*3.1. Quality of Lane Markings' Detection*

During night-time, 69% of all readings of the middle line were ranked as level 3, i.e., "high detection confidence". Around 18% of middle markings were not detected (level 0), almost 2% had "low detection confidence" (level 1) while 10% had "medium detection confidence" (level 2). On the other hand, night-time readings for the edge line are as follows: 31% level 3, 20% level 2, 1% level 1 and 48% level 0.

The overall results for the middle line show that the number of samples classified as level 3 and 0 differed around 2% between daytime and night-time with lower number of samples recorded during daytime. On the other hand, the number of samples classified as level 2 during daytime increased by reaching 3.5% difference compared to night-time, while level 1 varied slightly between the two conditions. A similar result was found for the edge marking. During daytime, the number of level 3 and 2 samples decreased compared to night-time, with differences of around 3% and 7% respectively. The number of level 1 samples increased during daytime and differed by 10.7% to the night-time, while level 1 stayed approximately the same.

The afore-presented results are shown in Figure 2.

(**a**)

**Figure 2.** Comparison of the Mobileye readings (quality level) between night-time and daytime quality measurements for middle (**a**) and edge lines (**b**).

Overall, when analysing all roads and lines together, on average 50% of daytime readings were classified as level 3, 13% as level 2, 2.2% as level 1 and 34% as level 0. On the other hand, those averages changed to some extent during night-time as visible from the Figure 3. Namely, the average number of level 3 readings decreased by almost 5% in daytime compared to night-time conditions. It is similar for level 2 where the decrease is 8% while the number of level 1 readings decreased slightly (0.28%). On the other hand, the number of level 0 readings in daytime increased by 12% compared to night-time.

**Figure 3.** Number of overall reading qualities during daytime and night-time as well as their differences (day–night).

When looking at each road separately, the results show that on all the roads the percentage of level 3 readings is higher during night-time when compared to daytime, reaching the maximum difference on road four—3.52%. On two roads (one and four), the percentage of level 2 daytime readings is higher compared to night-time and lower on other two roads (two and three). The percentages of level 1 readings remained approximately the same in both conditions while the number of level 0 readings is higher during daytime on three roads (one, two and three) with the maximum of 6.86%. The results for each road are presented in Table 3.

**Table 3.** Frequencies and percentages of samples by each marking quality rank for middle and edge lines during night-time.


In order to test the statistical difference between the detection quality of lane markings during daytime compared to night-time, a Wilcoxon signed-rank test was used. Table 4 presents the results of the aforementioned tests, separately for middle and edge lines on all four roads. Although the majority of samples had "equal readings" in both conditions (day = night), it can be concluded that for all four roads a statistically significant difference in the number of samples between visibility conditions still exists (*p* < 0.05). Roads 1 and 4 have more samples which had higher readings during night-time compared to daytime, while it is the opposite on roads 2 and 3.


**Table 4.** Results of Wilcoxon signed-rank test for middle and edge lines on analysed roads.

### *3.2. View Range of Lane Markings*

Overall, the absolute average of Mobileye's view range for the middle line during night-time was 34.07 ± 22.23 m. During daytime, the view range increased and averaged 39.42 ± 25.36 m. On the other hand, the range for the edge lines was lower compared to the middle lines in both daytime and night-time conditions, averaging 17.69 ± 24.38 m and 17.01 ± 20.48 m, respectively. Absolute averages of the view range for each road, line and visibility condition is presented in Table 5.

Furthermore, we calculated the average difference and the standard deviation of the view range when daytime reading quality was higher compared to night-time and vice versa for both markings (middle and edge). On average, the difference in the view range for the middle line when daytime had higher reading quality was around 29 m with the standard deviation of around 21 m. On the other hand, when reading quality was higher during night-time, a slight decrease of the view range difference and standard deviation was recorded. There is a similar trend for the edge line as well but the difference in view range for "Daytime > Night-time" is almost 40 m and "Night-time > Daytime" around 27 m as shown in Table 6.


**Table 5.** Absolute averages of the view range for each road, line and visibility condition.

**Table 6.** Average differences in view range of middle and edge lines for each road and condition.


In addition, a paired samples t-test was used to test whether a significant difference between night-time and daytime conditions for each line on each road exists. In total 7 pairs were made as shown in Table 7. The results of the t-test show that the view range between all pairs is statistically different (*p* < 0.05), i.e., that Mobileye view range differed for all test cases between daytime and night-time conditions.


**Table 7.** Results of paired samples *t*-test.

#### **4. Discussion**

Although several studies [11,14,15] investigated how different factors affect lane detection and thus proper functioning of LSS, gaps in literature still exist. Mainly, these gaps are related to determining the adequate levels of lane markings' visibility in different conditions. For this reason, an on-road test was conducted to determine and compare the detection quality and view range of machine-vision system during daytime and night-time.

