The function of road lighting is to enable traffic to function more or less as during the day. The effectiveness of road lighting is expressed in the reduction of nighttime accidents. For important urban roads, the installation of good road lighting will result in a reduction by 30% of nighttime injury accidents compared with no or very poor lighting. Accident studies have shown that the relative nighttime danger is reduced when the lighting is improved. The efficiency of road lighting is expressed in terms of supply-and-demand, both of which can be expressed in terms of conspicuity, i.e., the supplied conspicuity of a lighting installation and the conspicuity required by road users, both of which can be measured. This paper appeared in Transportation Research Circular No. 297, Providing Visibility and Visual Guidance to the Road User, TRB, 1985, p. 7-8.
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