We've all been there at some point. You're walking in a dark space at night, alone, maybe to your car in a parking lot. And a kind of fear seizes you. You can't see very well around you, so you don't know if there is someone waiting in the shadows. You get to where you're going as fast as you can, unlock your car door, get in and lock the door behind you. For a moment, you feel safe again.

My friend and colleague Nancy Clanton, a lighting designer based in Colorado, explains how the situation is even more acute for women. She told me about an incident once, some years ago, when she was in an unfamiliar city to attend a lighting conference. She had to walk alone at night along a dark stretch of road between the convention center and her hotel. Even with all her knowledge about lighting and darkness, she still felt uneasy being out there by herself.

She saw two people, both silhouettes, approaching. Like many people in the same situation, Nancy had to make a quick decision. Were they two threatening men? An innocuous couple? Something else? In her mind, at that moment, was born the idea of "lighting for reassurance". With a small amount of well placed light, she could have made a quick friend-or-foe determination. And if circumstances required, she could have made a quick exit. It would have changed the situation in a fundamental way.

There are many reasons that people use artificial light at night. For example, we light sidewalks and pathways to help people orient and find their way from Point A to Point B. We light roadways and "conflict zones" where different kinds of traffic come together, because we know that doing so saves lives. We like to add nighttime amenity to outdoor spaces, making them more inviting for activities like commerce. But we also liked it because many people believe that light at night deters or even prevents the incidence of crime.

Does outdoor lighting yield any real, positive influence on nighttime safety and security? Does it reduce criminal behavior? "The influence of outdoor light at night on crime is mixed," writes DarkSky International in its most recent annual report Artificial Light at Night: State of the Science 2024. "Some of the same studies that looked at lighting and traffic/pedestrian safety also considered nighttime crime incidence. Certain studies reported crime reduction when lighting is added to outdoor spaces. Others find either a negative effect, no effect, or mixed results."

​Whatever the ways in which lighting and crime interact, it's not like the effect of administering a drug to a person. That is, there is no sense that a dose X of the medicine always produces effect Y in the individual.

Crime and lighting seem to have a very context-dependent relationship. There are very many of what researchers call "confounding variables". These are influences unrelated to the study subject that might produce some kind of effect in the system. Without careful control of these variables, the researcher can draw an incorrect conclusion on the basis of observing the intended effect.

To continue the medical analogy, drug researchers try to remove differences among people in patient study samples. These include age, gender, geographic, location, and other factors that might produce a result that mimics what the drug should do. Failure to deal with these "confounders" means that one can be fooled into believing there is an effect from a drug that is in fact, unrelated to its mechanism of action.

It is also very difficult to carry out high-quality, well-controlled studies. Designing reliable (and replicable) lighting studies is challenging. There is not much research funding for this. Sometimes the sources of research funding are questionable. Researchers worry they may want to see some sort of pre-defined result from studies they support.

And there are instances where experimental design is just bad. For example, a 2019 study conducted in New York City housing projects claimed a strong relationship between high levels of light and decreased crime. The study looked at the effect of placing intense light sources on portable towers outside housing buildings and tracking changes in the incidence of crime in the immediate environment around them.

​To the surprise of few, crime dropped. But that's what happens when you treat people like criminals and light up their homes like they were prison yards: you get unreliable results.

What underlies concerns about outdoor lighting, public safety and crime is the unspoken element of fear. It is a powerful and underestimated motivator of human behavior. Our evolutionary heritage has given us limited abilities to see at night. As diurnal animals are active during the day, it made night a dangerous place for our early human ancestors.

Although we tend to think of fear of the dark as something that particularly afflicts children, many people as adults feel a deep-seated sense of terror toward dark places at night. How much we know about a place determines our sense of ease (or unease) about it. Because we are so dependent on our sense of sight, it's easy to feel that we lose control when we cannot see what is happening around us.

Psychologists have long understood that these influences have tremendous power in shaping our perception of the world. In recent years, social scientists have begun investigating what they call "feelings of safety". That is, they try to measure the degree to which people feel safe or unsafe in different situations that may involve danger.

It is important to know that feelings of safety are independent of whether people are actually safe. There are many instances in which people feel unsafe about a situation but the evidence shows that there was no actual danger.

