## 4 Discussion
### Angle of Stripes  
In terms of the angle of the stripes, visibility was higher in the oblique direction than in the horizontal and vertical directions, and among the oblique directions, 165\({}^{\circ}\) had the highest visibility. The other oblique directions, 150\({}^{\circ}\), 30\({}^{\circ}\), 135\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 15\({}^{\circ}\), had similar visibility, and the difference between 180\({}^{\circ}\) and 90\({}^{\circ}\) was obvious. In the natural world, most of the things we see are horizontal or vertical, and we may feel uncomfortable with things that are tilted; therefore, we thought that visibility in the oblique direction has increased. In this study, we investigated the effects of two colors, yellow and black, on clothing visibility. We found that the two colors, yellow and black, had the highest visibility when placed in a diagonal 165-degree stripe pattern, with a spatial frequency of 5.0. This study provides information pertaining to visibility for the study of base standards that may be considered for clothing and provides a first step in the research concerning this study. When creating high visibility clothing, the most important consideration is to ensure that the driver can see enough to know that a human is in his view. For example, even if the striped pattern, which was most visible in this study, is reproduced on a vest or pants as in the sample, to the driver, it may appear as a road sign or a roadway bollard, and it is not certain that the driver will recognize that a human is wearing the clothing. Field-based studies have reported that pedestrians were more easily recognized when made to wear black clothing with retroreflective materials at the wrist, ankle, elbow, and knee joints [12]. This was because the biological motion of the pedestrians became more pronounced, and they were more accurately recognized as humans. It is not enough to randomly put retroreflective materials on people as a brighter emphasis does not always directly translate into recognition as human [13]. Another factor that should probably be considered is the relative speed of the driver to that of the pedestrian. In a real-world scenario, both the driver and pedestrian move at different speeds in the same or different directions. It is difficult to ascertain if this affects the parameters ensuring maximum visibility of the pattern. However, since the relationship between perceived motion and spatial frequency is well reported [14,15], relative motion and velocity should be considered for more solid evidence. Nonetheless, it is difficult to consider every variable; thus, the limitations of this study should be considered and clarified. To our knowledge, no previous studies have examined high visibility clothing for the general public, but one study has assessed field-based high visibility clothing in fluorescent colors for road-workers [16], reporting that distance drivers observed no difference in visibility between road workers in fluorescent yellow-green and fluorescent red-orange colors. This is because both fluorescent colors are more luminous than general colors, and the brightness effect due to luminance is considered to play a significant role in visibility. Since no study has used general colors that are not fluorescent, the next step would be to study the difference in visibility using general colors that general people prefer, and whether general colors have the same visibility as fluorescent colors, based on the results of this study. Finally, we would like to evaluate the visibility of pedestrians wearing various colored clothes, and to thus devise and disseminate a standard for high visibility clothes. The following are available online at [[https://www.mdpi.com/article/10.3390/app11167229/s1](https://www.mdpi.com/article/10.3390/app11167229/s1)]([https://www.mdpi.com/article/10.3390/app11167229/s1](https://www.mdpi.com/article/10.3390/app11167229/s1)), Figure S1: Number of injured pedestrians by age group in 2019 by Japan's National Police Agency.