Swiss Cheese Model — Accident Causation Simulator

Defense Layers

Layers 4

Holes per layer 3

Hole size 0.32

Drift speed 0.15 rad/s

Hazard rate 2.0/s

Live Readout

Attempts

Blocked (near-miss)

Breakthroughs

Accident rate

0.0%

Theoretical alignment probability

0.0%

How it works

This is the Swiss Cheese Model of accident causation (James Reason, 1990). Each translucent plate is one independent line of defense — a procedure, a piece of equipment, a supervisor, a training standard. Every defense is imperfect and has weaknesses, the "holes". A hazard travelling through the system (the small spheres, flying along the depth axis) only turns into an accident if the holes in every layer happen to line up at the exact moment it arrives.

p_i  = min(1, N_i · π r_i² / A)      — hole coverage of layer i
P    = ∏ p_i   for i = 1..L          — chance all L layers align

N_i is the hole count and r_i the hole radius on layer i, A is the plate's usable area. The theoretical alignment probability above assumes the holes are independent and is compared live against the measured breakthrough rate (breakthroughs ÷ attempts) as hazards keep arriving.

Layers — more independent defenses shrink the alignment probability multiplicatively, even if each one is individually weak.
Holes per layer / Hole size — controls how porous a single defense is on its own.
Drift speed — holes slowly rotate around each plate, modeling how latent weaknesses shift over time (staff turnover, changing conditions, wear). A gap that was covered a moment ago can open up later — the core insight of the model is that alignment is usually transient, not permanent.
Hazard rate — how often a new hazard event is launched at the stack.

Cyan spheres are hazards in flight, amber flashes are breakthroughs (accidents), and dim red flashes are hazards absorbed by a layer that had no hole in the right place (near-misses, caught by the system).

Hazard in flight Breakthrough (accident) Blocked (near-miss)

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