This study investigated the legibility distance characteristics of variable speed limit signs and variable message signs under foggy conditions according to different luminance levels. In South Korea, the current installation standards for these signs are based on normal weather conditions, and empirical analyses of their visibility under adverse weather conditions remain limited. To address this issue, a controlled large-scale experiment was conducted at the Yeoncheon SOC Test Center, where artificial fog was generated in a tunnel environment. Seventeen elderly participants (average age: 70 years) participated in the experiment. They walked toward a sign to identify the distance at which it became legible. The experiment varied the fog visibility levels (50–80 m, 30–50 m, and 10–30 m) and display luminance (six levels). The results showed that as the fog density increased (that is, visibility decreased), the average legibility distance decreased. Conversely, higher luminance levels consistently improved legibility distance under foggy conditions. Under normal weather conditions, changes in luminance had a minimal impact on legibility. Compared with the minimum legibility distances calculated based on the design speed, many cases under foggy conditions failed to meet these thresholds, particularly at lower luminance levels. These findings indicate that the current luminance standards may not adequately ensure sign legibility under adverse weather conditions, underscoring the need for updated luminance guidelines that reflect environmental conditions. The results of this study provided quantitative data to support policy revisions and technical advancements aimed at improving road safety.

OBJECTIVES : A fog generated field test was conducted to analyze the relationship between different sizes of variable speed limit signs and the legibility distance under various fog density conditions. By using this study, appropriate sizes of signs can be selected depending on the density of fog. METHODS : An actual tunnel was selected as the area for this test, as other places cannot maintain the fog condition because of rapid air current. A total 121 subjects were recruited for this test, which took place over the course of four days. The test on the first day was conducted under normal weather conditions for comparison. Visibility-distance detecting sensor was used to measure the visibility distance due to the fog density time, simultaneously with the evaluation of legibility distance by subjects. RESULTS : The test results show the relationship between the different sizes of signs and the legibility distance corresponding to the visibility distance due to both non-fog and fog generated conditions. According to the technical test results, appreciable amount of reduction in legibility distance due to fog was resulted in all sizes of signs. Moreover, the legibility distance is reduced proportionately with the decrease in the visibility distance due to fog. CONCLUSIONS : The results of this study can be used to select appropriate sizes of valuable speed signs under fog conditions. Hence, drivers can expect to have more room to respond to adverse weather conditions, thereby reducing the risk of accidents.

OBJECTIVES : A fog generated field test was conducted to analyze the relationship between the luminance of variable speed-limit signs and the legibility distance under various fog density conditions. By using this study, appropriate luminance values can be selected depending on the density of fog. METHODS: An actual tunnel was selected as the area to conduct the test, as other places cannot maintain the fog condition because of rapid air current. Ninety-two subjects were recruited for this test, which took place during the course of three days. Visibility-distance detecting sensor was used to measure the visibility distance due to the fog density time, simultaneously with the evaluation of legibility distance by subjects. RESULTS: The test results show the relationship between luminance values and the legibility distance corresponding to the visibility distance due to fog. According to the technical test results, lower luminance value such as 7000 cd/m2 corresponds to less legibility distance compared to higher values such as 20000 cd/m2 or 40000 cd/m2. However, the amount of difference between 20000 cd/m2 and 40000 cd/m2 is negligible in our test. CONCLUSIONS: The results of this study can be used to select appropriate luminance of valuable speed signs under fog conditions. Hence, drivers can expect to have more room to respond to adverse weather conditions, thereby reducing the risk of accidents.