This study evaluated the influences of lamp separation and lamp intensity on the effectiveness of daytime running lights. Subjects performed two tasks. They rated discomfort glare from a pair of lamps, and evaluated the conspicuity of the same lamps. The same nine stimuli were used in both the glare and conspicuity portions of the study. These nine stimuli were obtained by the orthogonal combination of three levels of luminous intensity directed towards the subject’s eyes from the two lamps (14,000, 3,500, and 875 cd), and three levels of lamp separation (1.05, 0.65, and 0.25 m—edge to edge). The study was performed outdoors, on bright, sunny days. The viewing distance was 20 m. As expected, lamp intensity had strong effects on both discomfort glare and conspicuity. On the other hand, in the case of lamp separation, there was more evidence for an effect on conspicuity (narrowly separated lamps being more noticeable) than on discomfort glare (a statistically nonsignificant trend for narrowly separated lamps to be only slightly more discomforting). Consequently, if the possible increase in discomfort from narrowly separated lamps is counteracted by a slight reduction in intensity, there is still a netbenefit for narrowly separated lamps in terms of conspicuity. Conversely, narrowly separated lamps that are as noticeable as widely separated lamps of higher intensity would be less glaring. The present findings imply that there is a small net benefit in using narrowly separated daytime running lights.
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