Design of Everyday Things - Norman, Don [75]
A lockout device is one that prevents someone from entering a place that is dangerous, or prevents an event from occurring. A good example of a lockout occurs in stairways of public buildings, at least in the United States (figure 5.5). 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 are trapped. The solution (required by the fire laws) is not to allow simple passage from the ground floor to the basement.
In the building in which I work, at the ground floor the stairs seem to end, leading directly to the building’s exit door. To go down further requires finding a different door, opening it, and proceeding down the stairs. This safety feature is usually a nuisance: we have never had a fire, yet I frequently must go from a higher floor into the basement. It’s a minor nuisance, however, and it is worth the cost if it can save lives when there is a fire.
5.4 Use of an Interlock. The Nissan Stanza van was constructed with the access door for its fuel tank right in the path of the sliding passenger door (above). It could be dangerous for the door to be opened while someone was fueling the car. To overcome the problem, Nissan added a forcing function, a bar that prevented the sliding door from opening whenever the fuel tank was being filled. The bar is constructed in the form of an interlock: the cap to the fuel tank cannot be removed unless the bar is moved to its safety position (below). Furthermore, the fuel door cannot be shut again unless the bar is returned to its normal position. Finally, warning signals were added, so that if someone attempts to open the door during fueling, a buzzer sounds. All in all, a lot of effort was put into these forcing functions—which were needed only because of an unfortunate placement of the fuel tank access in the first place.
5.5 Lockout. A form of forcing function that prevents people from going down the stairs, past the ground floor, and into the basement. Although in normal times this is a nuisance, in times of fire, when people flee down the stairs in panic, the forcing function can save lives by preventing a mad dash into the basement. The bar encourages people to stop at the ground floor and leave the building.
Forcing functions almost always are a nuisance in normal usage. The clever designer has to minimize the nuisance value while retaining the safety, forcing-function mechanism, to guard against the occasional tragedy.
There are other useful devices that make use of a forcing function. In some public restrooms there’s a package shelf inconveniently placed 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 keeps it there. Why not supply a permanent shelf, always horizontal, placed so that it wouldn’t interfere with the opening of the door? There is room. A little thought reveals the answer: the shelf’s position is a forcing function. When the shelf is lowered, it blocks the door. So to get out of the cubicle, you have to remove whatever is on the shelf and raise it out of the way. And that forces you to remember your packages. Clever design.
It is common to forget items. Examples spring readily to mind:
• Making copies of a document, but leaving the original inside the machine and walking off with only the copy.
• Using a bank or credit card to withdraw money from an automatic teller machine, then walking off without the card. This was a frequent enough error that many machines now have a forcing function: you must remove the card before the money will be delivered. Of course, you then can walk off without your money, but this is less likely than forgetting the card because money is the goal of using the machine. The possibility exists so the forcing function isn’t perfect.
• Leaving a child behind at a rest stop during