The Design of Everyday Things

Author:Unknown
Language: eng
Format: mobi
Published: 2014-06-16T13:25:57.218627+00:00

The Born Loser, May 11, 1986. Copyright © 1986 NEA Inc.

FIVE: To Err Is Human 133

time ago, the button that activated the starter motor was separate from

the ignition key, so that it was possible to attempt to start the car

without the keys; the error was made frequently. In most modern

automobiles, the starter switch is activated by turning the key—an

effective forcing function that makes you use the key to do the opera-

tion.

There is no analogous forcing function for removing the key upon

leaving the automobile. As we have already seen, those automobiles

that have door locks that can be operated only by a key (from outside

the vehicle) do introduce a forcing function: if you want to lock the

door you can't leave the key in the car. If a forcing function is really

desired, it is usually possible to find one, although at some cost for

normal behavior. It is important to think through the implications of

that cost—to decide whether people will deliberately disable the forc-

ing function.

The history of seatbelts in autos provides a good example. Despite

all the evidence that seatbelts are an effective means of saving lives,

some people dislike them enough that they refuse to wear them, proba-

bly because the perceived risk is so much less than the actual, statistical

risk. For a short period, the United States tried a forcing function on

seatbelts: a special interlock was installed on each new car. If the

driver's and passengers' belts were not fastened, the car would not start

(and a buzzer would sound). This forcing function was so disliked that

most drivers had their mechanics disconnect it. The law was quickly

changed.

There seemed to be three problems. First, many people did not want

to wear seatbelts, and they resented the mechanical forcing function.

Second, the forcing function couldn't distinguish legitimate cases in

which the seatbelt should not be buckled from illegitimate ones. Thus,

if you wanted to carry a package in the passenger's seat, the weight-

sensing element in the seat registered a person, so the car wouldn't start

unless the passenger seat's buckle was fastened. Third, the mechanisms

were not reliable, so they often failed— buzzing, stopping the engine,

and being an overall nuisance. Those people who couldn't figure out

how to disconnect the forcing function simply buckled the belts per-

manently, fastening the buckle when the seat was unoccupied and

stuffing it under the seat. So if a passenger really wanted to use the belt,

it couldn't be done. Moral: it isn't easy to force unwanted behavior

upon people. And if you are going to use a forcing function, make sure

134 The Design of Everyday Things

it works right, is reliable, and distinguishes legitimate violations from

illegitimate ones.

Forcing functions are the extreme case of strong constraints that

make it easy to discover erroneous behavior. Not every situation allows

such strong constraints to operate, but the general principle can be

extended to a wide variety of situations. In the field of safety engineer-

ing, forcing functions show up under other names, in particular as

specialized methods for the prevention of accidents. Three such meth-

ods are interlocks, lockins, and lockouts.

An interlock forces operations to take place in proper sequence (figure

5.4). Microwave ovens

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