There is currently no generally accepted objective method for the determination of visibility of objects in highway traffic lanes at night under highway lighting conditions. Visibility appraisal methods are generally subjective in nature, with wide variance among observers. This paper provides an analysis of the problem of visibility assessment under highway lighting conditions and evaluates many of the factors under these conditions. The findings of this investigation have resulted in a mathematical analysis of visual assessment and permit the establishment of visibility criteria for highway lighting. The instrument described provides an objective determination of relative visibility. The device consists of a dual monocular system with suitable optical wedges arranged to reduce a 2 degree central portion of the roadway area under observation to a contrast threshold, while maintaining a constant eye adaptation, thus providing a relative index of visibility. The visibility index is calibrated in terms of the contrast between an object of fixed size and its background. The findings of this investigation provide rational design criteria for highway lighting, enable the prediction of the visual performance of a highway lighting design, and provide an objective visual appraisal method for the comparative evaluation of visibility of existing highway lighting systems. /author/
Abstract