Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 37

Fig. 3.26 Bright and dark patches on a wet road surface
will produce very bright patches on its surface alternating with large dark patches
(Fig. 3.26 ).
The bright patches can have luminance values of more than ten times the values at
the same area during dry conditions (Ekrias et al. 2007 ). The result is that the average
luminance of the surface increases while the overall and longitudinal luminance
uniformities decrease. While the increase in average road-surface luminance has a
positive effect on the adaptation state of the driver's eyes, the decrease in uniformity
has a much larger negative effect on overall visual performance. The most important
influence on the wet-weather quality of the road lighting is without doubt that exerted
by the nature of the road surface itself, although the light distribution of the luminaires
and the type of lighting arrangement also have a, somewhat smaller, influence. Open-
structure road surfaces that easily drain away water remain diffusely reflecting even
when wet, and thus have only a small negative impact on wet-weather road-lighting
quality. As will be shown in the Section “Standards and Recommendations”, some
standards give specific requirements for the value of the overall uniformity U o under
wet road-surface conditions.