Effect of Time and Foggy Conditions on Subjective Visibility

Abstract

In dense fog, the visibility of retroreflective traffic control devices is vital for driving safety. However, few studies have addressed the visibility of such devices in fog during a 24-h period (daytime, evening, and nighttime). This 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 200 m long and 7.5 m wide in Eastern Hokkaido, Japan. Visibility conditions varied by daytime, evening, and nighttime and by weather condition factor (clear, cloudy, and dense fog). Thirteen kinds of traffic control devices were used as targets: two types of pavement markings, three of delineators, four of warning signs, and four of chevrons. Various newly developed materials were tested, and color and size were varied. The 20 women who participated in the experiment were required to make subjective judgments within a few seconds of the visibility of each target and their feeling of safety, assuming that they were driving on that section at 50 km/h. Results showed that daytime fog had a major negative effect on subjective visibility evaluations, while nighttime fog did not have as great a negative effect as that in daytime. However, the subjective visibility values of targets under the nighttime cloudy condition were worse than those under the daytime dense fog condition. Also, subjective visibility values for target objects made of highly reflective materials or incorporating fluorescent colors were high under both daytime fog and nighttime fog conditions.