The Effects of Commercial Electronic Variable Message Signs ...

connected to the vehicle data bus so that additional vehicle-related measures of engine, braking,
and steering performance are also recorded. However, because of the unobtrusive nature of the
experimental technique, this method cannot support the use of extremely accurate head-mounted
or vehicle-mounted eye-tracking equipment. In the present state of technology, these accurate
eye movement instruments involve careful calibration procedures with the driver. With this
method, the eye-tracking system is mounted in the dashboard in a manner which involves little or
no driver interaction. Once the participant’s vehicle has been instrumented, data are collected by
means of automatic wireless downloads without participant awareness or involvement. The
instrumentation is left in the vehicle for a period of 3 to 6 months, during which time the
participant drives the vehicle for normal personal or business use.
The fact that participants drive their own vehicles for their own use reduces control and adds
uncertainty to the study. It is difficult to control where the participants are going to drive and
when. The study site must be selected carefully so that participants are likely to drive by at least
some of the target CEVMS installations. The participants must be selected carefully so that they
are likely to take the selected roadway with some reasonable frequency. As a result of this
increased uncertainty, the number of study sites must be increased to 4 and 5, the number of
research participants selected at each site must be increased to 50 and 75, and the duration of
measurement for each participant must be increased to 3 and 6. In this study, it is even more
important that there are several CEVMS installations at each study site. As was the case for the
on-road instrumented vehicle study, each study site needs to include CEVMS installations,
standard (non-digital) billboards, objects of casual visual interest (houses, barns, etc.), and
natural background control scenery (trees, fields, etc.).
B.2.2 Factors and Measures
As with the on-road instrumented vehicle study, the major factors or independent variables are
the presence or absence of CEVMS and other comparison visual stimuli (standard billboards,
buildings, control settings, etc.) along the driven path. If possible, the CEVMS should be turned
off and on or changed in some other way, according to a prearranged experimental design.
However, in this instance, the CEVMS would have to be turned off or changed for longer periods
of time because it is not certain when the instrumented test vehicles might pass. These are the
primary independent variables. Secondary independent variables could include the type of
vehicle (sedan, pickup, or SUV) and driver characteristics (age, gender, and route familiarity).
In addition, as much as possible, one or more of the primary CEVMS variables of interest to the
community concerned with outdoor advertising control should be represented by varying levels
in the selection of CEVMS stimuli.
As in the on-road instrumented vehicle study, the primary measure or dependent variable is the
frequency, direction, and duration of driver eye glances. The fundamental hypothesis of selfregulated
attention which needs to be concentrated on the roadway scene ahead remains the
same. As before, if the frequency and duration of eye glances away from the roadway ahead
exceed accepted norms or criteria, then driver safety is assumed be compromised. Thus, eye
glance behavior is the primary dependent variable in this study, as well. However, the particular
unobtrusive and disengaged dashboard-mounted eye-tracking device may not be capable of
making as accurate measurements of eye-movements as can other more delicate vehicle-mounted
or head-mounted devices which require periodic participant calibration. Consequently, this study
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