a distance — it is a time converted into a distance under one weather scenario, and a wind shift can put the fire past a line that was generous an hour ago. After this Concept you can construct and defend a trigger buffer from a spread run and an evacuation time estimate.
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