Research note: Describing average illuminance for P-class roads

Design criteria for lighting in subsidiary roads usually include a minimum average horizontal illuminance, widely assumed to be the arithmetic mean illuminance. Analyses of the illuminance distributions over thirty road sections shows that the distributions are not normal and hence the median is more appropriate than the arithmetic mean as a measure of central tendency: the medians are significantly lower than the arithmetic means but the two are highly correlated. Design recommendations should state whether it is the arithmetic mean or median and not just the ‘average’ that is required

Using obstacle detection to identify appropriate illuminances for lighting in residential roads

This paper uses data regarding detection of pavement obstacles to explore two approaches to establishing an appropriate illuminance for road lighting designed to meet the needs of pedestrians. A previous obstacle detection experiment was repeated using young observers under high pressure sodium (HPS) lighting. One approach was to identify whether there is a plateau-escarpment relationship between obstacle detection ability and illuminance – better detection with increasing light level until further increases bring little improvement: This suggested an appropriate illuminance of 5.7 lux. The second approach was to identify the size of an obstacle that a pedestrian should expect to be able to detect and the associated probability of detection: An obstacle of height 25 mm located 6 m ahead may require 1.8 lux to be detected with 95% probability

Lighting for subsidiary streets: investigation of lamps of different SPD. Part 1 - Visual Performance

British Standard BS5489-1: 2003 permits a trade-off between colour rendering and illuminance for lighting in subsidiary streets—if lamps of high colour rendering index such as metal halide are used instead of high- or low-pressure sodium lamps, a lower average illuminance can be used. A series of tests are carried out under mesopic conditions to validate the trade-off, and this paper reports on the new visual performance results. Four tests are carried out: acuity of achromatic and chromatic targets, achromatic contrast detection threshold, and colour identification, these being for on-axis targets. It is found that spectral power distribution (SPD) does not affect the performance of achromatic tasks except for an increase in contrast detection threshold under low-pressure sodium lamps. The performance of an acuity task using coloured targets displayed interaction between target colour and SPD. Colour naming accuracy is found to be significantly higher for metal halide lamps than for sodium lamps. For all tasks there is a reduction in visual performance at lower illuminances, and therefore a reduction in design illuminance leads to a reduction in the performance of some visual tasks which may not be offset by lamp SPD. Implications for the performance of real pedestrian tasks are discussed