Forcing Functions
A forcing function is the extreme form of physical constraint: failure at one stage prevents the next step from happening. (Example: a car needs a physical token — key/card — before it will start.) Three safety-engineering types:
Interlock
Forces operations into the proper sequence.
- Microwave/high-voltage devices cut power the instant the door/back is opened.
- Automatic transmission won't leave Park unless the brake is pressed.
- Dead man's switch ("driver's safety device") — equipment stops if the operator releases it (trains, chainsaws, mowers); variants resist being bypassed.
Lock-in
Keeps an operation active, preventing premature stopping.
- "Save your work?" prompt on exit (Norman uses it deliberately as a shortcut — an error message turned efficient).
- Literal: jail cells, playpens.
- Business lock-in: making your products interoperate only with each other to trap customers — "everyone loses except the dominant manufacturer."
Lockout
Prevents entering a dangerous space or an event from occurring.
- Stairway gate at the ground floor stops people fleeing a fire from running into the basement and getting trapped.
- Child-proof caps, outlet covers, fire-extinguisher pins.
Design caveat
Forcing functions are a nuisance in normal use, so people disable them (negating the safety benefit). The clever designer minimizes the nuisance while keeping the safety — e.g., the fire-stair gate is just enough restraint to signal "you're leaving the ground floor" without tempting people to prop it open.
Confidence
1.0 — explicit, enumerated.
Sources: ["DOET ch.4 'Constraints That Force the Desired Behavior' p.141-145"]