The Effects of Commercial Electronic Variable Message Signs ...

method depends more heavily on secondary dependent variables. Safety surrogate measures
associated with driver errors and other measures of driver performance (headway, lane deviation,
conflicts, and erratic maneuvers) become increasingly important in this method. Since the
participants will be driving according to their own personal schedules, additional dependent
variables may include the time of day (day/night), traffic conditions (peak and nonpeak), invehicle
distractions (eating and/or cell phone use), and state of fatigue.
B.2.3 Advantages/Disadvantages
The naturalistic driving method possesses one major advantage over the on-road instrumented
vehicle method: the driving scenario, driving task, and driving purpose are all completely
natural. The research participants drive their own vehicles (or ones loaned to them) on their own
personal schedules along personally selected routes to meaningful destinations. Although to a
lesser degree, the naturalistic driving method shares another advantage with the on-road
instrumented vehicle method: its ability to implement eye-tracking measurements. In fact, the
dashboard-mounted eye-tracking device is far less intrusive to the driver than the head-mounted
eye-tracking device sometimes employed in the on-road instrumented vehicle method.
Unfortunately, some dashboard-mounted eye-tracking devices may not be as sensitive and
accurate as a head-mounted device. Also, they may not be able to track extensive head
movements or measure subtle eye glances indicative of unconscious distraction. The useful field
of view can also be an issue with certain unobtrusive vehicle-mounted eye-tracking equipment.
Consequently, this experimental method may be less effective in its ability to probe the subtle
phenomena of unconscious and involuntary distraction as they relate to CEVMS exposure.
Another disadvantage of this method is its inherent lack of structured driving scenarios. Since
participants drive whenever and wherever they want, it is difficult to ensure adequate and
uniform exposure to CEVMS and other relevant visual stimuli. This lack of experimental control
and higher degree of uncertainty necessitate an increase in the number of study sites, research
participants, and duration of the study, which negatively impacts the productivity and cost
effectiveness of the technique. For example, this method typically requires the instrumentation of
a relatively large number of vehicles at any given study site instead of the instrumentation of just
one vehicle which is shared by many research participants. Another minor disadvantage is that
research participants are aware that they are participating in an experiment, even if the study is
minimally intrusive in terms of daily life routine.
B.2.4 Budgetary Cost
A rough budgetary estimate for conducting such a naturalistic driving study is between
$2 million and $4 million. The main cost drivers for this method include increasing the number
of study sites, installing instruments in a large number of vehicles at a single site, and collecting
and analyzing data covering a long period of time. The range in this budgetary estimate relates to
the number of study sites, adequacy of the sites, number of vehicles which need to be
instrumented at one time, number of research participants, difficulty in obtaining research
participants, driving patterns of the research participants, length of the study at any given site,
ability to turn the CEVMS off and on, and numerous other factors which cannot be determined
without further planning.
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