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The authors describe a practical method for specifying the roadway lighting required for the performance of any driving task designers agree to use as a criterion. The method was developed from extensive studies of the visibility of objects on a 15 to 1 scale model roadway, using the quantitative metric of visibility, effective visibility level, described in a report on visual performance by the Commission Internationale de I'Eclairage (CIE). Using a particular study in which the effective visibility level of the relevant target was measured, the authors illustrate how visibility requirements can be combined with the visibility from a given lighting system to give a task performance probability index for that system.

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