The Effect of Pavement Material on Road Lighting Performance

Abstract:

The primary purpose of road lighting is to make people, vehicles and objects on the road visible by revealing them in silhouette against the road surface. As a result, road lighting standards are expressed in terms of three luminance metrics, average road surface luminance, overall luminance uniformity ratio and longitudinal luminance uniformity ratio. The luminance of any point on a road surface is a function of the illuminance on, and the reflection properties of, the pavement material. The reflection properties of the road surface will be determined by the pavement material used, whether it is wet or dry, and how much use the road has had.

Despite the existence of these variables, the recommended design method for road lighting in the UK uses one set of data for characterizing the reflection properties of road surfaces, called the representative British road surface, although this is modified for concrete roads. Quantitatively, the reflection properties of a road surface are given by a reduced reflection coefficient table, called an r-table. This r-table is summarised by two metrics; Q0, this being a metric of the diffuse reflection, and S1, this being a metric of the specular reflection. The representative British road surface design method has been applied for many years to roads constructed with such established pavement materials as hot rolled asphalt and brushed concrete. However, there are now a number of new asphalt-based pavement materials available, such as porous asphalt, stone mastic asphalt and a number of proprietary thin surfacings together with one new concrete-based pavement material, exposed aggregate concrete. The first objective of this report is to determine whether these new pavement materials can be accommodated within the representative British road surface road lighting design system. If they cannot, the second objective is to suggest what should be done to ensure the accurate design of lighting for roads where these new pavement materials are used.

The first part of this report summarizes the development of the representative British road surface and describes how it is used in the calculation of road lighting luminances. Then, the magnitude of the errors inevitable in using a single r-table to describe many different pavement materials is examined, as is the effect of use on the reflection properties of pavement materials. The reflection properties of a pavement material change markedly over the first six months of use, this change contributing to the large discrepancies that can occur between the luminance metrics calculated using the representative British road surface and r-tables specific to different pavement materials.

From a consideration of the calculations made and the caveats expressed, the following actions are recommended:

1. Action should be taken to confirm the validity of the Q0 values for both established and new pavement materials given in Cooper et al. (2000). This should be done in two stages. The first is to identify a laboratory based measurement system capable of giving consistent results for the same pavement material sample. The second is to use the identified measurement system to measure r-tables for all pavement materials frequently used in the UK, the materials being dry and at an appropriate state of wear.
2. If the Q0 values given in Cooper et al. (2000) are shown to be valid, a decision has to be made on whether or not to accept errors in the average road surface luminance of the magnitude found here, for both new and established pavement materials. If such errors are acceptable, then the representative British road surface approach can be applied to the new pavement materials without change. If such errors are not acceptable, the representative British road surfaces in BS5489 should be abandoned as a basis for road lighting design.
3. If the representative British road surfaces in BS5489 are to be abandoned, they should be replaced with two new r-tables, one for asphalt-based pavement materials and one for concrete-based pavement materials. These two new r-tables might be formed from the current C2 r-table but with every cell adjusted so that one r-table has Q0 = 0.050 and the other r-table has Q0 = 0.085. The former r-table would be taken as representative of asphalt-based pavement materials. The latter r-table would be taken as representative of concrete-based pavement materials.
4. To avoid any consequent increase in costs for road lighting following such a change in recommended r-tables, the soundness of the current luminance recommendations used for road lighting design in England and Wales should be assessed.
5. To avoid any consequent increase in costs for road lighting following such a change, the practicality of increasing the amount of light reflected from pavement materials by incorporating brighteners into the material mix should be evaluated.
6. The practicality of measuring road luminance metrics from a moving vehicle should be investigated. Equipment designed to do this already exists. Its use would provide a means for determining compliance with contract and for identifying the need for maintenance.

1. Background
2. The route to the representative British road surface
3. Errors inherent in the use of the representative British road surface
4. Road reflection properties and pavement recipes
5. New pavement materials and their reflection properties
6. Questions to be addressed
7. Discussion
8. Caveats
9. Recommendations
10. References
11. Acknowledgements
12. Appendix

Publication Year:

2005

Publisher:

Department for Transport

DOI:

No DOI minted

Author(s):

Fotios, S., Boyce, P. and Ellis, C.