Eye movements were monitored in many different ways. The apparatus of eye tracking can be divided into three types according to the techniques they rely on: (a) the surface electrodes placed around the eyes, which are good at measuring saccade behaviour but accuracy of the gaze location measuring is deficient; (b) the contact lenses attached to the surface of eyes; (c) the video-based non-contact, corneal reflection method, in which, infrared light is reflected from the eyes and sensed by a video camera or some other specially designed optical sensors. The third one is widely used in researches because it is non-invasive and inexpensive. The FaceLabTM system used in this research is based on the third video-based method. The
have been reviewed intensively in this section. Although there has been research concerning the measurements, evaluation, and reporting of eye movement data, no standard for eye movement measurements have been adopted. Therefore, some of the most commonly used measurements in secondary task evaluation are introduced below. The ones already described in Section 2.1.6 are not repeated here.
2.2.3.1 Percentage of Road Centre
The percentage of eye fixation on the road centre is probably the most commonly used parameter in the research for drivers’ workload, in-vehicle system assessment, road condition evaluation, and driving performance. Many studies have shown that drivers look to the road centre region (at a few degrees down from the horizon gaze angle) about 80–90% of the time, and only rarely look at the lane markings near the vehicle during the normal driving (Harbluk, Noy & Eizenman, 2002; Recarte & Nunes, 2000; Rockwell, 1972; Wann & Swapp, 2001; Wierwille, 1993a, 1993b).
Victor et al. (Victor, Harbluk & Engstrom, 2005) have found that the percent of road centre viewing was decreased according to the increasing difficulty of the in-vehicle visual task, while it was increased during the auditory task. The sensitivity of gaze concentration towards the road centre to the secondary tasks was also found (Jamson and Merata, 2005; Engstrom, Johansson, and Ostlund, 2005).
This parameter is generally taken as “the higher the better” based on the assumption of “looking away from the road ahead at any time is harmful”. However, this does not always hold true. For example, when the road condition itself is less demanding, drivers may have spare resource to look into peripheral areas more often. Another example is that compared to novice, an
experienced driver looks less at the road ahead (Crundall, Underwood & Chapman, 1999a; Underwood, Crundall & Chapman, 2002), because they can use their peripheral vision more efficiently and be more selective in the mirror checking. Therefore, this parameter needs to be considered in relation to driving situation and task condition.
2.2.3.2 Total Number of Times of Fixation on Display
The total times of fixation is the total number of fixations made to the in-vehicle display during a visual task, where each fixation is separated by at least one glance at a different target. This measurement shows the number of times that drivers need to look at the display in order to perform a specific visual task. This measurement is widely used and intensively debated in safety research and in-vehicle system design. Initially, accumulated fixations of more than 9
times on an in-vehicle display, were suggested to be unsafe (Hankey et al., 2000). Later Blanco et al. (Blanco et al., 2006) investigated many of the visual tasks, and found actually almost half of the participants have to make more than 9 fixations at most displays to perform each task. Therefore, some other research suggested that the total number of fixations is not important, and even values greater than 10 are acceptable and routinely used (Chiang et al., 2004), and they suggested that it is more important to guarantee the task to be “chunkable” so that it can be finished in separate incremental steps, as long as each glance duration is typically less than 2 seconds.
2.2.3.3 Accumulated Fixation Duration
Accumulated or total fixation duration is the total amount of time in which fixations are made on the in-vehicle display. It is a parameter which suggests in total, how long a certain amount of information takes to be extracted by the driver. Accumulated fixation duration is an efficient measure which reflects how complex and demanding a given visual task is.
2.2.3.4 Total Task Duration
Total task duration is not an eye movement measurement, but it is listed here because of the tight correlation with visual searching behaviour. A shorter task time is desirable, as suggested by previous studies (e.g., Hashimoto and Atsumi, 2001). There were a number of generally accepted proposals. Green et al. (Farber, 2000; Green, 1999a, 1999b) proposed a “15-second rule” for in-vehicle navigation systems, which suggested that a Human-Machine Interface (HMI) task should not be any longer than 15 seconds. Later, Green suggested that to minimize the risk of distraction/overload, the criteria need to be dropped to 10 seconds (Green, 2008). Other researches argued a much longer duration, for example, Chiang (2004) suggested that
acceptable total task times can be in the range of 30-40 seconds, or greater. Nevertheless, it is still generally believed that the longer task duration has a negative effect on workload and driving performance (Angell et al., 2006).
2.2.3.5 Fixation Duration
Fixation duration is the measure of interest when evaluating in-vehicle displays. A study by Chiang et al. (Chiang, Brooks & Weir, 2004) showed that, when involving in manual input, drivers’ mean fixation duration on a display is one second when a single keystroke is entered, and about 1.5s when 2 keystrokes are entered in one chunk. Also, 94% of all display fixations in the cases of 1 or 2 keystrokes are less than 2s. It was therefore suggested that the fixation durations in the secondary task should be typically less than 2s (Chiang, Brooks & Weir, 2004). When a decision making process is involved, a maximum single fixation time during a task can
be longer, about 2.5s (Blanco et al., 2006). At the same time, during the destination entries, the road was fixated for an average of 0.47s each time, with 95% of the roadway fixations being less than 1.2 s (Chiang, Brooks & Weir, 2004).
2.2.3.6 Percent Fixation Durations Exceeding 2 Seconds
Based on the research on the fixation duration, it was believed that durations longer than 2s have an impact on safety and therefore needs to be specially investigated. It was found that, when the in-vehicle display is more complex, drivers’ percent fixation durations which are longer than 2 s increased (Victor, Harbluk & Engstrom, 2005). This measurement might be more significant than fixation duration solely. However, the 2s threshold needs to be more carefully proven.
2.2.3.7 Standard Deviation of Fixation Duration
Standard deviation of glance duration is the standard deviation of the durations of all fixations made to a display during a visual task. Victor, Harbluk, and Engstrom (2005) investigated several eye-movement measures, including the mean fixation duration, the standard deviation of glance duration, the glance frequency, the total glance duration, and the percent on the road centre. In all of these measures, the standard deviation of fixation was found to be more sensitive, more robust, more reliable, and easier to calculate than the established glance-based measures. The standard deviation of fixation duration is increased with the increasing difficulty of visual task, which means looking at the display for more varied durations. It was found that a similar increase occurred during auditory tasks.