Road Weather Management Program
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Best Practices for Road Weather Management Version 2.0


Effect of Time and Foggy Conditions on Subjective Visibility: Evaluation of Retroreflective Traffic Control Devices


In dense fog, the visibility of retroreflective traffic control devices is fundamentally important to driving safety. However, few studies have addressed the visibility of these devices in fog during 24 hours (daytime, evening and night). The present study measures the subjective visibility of the latest traffic control devices under various visibility conditions. An experiment was performed during a 2-month period in 2003 under natural fog at a test road in Eastern Hokkaido, Japan. Visibility conditions varied by time of day (Daytime, Evening and Night) and weather condition factor ("Clear", "Cloudy" and "Dense fog"). The results showed that daytime-fog had a large negative effect on subjective visibility evaluations, while nighttime-fog had not as large a negative effect on subjective visibility evaluations than that under daytime. However, the subjective visibility values of targets under nighttime cloudy condition were worse than those under daytime dense fog. It was also found that subjective visibility for target objects made of highly reflective materials or incorporating fluorescent colors is high under both daytime fog and nighttime fog.


84th Transportation Research Board (TRB) Annual Meeting, Civil Engineering Research Institute of Hokkaido and Hokkaido University in Sapporo (Japan). For an electronic copy of this resource, please direct your request to

Date: 2005


Munehiro, Tokunaga, Asano, Hagiwara


Traffic control

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