Road lighting

Historically, the lighting of roads and streets was introduced to combat the crime rate. Although this is still a major consideration in justifying the installations of road lighting, the chief criterion now is the reduction of the night-time accident rate. Visual performance is considerably poorer at night than by day, particularly with respect to contrast sensitivity. Road lighting is, therefore, designed to maximise the contrast of objects on and near the road by producing as high a luminance of the road surface and surroundings as possible. Objects then generally appear as silhouettes against a bright background. The performance of a road-lighting installation depends on the lantern light distribution, the light source, the road-surface reflection characteristics and the installation geometry. It is possible to calculate the performance of the lighting in terms of road-surface luminance and uniformity and of glare. Direct measures of visibility have also been proposed. Design methods and standards/codes of practice to ensure a reasonable quality of road lighting vary considerably from one country to another. The British Standard is in course of revision, and some radical changes in design method may be adopted. A certain variation in performance of the resulting installations, however, is inevitable, principally because of the differences that exist in the reflection characteristics of road surfaces. Economic considerations mean that only two light sources are serious contenders for modern traffic-route lighting in the UK: low- and high-pressure sodium. Capital cost, luminous efficacy and life are key factors in costing road lighting. Finally, one form of road lighting that poses quite different problems is that for tunnels. The determination of adequate lighting by day has prompted a considerable amount of research.