Civil Engineering Reference
In-Depth Information
typical of tasks that involve watching over long periods of time to detect intermittent and
unpredictable signals, which may happen once every half hour or so. It is a very
monotonous task and operators have a tendency to fall asleep on the job when it gets too
boring (reference). In addition, the job involves total isolation from coworkers, which
makes it even less stimulating. Many studies have been conducted to analyze vigilance
problems (see Figure 5.9).
Figure 5.9 shows that at the beginning of a task the vigilance level remains adequate
for some time, but starts dropping after 20 to 30 minutes. The loss in performance over
time is called the vigilance decrement, whereas the steady state level of vigilance
performance is called vigilance level. The problem with vigilance increases if the signal
has low strength; if there is spatial or temporal uncertainty, meaning that one cannot
predict where or when the signal will occur; and if only a few events happen in the
background.
EXAMPLE
At one time I worked for Human Factors Research, Inc., in Santa Barbara, U.S. This
company was famous for vigilance research, and the project described here was one of
many. It had been observed that security guards at some military installations in the U.S.
had a tendency to fall asleep at work. They were sitting all day long looking at video
images from security cameras that overlooked the fence and the entrance to the building.
Nothing ever happened, so it was a boring job. The objective of the study was twofold:
1. To demonstrate that people could actually break in unseen by the guards
2. To suggest solutions to solve the vigilance problems
As part of the study, my colleague Selz was given the task of breaking into this secure
environment without being observed by the security guards. He crawled along the fence,
found an opening, and eventually managed to get inside the building. At some time the
security guards looked at the display and noticed something moving—Ah! A groundhog!
(This became a new nickname for Selz.) The break-in was a dangerous undertaking; he
could have been caught and shot at by the security guards. By successfully breaking in,
Selz proved a very important point: vigilance decrements constitute a very serious
problem. The military organization in charge of these highly classified installations had
to come up with new ways of solving the vigilance problems.
To reduce vigilance decrements, one can do many things:
• Show examples of targets on the screen and thereby increase mental availability.
• Increase target salience—for example, the size of the target.
• Remove social isolation.
• Add irrelevant tasks to increase the physiological activation level, such as playing
games.
• Provide trial testing with feedback on hits, correct rejections, false alarms, and misses.
• Schedule work and rest periods so as to reduce fatigue. For example, there can be a
rotating job assignment so that operators spend at the most one hour at a time in front
of the screen.
