Lockouts whereas a lock in keeps someone in a space

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LOCKOUTS Whereas a lock-in keeps someone in a space or prevents an action until the desired operations have been done, a lockout prevents someone from entering a space that is dangerous, or prevents an event from occurring. A good example of a lockout is found in stairways of public buildings, at least in the United States (Figure 4.7). In cases of fire, people have a tendency to flee in panic, down the stairs, down, down, down, past the ground floor and into the basement, where they might be trapped. The solution (required by the fire laws) is not to allow simple passage from the ground floor to the basement. Lockouts are usually used for safety reasons. Thus, small children are protected by baby locks on cabinet doors, covers for electric outlets, and specialized caps on containers for drugs and toxic substances. The pin that prevents a fire extinguisher from being activated until it is removed is a lockout forcing function to prevent accidental discharge. FIGURE 4.7. A Lockout Forcing Function for Fire Exit. The gate, placed at the ground floor of stairways, prevents people who might be rushing down the stairs to escape a fire from continuing into the basement areas, where they might get trapped. four: Knowing What to Do: Constraints, Discoverability, and Feedback 145 Forcing functions can be a nuisance in normal usage. The result is that many people will deliberately disable the forcing function, thereby negating its safety feature. The clever designer has to minimize the nuisance value while retaining the safety feature of the forcing function that guards against the occasional tragedy.
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The gate in Figure 4.7 is a clever compromise: sufficient restraint to make people realize they are leaving the ground floor, but not enough of an impediment to normal behavior that people will prop open the gate. Other useful devices make use of a forcing function. In some public restrooms, a pull-down shelf is placed inconveniently on the wall just behind the cubicle door, held in a vertical position by a spring. You lower the shelf to the horizontal position, and the weight of a package or handbag keeps it there. The shelf s position is a forcing function. When the shelf is lowered, it blocks the door fully. So to get out of the cubicle, you have to remove whatever is on the shelf and raise it out of the way. Clever design. Conventions, Constraints, and Affordances In Chapter 1 we learned of the distinctions between affordances, perceived affordances, and signifiers. Affordances refer to the potential actions that are possible, but these are easily discoverable only if they are perceivable: perceived affordances. It is the signifier component of the perceived affordance that allows people to determine the possible actions. But how does one go from the perception of an affordance to understanding the potential action?
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