This blog post draws heavily on “The State of the Science” report by John Barentine, available here.
The belief that outdoor lighting improves traffic safety and discourages or prevents crime is common. It may explain in part the rapid growth in the use of outdoor light at night in recent years and decades. There are cases where the careful application of outdoor lighting may improve night-time safety, but there is no general benefit supported by scientific evidence.
Crime, Safety and Traffic
There is conflicting research on this topic and no consensus that adding or increasing lighting reduces crime, or traffic accidents, (except on busy urban junctions, where sensible lighting is shown to reduce accidents.)
Researchers have not been able to accurately predict or model the way that lighting might affect actual crime figures. There are issues with the scientific reliability and honesty of that show a reduction in crime rates where lighting is installed, e.g. one study concluded that putting additional lighting did reduce crime, but the researchers failed to report that in those areas, additional policing was also implemented, which was not in the areas with no or low lighting.
Confounding factors and variables have subtle effects on research that add up to important, erroneous conclusions because responsibility can easily be assigned to lighting even though it contributed very little. As a result, many of the claims about outdoor lighting and its impact on crime and traffic safety – for better or worse – may be fundamentally wrong (266, 267).
The Hazards of Glare
Glare from bright artificial light sources can decrease night-time safety. Intense light directly entering the eye from unshielded sources scatters inside the observer’s eye, reducing the contrast between foreground and background and reducing peripheral and night vision. Additionally, the pupil of the observer’s eye contracts, reducing total visibility by dimming the appearance of the entire scene. These effects make it difficult to see objects, such as cars or potential attackers, as distinct from what surrounds them.
Are We Afraid of the Dark?
Although the data on crime statistics is variable, how we feel about crime in relation to the darkness is not. People in Western, urban societies tend to feel safer when there is more light, but how much more is needed for this effect? The amount of light used in outdoor spaces at night may not reflect public expectations for feelings of safety and comfort (270), and artificial light itself may influence the human perception of fear (271). In some cases, over-lighting can itself become the source of safety hazards (272). However, properly designed lighting can reduce light pollution and save energy without compromising on public feelings of safety in outdoor spaces at night (273).
Conclusions for Crime and Safety.
The assumption that adding bright lights reduces crime is not supported by evidence. However, adding appropriate, sensible, thoughtful lighting does reduce our fear of crime at night, and also reduces the obvious safety issue of glare.
Lighting and Social Justice
We know very little about how light pollution affects people in social contexts. Light at night may be used in ways that affect neighbourhoods according to the race of the people who live in them. That may make light at night use a matter of social and environmental justice.
Light Pollution, Racism and Poverty
The only comprehensive study to date on this topic looked at the social aspects of lighting in the U.S. only (292). Researchers found that Americans of Asian, Hispanic and Black descent tend to live in brighter neighbourhoods (Figure 7). In these areas, skyglow is about twice as Figure 7. Average exposure to light pollution in the continental United States by racial/ethnic group. The bars show population-weighted average zenith night sky brightness levels in units of millicandelas per square meter. Figure 4 in Nadybal, Collins and Grineski, 2020 (292). high as in predominantly white neighbourhoods. They further note that lower socioeconomic status is also associated with higher night-time light exposures. These conditions can add to other social and environmental stressors such as poverty and exposure to air and water pollution, affecting quality of life.
Closely related to this is the idea that light pollution is harmful to people whose religious or cultural practices rely on access to the night sky. The erasure of the stars from view due to skyglow separates people from this resource. Some argue that it threatens Indigenous traditions and knowledge systems based on accessibility of the natural night sky (300).
- Marchant, P. Why lighting claims might well be wrong. International Journal of Sustainable Lighting, 19(1):69–74, jun 2017. doi: 10.26607/ijsl.v19i1.71.
- Marchant, P. Do brighter, whiter street lights improve road safety? Significance, 16(5):8–9, oct 2019. doi: 10.1111/j.1740-9713.2019.01313.x.
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- McGlashan, E.M., Poudel, G.R., Jamadar, S.D., Phillips, A.J.K. and Cain, S.W. Afraid of the dark: Light acutely suppresses activity in the human amygdala. PLOS ONE, 16(6): e0252350, jun 2021. doi: 10.1371/journal.pone.0252350
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- Nadybal, S.M., Collins, T.W. and Grineski, S.E. Light pollution inequities in the continental united states: A distributive environmental justice analysis. Environmental Research, 189: 109959, oct 2020. doi: 10.1016/j.envres.2020.109959.
- Hamacher, D.W., de Napoli, K. and Mott, B. Whitening the sky: light pollution as a form of cultural genocide, 2020.