Investigating Non-Image-Forming (NIF) effects of lighting for pedestrians after dark

A key purpose of road lighting is to allow road users to proceed safely. On minor roads, road lighting is designed to meet the needs of pedestrians and cyclists. Current guidelines for designing outdoor lighting focus on their visual needs: it is not known whether they are in line with the ongoing understanding of non-image-forming (NIF) effects and the unwanted side effects of road lighting. The NIF effects of light are psychological, physiological and behavioural effects of light. One such effect is that light with a peak around the blue range of the light spectrum can improve alertness and cognitive performance. Alertness is the state of being ready to see, understand, and act in a particular situation. Enhancing alertness could make pedestrians more alert to potential hazards, enhance visibility and improve visual perception in detecting dangerous objects. A lack of alertness may contribute to road traffic collisions (RTCs) and pedestrians sustaining injuries from falls.

In this thesis, two laboratory experiments were conducted to investigate the NIF effects of road lighting in a context representing pedestrians. The aim of the experiments was to explore the effect of lighting on pedestrian alertness and melatonin levels in the evening, using lighting conditions typical of outdoor lighting. 80 participants were recruited, and trials were conducted in pairs. Four dependent variables (DVs) were measured: reaction time (measured via Psychomotor Vigilance Task (PVT)), subjective alertness (assessed via Karolinska Sleepiness Scale (KSS)), melatonin levels (collected via saliva samples) and skin temperature (measured via iButtons).

The results of the first experiment did not suggest an effect of changes in lighting on any of the four DVs. The null finding supports the results of Bhagavathula et al.113 and Gibbons et al.114 However, it was not certain whether the null finding meant no effect, or rather that the experiment was insufficient to reveal an effect. This was tested in the second experiment by using more extreme variations in test conditions. The same protocol and DVs were used as in the first experiment, excluding the skin temperature measures. The results of the second experiment suggested that for melanopic equivalent daylight illuminance (EDI) of up to 10.7 lx, there was no effect on alertness or melatonin level, confirming the findings of previous studies.113,114 Increasing the melanopic EDI to almost 100 lx revealed a significant reduction in reaction time, an increase in subjective alertness and a decrease in melatonin level. Changing position from sitting to standing increased melatonin concentration. Walking at a moderate speed significantly shortened reaction time and enhanced subjective alertness.

The results of this thesis do not suggest that lighting on minor roads has a significant impact on the alertness or melatonin level of pedestrians and thus would not affect the choice of optimal lighting conditions.