Abstract
This study was to investigate the visibility effectiveness of the hemisphere pavement markers on various curved road conditions at night. The corner cube retroreflective pavement marker has been used to enhance the visibility of road delineation at night for long time. Although its retroreflection efficiency is high, its retroreflection index is highly depends on entrance angle. As the entrance angle increases, its retroreflection index drops significantly and its visibility decays. Therefore it may not be suitable for curved road driving conditions. The newly introduced hemisphere retroreflective pavement marker has a retroreflection index independently of entrance angle. This retroreflection property was considered to be better visually effective than the corner cube on curved roads. This study was undertaken to use the corner cube as bench marker, to investigate the visibility effectiveness of the hemisphere on curved road driving conditions.This study consisted of two phases: the field experiment and the visibility effectiveness evaluation. The field experiments were to investigate the visibility effectiveness of these two markers on a curved road of typical small curvature. The visibility effectiveness evaluation was to evaluate the visibility effectiveness of these two markers on various curved roads with different radii based on the results of the field experiments.The field experiments consisted of two tests. The first was to investigate the preview effectiveness of hemisphere for different types of cars. The second was to investigate the relative superiority between two marker types for different types of cars. The criteria of visibility effectiveness were preview effectiveness and relative superiority. Preview effectiveness was to compare the visibility length against the required preview length for safety driving. Relative supervisory was to compare the visibility length between these two markers. Thirty-two cars, consisting 22 sedans and 10 trucks, participated in the first test, among them, 5 cars were selected for the second test. An experimental U-shaped test track of 30 m in radius was set up to simulate the curved road driving conditions. The U-shaped track was used to simulate both right turn and left turn driving conditions. The subject was asked to report the appearance of a pre-determined target marker when he/she was just turning to the curved section from the straight leading section. The result indicates that both markers are visibility effective for both sedans and trucks, and the hemisphere is relatively superior to the corner cube regardless of car types.The visibility effectiveness evaluation was to evaluate the visibility effectiveness of these two markers on eight levels of curved roads of different radii. Visual threshold was derived first as a key measure for evaluation. The results indicate that marker types do not result in difference in visual threshold, but directions of turning do. The left-turn had a significantly higher visual threshold, 1.70×10-6 lx, and was determined as the resultant visual threshold. It is significantly higher than the recommendation of Commission Internationale de l’Eclairage (CIE). The evaluation indicates that both pavement markers meet the required 3 seconds preview time as suggested by CIE. Although they are both visual effective, the corner cube is superior on roads with radii greater than 60 meters, and on the other hand, the hemisphere is much better for the roads with radii smaller than 60 meters.