The overall results show that the number of each quality level (0–3) as output from the camera differed between visibility conditions. Namely, the average number of level 3 and 2 readings decreased by 5% and 8% in daytime compared to night-time conditions. The share of level 1 remained approximately the same (slight decrease—0.28%) while the number of level 0 readings in daytime increased by 12% compared to night-time indicating potential failures in lane detection during daytime. Although the conducted tests show that a significant difference of quality readings between visibility conditions exists, it has to be noted that differences are relatively small and may not influence the functioning of the lane detection system. The main concern from the practical point is related to the 12% increase of level 0 readings during daytime compared to night-time. Since level 1 readings in both conditions stayed approximately the same, it is reasonable to conclude that in some cases markings were not detected by Mobileye (level 0) during daytime while during night-time they were detected with medium (level 2) or high detection confidence (level 3). Since the driving speed, the sun direction in daytime conditions and the impact of road lightning in night-time conditions were controlled, the potential reason for such results is related to the difference in visual complexity between the two conditions (day vs night-time). Due to the complexity of visual clutter during daytime, the contrast ratio between the marking and the road may differ and decrease thus affecting the proper functioning of lane detection. This is also suggested by previous studies in which it was found that the optimum contrast between marking and road surface should be around 3:1 during daytime [11,19]. On the other hand, the aforementioned contrast ratio during night-time is generally much higher due to the fact that the surrounding environment is dark and the visibility of lane markings is achieved with the use of retroreflective materials, i.e., glass beads which return the incoming light ray from the vehicle headlights back to the source (driver) [24]. In addition, the number of samples classified as level 0 detection quality was much higher for edge markings compared to middle markings, both during daytime and nigh-time. This suggests that the quality of the marking plays an important role. On three roads the middle line was relatively new (<6 months,) and on one road it was older

than one year. On the other hand, edge markings on all three roads were older than one year. Several studies proved that age affects visibility properties (daytime and night-time visibility) of lane markings, especially for paint markings [25–28]. Since the service life of paint markings is usually around one year, it is reasonable to conclude that the visibility properties of edge lines in this study were lower compared to newer middle lines.

Although the detection quality slightly decreased in daytime compared to night-time, the view range increased, however differently for middle and edge lane marking: the absolute average and the standard deviation of the view range for the middle line during daytime was 39.42 ± 25.36 m compared to 34.07 ± 22.23 m in night-time conditions. The range for edge lines was shorter compared to middle lines in both daytime and night-time conditions averaging 17.69 ± 24.38 m and 17.01 ± 20.48 m, respectively. Such results are lower compared to the study [17]. However, this is mainly due to the different methodology. The aforementioned study tested lane detection in a static environment with consistent parameters (new markings, lighting, road surface retroreflectivity, etc.) and used older version of Mobileye (560). Overall, form the practical point of view, the differences of the results between visibility conditions are, as in the case of detection quality, relatively small. However, there is a much higher difference between the view range of middle and edge markings. This may be due to the markings' age. As described in previous paragraph, the age of the marking affects its visibility properties (daytime and night-time visibility). Since edge lines in this study were older compared to the middle lines, it is reasonable to conclude that their visibility properties were lower compared to newer middle lines. Furthermore, relatively high standard deviations of the view range (around 20 m) for all markings suggest that, beside the markings age and its visibility, road geometry plays an important role in lane marking detection. Since Mobileye camera is fixed to the vehicle windshield, it is not moving like human drivers' head and eyes would, meaning that it "looks" at a relatively fixed area at the more or less the same angle, i.e. the viewing area and angle are changed only with the change of the vehicles movement. This indicates that with the change of road geometry such as curves, dips, slopes etc., middle and edge lane markings will be "seen" by the "fixed" machine-vision at different distances. The aforementioned findings further support the findings from literature [11,14,15].

Although this study provided valuable results, there are several limitations. First, lane markings' visibility properties were not taken into account due to the fact that the main objective of the study is to compare the detection quality and view range of a machinevision system during daytime and night-time and to analyse how these values change depending on the visibility conditions alone. Further research is needed to determine the relationship between daytime and night-time visibility on the detection quality and view range of LSSs. Furthermore, we did not evaluate how different road geometry and lane markings' configurations (dashed vs solid) affect the detection of markings. Existing literature indicates that both road geometry and configuration of lane markings are potential influencing factors [11]. However, due to the lack of data related to the road geometry (curves, dips, slopes etc.) and exact location of each type of dashed lines, such analysis was not conducted. Also, the daytime measurements were conducted between 10:15 h and 13:00 h which may have, to some extent, affected the results since the sun direction is one of the factors that affects lane markings' detection by machine-vision systems [11,14,15]. However, all measurements were conducted on the same day under the clear sky and on roads whose direction is not in the direction of the sun (north-south direction) so the effect of the aforementioned factor is limited and negligible. The effect of driving speed was also not analysed, although the literature suggests that vehicle speed has a varying impact on machine-vision systems—some improve at higher speeds and some degrade [11]. Since the tests in this study were conducted on rural roads, the driving speed was in accordance with the speed limit (between 60 km/h and 80 km/h) and thus the impact of speed was controlled. Lastly, only one machine-vision system (Mobileye 630) was tested and overall results may not be applicable to other such systems.

Since the differences between lane markings' detection found in this study are relatively small, more studies are needed to further validate here presented results. Besides determining the relationship between daytime and night-time visibility on the detection quality and view range of LSS, future studies should focus on detailed investigation of the influence of other factors on the LSS such as weather conditions, road geometry (curves, dips, slopes etc.), markings' characteristics (colour, width, configuration), driving speed, road surface characteristics (type, condition and texture) etc., as well as combinations of these factors. Therefore, we propose that future research combines the field and on-road tests in order to further expand the existing knowledge and thus improve the overall quality of machine-vision systems as well as to determine adequate properties of lane markings.
