Visibility Distance with Headlights: A Functional Approach

This paper reports a novel approach to quantitative prediction of nighttime visibility with headlight illumination. Unlike previous efforts to model nighttime visibility, our goal is to provide an index of visibility that can be understood by non-specialists, as well as scientists and engineers, as a useful approximation of visibility distance for ordinary objects. Specification of the functional range of recognition vision is based on the concept of Civil Twilight. Early astronomers defined Civil Twilight as the time period following sunset or before sunrise when normal outdoor activities can be conducted without supplementary illumination. Psychophysical research later showed that visual functions such as acuity and contrast sensitivity deteriorate rapidly over this range of Illumination, thus providing scientific validation of the astronomers' observations. In the present approach, the headlight beam is treated as a spatial analogue of changes in illumination during Civil Twilight. Thus, the practical limit of useful recognition vision is defined as the Twilight Distance, which is the point that headlight illumination is equal to the darker limit of Civil Twilight. Twilight Distance distributions for low and high beams of different headlight systems are presented, and implications for driver performance are discussed.