Recent developments in road lighting.

The design method for road lighting has changed greatly in the last 15 years and is about change again over the next few years. Initially road lighting was designed using pavement illuminance as the criterion: the design goal was to achieve a minimum specified average pavement illuminance and a minimum level of uniformity of illumination. This, of course, did not correlate with what we see - for that the method had to be luminance based. Ready access to computers and an international effort to collect data on pavement reflectance, have made it possible to predict pavement luminance for a given lighting installation. However, there are many ways to calculate the pavement luminance and its uniformity, so the CIE set down some rules and methods and a computer program so that all users would obtain the same results with identical lighting installations. But we don't see by virtue of the level of luminance alone. Our visual performance is based on detecting differences in luminance: we see by virtue of contrasts. There is now a push to design road lighting according to a visibility criterion. This is a more valid way of designing road lighting as the purpose of road lighting is to improve visibility in the night-time road environment and hence the visual performance of drivers. While it is clear that road lighting does make driving safer at night (CIE 1992), how it does this is not at all clear. Because the mechanism by which safety is increased is uncertain, it then becomes a matter of discussion as to how a visibility-based design method would be implemented. There is currently a proposal in the USA for calculating the visibility level (VL) of a road lighting installation using small target visibility (STV). The proposal has not yet achieved consensus and discussion is lively. One of the reasons for its contentiousness is that in many instances it recommends a lighting installation for a particular piece of road which is quite different to that which would be installed based on the currently accepted method using pavement illuminance. This paper is a brief summary of some of the papers presented at the 2nd international symposium on Visibility and Luminance in Roadway Lighting held in Orlando Florida on October the 26th and 27th 1993. (Author/publisher)