The Effects of Commercial Electronic Variable Message Signs ...

The primary measure or dependent variable in this study is the frequency, direction, and duration
of driver eye glances, which serves as an indication of visual attention and distraction. The
fundamental hypothesis is that drivers have limited attention; they self-regulate their attention to
perform demanding tasks. In the case of the driving task, a certain proportion of their attention
needs to be concentrated on the roadway scene ahead. To the degree that eye glance behavior can
serve as a measure of visual attention, eye glances need to be concentrated on the roadway
ahead. If the frequency and duration of eye glances away from the roadway ahead exceed
accepted norms or criteria for keeping a driver’s eyes on the road, then driver safety may be
compromised. Thus, eye glance behavior is the primary dependent variable in the study. Eye
glance behavior has an intuitive connection to visual attention and is sensitive to subtle visual
search strategies, including those which are below the level of conscious awareness (see
section 2.7.2). Depending upon the type of eye glance measuring instrumentation selected, the
act of measuring eye glance behavior may prove to be a more or less significant distraction to the
driver in itself. This experimentally-induced artifact can be controlled by selecting a minimally
intrusive measurement method or by ensuring adequate adaptation to the instrumentation on the
part of the research participant.
This study includes another class of secondary dependent variables. These are safety surrogate
measures associated with driver errors and other measures of driver performance, such as speed
changes, headway, lane deviation, and traffic conflicts. These secondary variables can be
measured by instrumentation in the vehicle in terms of speed, acceleration, and lane position.
These secondary variables can also be directly observed and noted by the experimenter and/or
safety observer in the instrumented vehicle for later analysis in terms of sudden braking,
inadequate headway, swerving, and conflicts. Thus, events indicative of possible driver error or
other maladaptive behavior can be flagged by human observers. Also, for these events, only
objective vehicle performance data needs to be analyzed, saving considerable effort and expense
by eliminating the need to analyze large amounts of continuous vehicle performance data.
B.1.3 Advantages/Disadvantages
One advantage of this method is its ability to implement accurate eye-tracking measurements
which afford the opportunity to observe subtle and often unconscious eye movements. This
ability to measure unconscious eye movements correlates with unconscious distraction facilitates
incorporation of the notion of self-regulated attention into the experimental paradigm. When a
driver is attempting to concentrate on the roadway ahead, a distractor, which unconsciously
diverts attention away from the roadway against the driver’s will, may have a more severe safety
consequence than a distractor which can be maintained under conscious and voluntary control.
Thus, in addition to being able to measure distraction which is both conscious and voluntary,
accurate eye-tracking determinations have the potential to probe other phenomena, such as
unconscious and involuntary distraction as they relate to CEVMS exposure.
Another advantage of this method is the ability to structure driving scenarios to have an
appropriate number of CEVMS, standard billboard, and other visual stimuli all located on a
controlled course, which all research participants drive in a consistent manner. The ability to
choose and structure the test drive assures adequate and uniform exposure to CEVMS and other
relevant visual stimuli. The ability to exert experimental control is a valuable asset to this
method. It facilitates a clean and robust statistical analysis of the data because all of the
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