EVALUATION OF FOG-DETECTION AND ADVISORY-SPEED SYSTEM

This paper presents findings from a study of traffic crashes due to fog in southern Saudi Arabia. The main aim was to assess the effectiveness of a fog detection and warning system on driver behavior regarding speed and headway. The warning system includes visibility sensors that automatically activate a variable message sign that posts an advisory speed when hazardous conditions due to fog occur. The system was installed on a 2-km section of a 2-lane, rural highway. A data set of 36,013 observations from both experimental and control sections at 2 study sites was collected and analyzed. Although the warning system was ineffective in reducing speed variability, mean speed throughout the experimental sections was reduced by about 6.5 kph. This reduction indicates that the warning system appeared to have a positive effect on driver behavior in fog even though observed mean speeds were still higher than the posted speed. From relationships found in the literature between mean driving speed and number of accidents, a speed reduction of only 5 kph would yield a 15% decrease in the number of accidents.