# Parallels of Head-Up Displays and Visual Safety for Driving
## 4 Discussion  
Among the four parallax variations used in this experiment, overall horizontal parallax and overall vertical parallax did not show significant differences between the parallax conditions in subjective fatigue perception or D2 performance tasks. However, when there was a small parallax, visual comfort perception was optimal, and the trend of this result is consistent with the findings of related studies on the design and analysis of HUD parallax [8, 9], especially the effect of the horizontal parallax being the most obvious. In this study, we found the optimal horizontal parallax of visual perception was 3.31 mrad, which is smaller than the optimal parallax of the conventional 3D display [8, 9]. This result might be related to the fact that the middle forces in the different positions of the 2D display screen are at the same parallax level, and when the human eye adapts, the focus of the glasses is always on a horizontal plane and does not require frequent adjustments, resulting in no significant differences in visual fatigue perception or task performance. Meanwhile, as a laboratory simulated research, the results did not consider real situations in driving scenarios but only simulated similar variables for observation. This might bring some bias as well. However, regarding only the effect of parallax, our results can still indicate that presenting a smaller parallax value in the overall horizontal parallax could improve the performance of visual comfort perception [8]. In the stepped horizontal parallax test, the horizontal parallax showed a step change in the horizontal direction, and there were significant differences in the visual performance task for different values of parallax in neighboring positions: worse task performance was found with greater values of the neighboring stepped parallax. However, significant \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Parallax (Median)** & **TN** & **E1** & **E2** & **TNE** & **CP** & **FR** \\ \hline
0 (\(n=17\)) & 642.000 & 1.850 & 0.470 & 617.000 & 180.000 & 5.000 \\
0.53 (\(n=17\)) & 625.000 & 2.080 & 0.610 & 603.000 & 167.000 & 6.000 \\
1.71 (\(n=17\)) & 631.000 & 2.110 & 0.470 & 608.000 & 174.000 & 4.000 \\
2.24 (\(n=17\)) & 635.000 & 2.910 & 0.910 & 608.000 & 165.000 & 6.000 \\
2.88 (\(n=17\)) & 619.000 & 3.180 & 0.760 & 590.000 & 157.000 & 8.000 \\
3.41 (\(n=17\)) & 606.000 & 4.140 & 0.560 & 566.000 & 157.000 & 9.000 \\
4.05 (\(n=17\)) & 630.000 & 3.960 & 1.080 & 564.000 & 146.000 & 12.000 \\ Kruskal-Wallis test statistic H-value & 6.031 & 9.798 & 12.370 & 11.476 & 14.293 & 15.079 \\ \(p\) & 0.420 & 0.133 & 0.054 & 0.075 & 0.027 * & 0.020 * \\ \hline \end{tabular}
\end{table} 
Table 4: Results of nonparametric test analysis of D2 test results. Figure 18: Results of the variability analysis of total performance and concentration under different conditions of vertical parallax step change. **(a)** When the vertical stepped parallax value was greater than 2.24 mrad, the CP for D2 task performance was significantly different from conditions less than that value. **(b)** When the vertical stepped parallax value was greater than 2.24 mrad, the FR for D2 task performance was significantly different from conditions less than that value. * \(p<0.05\), **\(p<0.01\). differences in performance task scores occurred only when the stepped parallax value was greater than 3.31 mrad. We can take the step parallax value of 3.31 mrad as the maximum acceptable value. The reason that step parallax can have a significant effect on visual tasks is that under the stepped horizontal parallax condition, the eye's focus of attention must be switched frequently in different horizontal parallax planes. Increasing the frequency of visual convergence adjustment and also increasing the eye focusing time, which increases eye fatigue and larger motion fusion [13; 29], makes it more difficult for the eye to process information.