Influence of pavement reflection on target visibility

Abstract

The properties of light reflecting from pavement surfaces have been studied for several years in both Europe and North America. The data has been used for the calculation of the luminance of the pavement in fixed roadway lighting design. With the proposed IESNA Standard Recommended practice RP-8[1997], the visibility of objects on the roadway has been added as a design quality criterion rather than the pavement luminance only.

To calculate the visibility of objects on the pavement, a model of visual sensitivity is used to calculate the required contrast of an object to its background. The required contrast, called the contrast threshold, is then compared to the actual contrast of the object. The ratio of these two values is called the visibility level. The average of this visibility level, calculated for several points on the roadway, is then established as the criteria for the quality of the roadway lighting design. In order to calculate the contrast threshold and the actual contrast, both the background luminance of the target and the target luminance must be calculated. The background luminance of the target is based on a calculation which has been established for several years. The target luminance, however, is not as simple. It is calculated from both the direct light from the luminaires and the indirect light reflected from the pavement surface. Both of these calculations, the background and target luminance, require knowledge of the reflection properties of pavement surfaces.

The pavement reflection data which currently exists is valid for a one degree observation angle only and can be used only for the background pavement luminance. It is not valid for the target luminance calculation. This research is the investigation of the surface reflection properties in order to fully describe the scatter of reflected light in all directions. Using samples of several pavement types, the reflection properties of the surfaces were measured. These measurements were then analyzed in terms of the surface characteristics.

An investigation of the surface roughness of the sample was also undertaken to investigate the possibility of relating the reflection properties to the physical properties of the pavement. No relationship was found to the reflection data but the roughness was used towards the development of an analytical model of the reflection properties.

Several methodologies have been investigated for an analytical solution of the reflection data. Some work towards a solution has been undertaken but the final model requires further investigation into the physical characteristics of the pavement.

This reflection data has been used in the development of a computer program which calculate s the pavement luminance, the target luminance and the visibility level. The visibility level for several installations have been calculated. These test installations which have been studied by other researchers, have provided actual target luminance and background luminance measurements. The results of the calculations have then been compared to the data provided. It was found that the calculated contribution of the reflected light to the target luminance follows the same trend as the measured results. This verifies both the calculation methodology and the nature of the reflection data.

The influence of the reflected light calculation on the design weighted Visibility Level of the roadway is very small. The impact was calculated for differing road surfaces and installation types and very little influence was found. The impact of the reflected light is very significant to individual target locations. For some individual targets, the addition of the reflected light reversed the calculated target contrast and increased the visibility by as much as 4 visibility levels. The effect is very similar to that found by previous research into the verification of the visibility level calculation. Further comparisons of the calculated target luminance to actual target luminance is required to fully verify the calculations of IESNA RP-8.