There are ways in which feelings of safety can increase in dark places at night that lead people to accept lower light levels. This relates to a sense in which people feel trapped by their surroundings. They are looking for the exits, so to speak, in case danger suddenly emerges. Research shows that lower light levels are less acceptable to most people in any situation where they feel trapped. If there are clear opportunities to escape if needed, the acceptability of lower light levels begins to rise. Other work shows that most people prefer somewhat higher levels of outdoor light at night. Most prefer light that is cooler than warmer in color appearance.

​But the value of this extra light diminishes very quickly as light intensities increase. The relationship between feelings of safety and light intensity appears to be logarithmic. What that means is that as light levels increase, the amount of an increase necessary to produce some certain amount of increase in feelings of safety becomes larger.

​The biggest increases in feelings of safety happen in moving from situations where there is no light to those in which there is a very small amount of light. To raise feelings of safety by again as much requires increasing the light intensity by much more, often more than a factor of ten.

​The increase in feelings of safety flattens out at highlight levels. We don't quite understand why. One theory is that high intensity lighting makes people feel insecure by promoting the sense that they are on display. Bright lighting can create deep, dark shadows between objects that obscure those spaces. A person moving through an outdoor space at night may not be able to tell whether there is a threat hiding in those shadows.

​The glare from very intense lighting sources also has a disabling effect on the viewer. Glare causes the pupil of the eye to contract, which reduces the depth of field of vision. It conveys a distinct disadvantage to the viewer. And it's the same "prison yard lighting" effect in the New York study mentioned earlier.

That's where "lighting for reassurance" comes in. It's an outdoor lighting ethic still new in the design community. While traditional design holds that more is better, lighting for reassurance asserts that better is better. Often that's less light chosen with a strategy in mind. The goal is to improve outdoor visibility at night and help users of spaces discern where threats might be.

​In particular, it makes use of the incredible properties of human vision, even at low light intensities, to see very small changes in contrast. It ensures that all potential safety hazards are clearly indicated. It also means not going overboard with lighting levels to the point that it becomes a disability for the viewer.

It also makes outdoor spaces at night more inviting to other users of those spaces. People generally tend to feel more unsafe when they are alone in such places at night. As more people fill outdoor squares, streets and other places, they begin to feel more safe in the presence of a crowd. This couples with the popularity of using outdoor light at night for aesthetic purposes. This can include lighting of building façades, statues and monuments, and other landscape features to enhance the sense of nighttime placemaking. And it avoids creating glare to the greatest practical extent.

The task is now to communicate this to more lighting designers. They don't usually learn it either in their formal education or as part of their praxis. We also have to change how lighting designers and engineers think about the concepts of minimum and maximum lighting levels. Right now, lighting standards generally tend to indicate only minimum lighting levels for various lighting applications. This is a belief, not always supported by evidence, that only certain minimum values are "safe". But it is often not clear why those values are "safe" while other values are not.

​It also does not take into account the idea that lighting can be too bright in some cases. That can create its own security problems. Organizations that make outdoor lighting standards are just beginning to embrace the notion that along with recommended minimum lighting values should come corresponding maximum values. The right lighting levels would make Goldilocks herself feel safe at night because they're neither too low or too high. They're just right.

What can we learn from all this? For one thing, many people experience fear of nighttime darkness, and that fear is visceral. We shouldn't tell them that they are wrong to feel fear because we have data that somehow show that their fears are unfounded.

It's also the case that feelings of safety are very powerful. We should recognize this and leverage that fact in outdoor lighting design. Furthermore, the human eye is an underrated detector of faint light at night. That said, lighting design often does not fully exploit its amazing properties. Instead, and as lighting has become much cheaper than ever to consume, design has pushed light levels very high.

The result is that the way we do lighting design now often does more harm than good, even though its practitioners clearly want to do good. What we can do for them is to help them put in place better design through lighting for reassurance. It combines the best of what scientists tell us about lighting with the power of psychology.

​Together, we can make outdoor spaces at night that are truly more safe while also helping people in them feel safer. If we overcome the fear that now leads to demand for brighter outdoor spaces, we'll increase support for measures that gradually draw down light pollution. To do so would be a win for all involved: for people, for the environment, and for the night sky.

Summary: Although subject to widely held folk beliefs, the truth about how outdoor lighting and crime interact is very unsettled scientifically. This post examines the evidence, identifies shortcomings in current research and practice, and suggests the factors and considerations that really matter.