Trigger criteria are conditions fixed in advance that, when met, mean leave now with no further discussion, and each must fire early enough for the group to finish leaving before the hazard arrives. If leaving takes 6 minutes and the hazard approaches at a walking pace of 1.4 m/s, the trigger must fire when it is still 504 m away, which is beyond a 300 m outer zone, so either detect farther out or shorten the time you need.
Part of Escape Route Planning
Trigger criteria are the pre-agreed events or measurements, hard ones that require immediate evacuation and soft ones that require assessment, which start an evacuation without debate.
A Trigger Is A Deadline Worked Backward From The Hazard
The parent article says to decide in advance what triggers evacuation, because deciding under fire is too late. It lists immediate triggers: an uncontrollable structural fire, an armed group breaching the perimeter with force you cannot match, chemical or biological contamination, and natural disaster such as flood, landslide or an approaching wildfire. And planned triggers: resource depletion, a disease outbreak that cannot be managed in place, an escalating external threat, seasonal conditions beyond the shelter. The phase-0 article separates hard triggers (act at once, no discussion) from soft ones (assess, then decide): Escape Routes.
Which condition is on the list matters less than whether it fires early enough. That is arithmetic.
What You Need
- The time the group needs to be out: measured in drills, from Evacuation Procedures.
- The speed of each hazard: a walking person, a spreading fire, a rising flood.
- The distance at which you will detect it, from Outer Detection Zone.
- A named person for each kind of trigger with authority to call it.
Method: Work Backward From The Hazard
Step 1: Measure the time to leave. T_need = time to decide + time to grab and go + time to reach the point where you are no longer reachable.
Step 2: Estimate the hazard’s speed v.
Step 3: Compute the trigger distance D = v × T_need. The hazard must be farther than D from you when the trigger fires.
Step 4: Compare with the detection distance. If you only see the hazard closer than D, shorten T_need or detect it farther out.
Step 5: Write each trigger as a rule with a threshold, an owner and an action, and post it.
Step 6: For slow hazards, trigger on the reserve. Leave when the remaining supply falls below the time you need to reach the next source, plus a margin.
Worked Example: Fast And Slow Triggers
Armed approach on foot (example inputs). Decision by rule: 60 s. Grab the bag: 60 s. Walk to the initial rally point, 200 m at 3 km/h (50 m/min): 4 min = 240 s. T_need = 60 + 60 + 240 = 360 s = 6 min. A walker’s speed is 1.4 m/s (5 km/h, recalled from Tobler’s function, see Distance and Timing). D = 1.4 × 360 = 504 m.
The outer zone ends at 300 m, so a sighting at its edge leaves 300 ÷ 1.4 = 214 s = 3.6 min: not enough for the 6 min needed. Either detect at 504 m or more (a lookout with a long view, a forward post) or cut T_need: a decision made earlier, a bag grabbed in less than 60 s, or a muster point closer than 200 m.
Slow hazard: water. Suppose 8 people use 2 L each per day (the corpus’s survival minimum) = 16 L a day, with 224 L stored, which is 14 days. The trip to the destination takes 1 day, and you want a margin of 3 days: trigger when the remaining water falls below (1 + 3) × 16 = 64 L. At 14 days’ stock, the trigger fires after 224 - 64 = 160 L has been used, which is 10 days.
Common Mistakes
- Waiting for certainty. A trigger set at “we are sure” fires after the deadline.
- A trigger nobody owns. Name the person.
- Punishing a false trigger. The next person will wait.
- Hard triggers needing a meeting. Hard means no meeting.
- Fire: fighting before leaving. I recall the fire-service rule that you fight only a small fire with your exit behind you; otherwise leave, and call it early.
Safety
Late departure is the commonest fatal error
Hesitation, collecting belongings and trying to fight a fire that has outgrown a bucket are the usual ways people die in evacuations. Leave on the trigger, take only the bag, close doors behind you if you can do so without delay, and never go back in. For chemical or smoke triggers, move across the wind, not downwind, and see Environmental Hazards for signs.
What’s Next
- Learn next: Movement Order — how the group moves once it has left
- Combine with: Danger Indicators — the signs that should raise the alert