This paper examined what might be done to improve the energy efficiency of lighting whilst maintaining the benefits of lighting, looking at changes in technology, patterns of use, design standards, and the basis of design. Considering carefully what problem road lighting is intended to solve and whether or not road lighting is the best answer is the key to minimizing the energy consumption of road lighting without diminishing road safety.
The 2013 version of BS5489-1 changed the way in which light spectrum was considered in minor roads. Previously, it was permitted to drop light levels by one class when using lamps of high colour rendering (Ra>60) but that decision was based on incomplete research. The 2013 version introduced a sliding scale reduction, as characterised by the S/P ratio. This article describes the reasons behind that proposal.
Fotios S, Goodman T. Corrigendum to Fotios S and Goodman T. Proposed UK Guidance for Lighting in Residential Roads. Lighting Research and Technology, 2012; 44(1); 69-83. Lighting Research and Technology, 2012; 44(3): 377-377
This paper presents a review of road lighting research and was included in the special issue to mark the 50th volume of Lighting Research and Technology. The article focuses on the basis of quantitative recommendations given in guidance documents, primarily how much light should be given. A key conclusions was that “Recommendations for the amount of light do not appear to be well-founded in robust empirical evidence, or at least do not tend to reveal the nature of any evidence.” If the basis of current guidance is unknown, then it is difficult to establish if the intended aims of road lighting are being met when the guidance is followed.
Some road lighting standards offer semi-cylindrical illuminance as an alternative design target. This paper presents the work undertaken to establish the basis for this recommendation. It concluded that there is no basis for using semi-cylindrical illuminance, or at least there is currently no credible research which supports why it should be used rather than horizontal or vertical illuminance.
BS5489-1:2013 permits the illuminance of P-class road lighting to be reduced when using lighting of higher S/P ratio. One reaction to this is that higher S/P ratio demands lighting of higher CCT, with an accompanying declaration of doom. This paper compared the CCT and S/P ratio of about 300 light spectra. While there is a strong association between CCT and S/P ratio, it is possible to vary S/P ratio at a given CCT, in particular where it is permissible to change the type of light source.
To determine mesopic luminance for a given photopic luminance and lamp spectrum, the CIE system for mesopic photometry uses either an iterative equation (accurate but complex) or a look up table (simplified and easy). This paper proposed an alternative equation which maintains sufficient accuracy (an average error of 0.28% compared to the iterative approach) but without the next for iteration.
This article reviewed the basis behind current design standards for P-class lighting. A key problem concerns the components of the points table, as used to establish a lighting class. To some degree they are common sense, although they do not match the elsewhere-stated purposes of road lighting. The main problem is that we do not know if the change in lighting class actually resolves the apparent issue. Consider adding a point (and thus a change of one lighting class) if assuming traffic volume is very high rather than moderate. There is no evidence that that change in lighting will have any effect on the increased RTC risk associated with greater traffic volume. It may also be the case that the original light level was already satisfactory to meet the needs of a very high traffic volume, and the higher illuminance does little more than increase energy and sky glow. On the other hand, it may be the case that neither lighting class was sufficient for either case. Designers may be being misled by the points table.
Fotios S. A review of design recommendations for P-class road lighting in European and CIE documents - Part 1: Parameters for choosing a lighting class. Lighting Research and Technology. Online first, 04/10/2019. DOI: 10.1177/1477153519876972
Most lighting design standards or guides refer to average illuminance without clearly stating whether that should be the arithmetic mean or median illuminance. The two can lead to widely different estimates of ‘average’. In this paper we compared means and medians for a sample of P-class roads.