## 2 Driver Gaze Behaviour on Real Roads
### Results
#### 2.3.1 Gaze Distributions During Night  
Looking at the overall gaze distributions did not lead to any new findings. Therefore, this section will only show selected situations and road categories. Furthermore, investigating the overall gaze data did not necessarily lead to the desired insight since a large portion of these recorded gaze vectors were just scanning gazes not leading to any information gathering for the driver. For this reason, the data presented here focus solely on fixations. For this paper, a fixation is defined as a gaze direction that stays within a 2.0\({}^{\circ}\) area for more than 200 ms [26]. The 2.0\({}^{\circ}\) area is, for this evaluation, set to be around the moving mean value of the last 50 ms to ensure that the behaviour of tracking objects will be evaluated as well [27; 28].  
One important remark here is that from the overall collected gaze dataset, 24.4% of the data are correlated to fixations during the night, while only 18.3% of the daytime data are correlated to fixations. This indicates that the additional light available during the day may lead to more scanning gaze behaviour, while the limited light available during the night leads to a more fixation-oriented gaze behaviour.  
While the overall fixation distribution did not lead to any particular insight, the most important metrics are given in Table 1. As a comparison, the same data are presented for the daytime driving. Additionally to the median values, the standard deviation of both the horizontal and vertical fixations as well as the median fixation distances, where the fixation vectors intersect with the road surface, are given.  
Without going into too much detail, the difference between the fixation distributions between daytime and nighttime driving mainly appears to be the gaze distance (\(d_{med}\)), which is more than double the distance during the day. The nighttime fixation distributions also appear to be slightly narrower than the overall daytime distribution.  
Both of these effects are to be expected due to the limited amount of light available during the nighttime. Without going into more detail about the daytime fixations, Table 2 shows the same information for nighttime driving, but divided into the three major road categories.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Day** & **Night** \\ \hline Median Horiz. & \(-0.86^{\circ}\) & \(-0.82^{\circ}\) \\ \hline Std (\(\sigma\)) Horiz. & \(7.65^{\circ}\) & \(-7.34^{\circ}\) \\ \hline Median Vert. & \(-0.80^{\circ}\) & \(-2.04^{\circ}\) \\ \hline Std (\(\sigma\)) Vert. & \(-3.10^{\circ}\) & \(-3.01^{\circ}\) \\ \hline \(d_{med}\) & \(85.9\) m & \(33.5\) m \\ \hline \(d_{med-\sigma}\) & \(17.7\) m & \(13.5\) m \\ \hline \end{tabular}
\end{table}
Table 1: Fixation data for daytime and nighttime driving. The distance d is calculated as the intersection of the fixation vectors with the road surfaces.  
The data shows, when compared to the average speed that is being driven in each of these road categories, that the orientation distance is highly speed dependent. The higher the speed, the higher and smaller the fixation areas are. The relatively far distance of 229 m for highway driving is mainly due to the focus on preceding vehicles, and not on the road ahead. Therefore, while the fixation vectors might intersect the road 229 m ahead of the driver, the actual fixation, in terms of other vehicles, could be much closer. However, an exact distance cannot be derived from this analysis.