Dominant contrast as a metric for the lighting of pedestrians

CIE Publication 115 and ANSI/IESNA Recommended Practice 8-00 both use vertical illuminance 1.5 m above the ground as a design criterion for the lighting of pedestrians. While vertical illuminance has the advantage of being easy to calculate and measure, visibility is based primarily on target contrast. A central question related to the visibility of pedestrians is whether drivers need to see the whole pedestrian or can they infer the presence of a pedestrian by recognizing any part of the pedestrian's shape. The objective of this work was to first explore various pedestrian contrast profiles that could exist and then to find a simplified approach to characterize pedestrian night-time visibility. The problem was addressed through theoretical analyses and computer simulations. Pedestrian contrast was found to be bipolar and dynamic. From the contrast profiles, we developed the concept of dominant contrast, which is defined as the contrast of any part of the pedestrian that provides the highest visibility. Dominant contrast was examined as a metric for street lighting design and night time visibility for (a) an unlit street with car headlights, (b) a lit street without car headlights and (c) a lit street with car headlights. Dominant contrast was found to be a viable metric for street lighting design and night time visibility studies.