## 4 Discussion  
This section reviews the studies on object detection in road traffic at night in non-urban areas by addressing and answering the research questions derived in the introduction of this article.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Offset to Longitudinal** & \multicolumn{4}{c}{**Distance to Vehicle in m**} \\
**Axis of Vehicle in m** & **40** & **60** & **80** & **100** \\ \hline \(-9.00\) & 2.585 & 25.439 & 22.967 & 20.100 \\ \(-6.00\) & 19.58 & 24.755 & 5.219 & 10.879 \\ \(-3.00\) & 5.000 & 13.215 & 6.772 & 20.119 \\ \(3.00\) & 5.750 & 12.670 & 4.893 & 6.243 \\ \(6.00\) & 4.715 & 14.960 & 6.052 & 7.746 \\ \(9.00\) & 4.453 & 14.434 & 4.817 & 11.341 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Required Visibility Level on the different object positions on the closed test site.  
Figure 13: Relationship between the Visibility Level VL and the proportion of object positions on the closed test site at which the gray card is reliably detected; while a Visibility Level of 25 is required to reliably detect the gray card at 90% of the object positions, at the Visibility Level of 13.35 determined in the light tunnel, the gray card is detected at more than 70% of the object positions.  
What Visibility Level is required for reliable object detection with a probability greater than 90%?  
The studies carried out demonstrate that the required Visibility Level for reliable object detection depends on the ambient conditions. Thus, in non-urban areas under controlled ambient conditions in the light tunnel, a Visibility Level of 13.35 is required to detect the gray card with a reflectance of about 4% on 90% of the considered object positions with a probability of 90%. This value is comparable to the field factor of 12.7 that [PERSON] [18] determined in his laboratory and field studies.  
On the closed test site in non-urban areas, the required Visibility Level for reliable object detection on 90% of the object positions increases to 25. This is due to the environmental conditions, which are not as strictly controllable as in the light tunnel and thus create more complex detection situations. This confirms the findings of [PERSON] [12; 13; 31], [PERSON] [18], [PERSON] [48; 49] and also [PERSON] [34], in which the required luminance difference increases with increasing complexity of the traffic situation; thus, the definition of a Visibility Level for such situations turns out to be useful. Nevertheless, the Visibility Level of 13.35 determined in the light tunnel ensures a reliable detection of the gray card at more than 70 percent of the object positions on the closed test site.  
What influence does the object's distance and angle have on the required Visibility Level?  
In order to determine the angle and distance dependence of the required Visibility Level, strictly controllable conditions, such as those found in the light tunnel, are necessary. The detection study carried out in the light tunnel shows that the Visibility Level required for reliable object detection has a Gaussian shape. The maximum of the Gaussian shape of the Visibility Level is in the foveal area around 0\({}^{\circ}\). This correlation, which has been demonstrated by studies of [PERSON] [18] and [PERSON] [34], can be explained by the distribution of rods and cones on the human retina. Thus, the higher density of the more light-sensitive rods in the peripheral visual field provides better contrast perception, which is not available in the foveal area due to the higher cone density [44; 45; 46; 47]. Since the distance to the object is already taken into account by determining the object size in angular minutes, the distance dependence is already included in the calculated Visibility Level. An additional distance dependence of the required Visibility Level cannot be extracted from the study results.