PURPOSES : LED based Variable Message Signs(VMS) have been widely used to inform safety messages to the drivers in advance. Legibility Distance of VMS is the most important factor to provide the safety messages to drivers in timely and effective way. However, current National Standards on legibility distance design considers letter size only even there is a difficulty to read the signs at adverse weather conditions such as heavy fog. So, this study examined the legibility issue under fog by evaluating the legibility distance with two design factors such as letter size and luminance. METHODS : Two foggy weather conditions, intermediate and heavy fog, were simulated at real-road-scale Proving Ground. Legibility distance at daytime and nighttime was evaluated by test subjects. Subjects were asked to fill the legible distance on the test sheet and statistical significant was analysed at the lab. RESULTS : The legibility distance(LD) under fog was observed only 22 to 41% of LD observed under normal weather condition at daytime, and 26 to 45% at nighttime condition. Study results showed a consistent increase in LD with higher luminance even at same letter size and vice versa conditions, However, statically significant difference between groups was only revealed when both letter size and luminance level increased conditions. In order to apply the test results in terms of engineering benefits, LD results from significantly different groups was evaluated with relative to Stopping Sight Distance(SSD) within conceptual frame suggested in this study. CONCLUSIONS : From the study results, current National Standard on legibility distance design needs to consider letter size and luminance simultaneously to response the legibility issue in adverse weather conditions.

1. 서론
    1.1. 연구 배경 및 목적
    1.2. 연구 범위
    1.3. 연구 방법 및 개념적 틀
2. 문헌고찰
    2.1. 현 VMS 기준 한계점 검토
    2.2. 선행연구 고찰 및 본 연구 차별성
3. 실험방법
    3.1. 피 실험자
    3.2. 안개 발생장치와 강도제어
    3.3. 실험 시료
    3.4. 실험 시나리오 : 문자크기*휘도 조건결정
4. 분석결과
    4.1. 주간 실험결과
    4.2. 야간 실험 결과
    4.3. 실험결과의 공학적 의미 검토
5. 결론 및 향후연구
    5.1. 연구결과
    5.2. 연구한계 및 향후연구
감사의 글

• 이석기(정회원 · 한국건설기술연구원 연구위원) | Lee Sukki
• 박원일(정회원 · 한국건설기술연구원 전임연구원) | Park Wonil
• 박기수(정회원 · 한국건설기술연구원 UST학생연구원) | Park Kisoo
• 김용석(정회원 · 한국건설기술연구원 연구위원) | Kim Yongseok (Research Fellow Department of Highway & Transportation, Korea Institute of Civil Engineering and Building Technology, 283, Goyangdae-ro, Goyang-Si, Gyeonggi-do 10223, Korea) 교신저자