When a community feels unsafe, the first two things proposed are almost always the same: better lighting, and more CCTV cameras.

Both belong to an idea known as CPTED, crime prevention through environmental design, which holds that it is possible to use the built urban form to reduce opportunities for crime.

Notably, the idea does not rest on four principles, as it is often described. A review in the Journal of Planning Literature records that CPTED is based on seven key strategies: territorial reinforcement, surveillance, image, access control, legitimate activity support, target hardening, and geographical juxtaposition, meaning the surrounding environment.

The term was coined by C. Ray Jeffery in 1971, building on foundations laid by Jane Jacobs and Oscar Newman.

"The question is not whether the idea sounds reasonable, but what it delivers when measured: by how many per cent does crime fall, and which kinds of crime fall at all?"

Source: https://prohic.nl/wp-content/uploads/2021/02/166-15feb2021-CPTEDReview.pdf

 

Lighting Works, But Not on Everything

Before the numbers, a word about what kind of study these are.

A systematic review gathers every study ever done on a question that meets a set of stated criteria, and pools their results into a single calculation. This is more reliable than picking one study with an attractive result, because a single study can produce such a result by chance.

The review of street lighting published in 2022 covers almost half a century of research, assembling 21 evaluations from four countries, the United States, the United Kingdom, Brazil and South Korea, spanning 1974 to 2021.

These studies work by comparing an area where lighting was improved against another area of similar character where it was not, known as a comparison area. A comparison area is necessary because crime rises and falls on its own with the seasons and with the economy. Looking only at the lit area would leave no way to tell whether a fall was caused by the lighting or by something else entirely.

Pooling the results of 17 studies, crime fell by 14 per cent in the treated areas compared with the comparison areas.

Split by crime type, the picture changes. Property crime such as theft and burglary fell significantly. Violent crime such as assault did not fall significantly at all.

Not statistically significant here does not mean the figure was zero. It means the difference measured was too small to separate from the ordinary fluctuation in crime numbers. In plain terms, it cannot yet be concluded that there was a real effect.

Source: https://prohic.nl/wp-content/uploads/2022/05/389-StreetLightingEffectivenessPreventingCrimeSystematicReview.pdf

 

The Strangest Finding in the Lighting Research

The reviewers divided the studies into two groups according to how they counted. The first group counted crime during both day and night. The second counted night-time crime only.

The result runs entirely against expectation. In the group counting day and night, crime fell by 18 per cent. In the night-only group it fell by just 3 per cent, too small to conclude there was a real effect.

Consider why that is strange. If lighting works by making things easier to see, the effect should be clearest at night, because daylight is already there and additional street lamps change nothing during the day.

Yet the data says the opposite: including daytime in the count made the figure better, which means daytime crime fell as well, in hours when the lights were not even on.

If daytime crime fell too, then whatever produced the fall is probably not the light itself, but something that arrived along with the installation.

The reviewers therefore suggest an explanation: that authorities coming in to fit new lighting may make residents feel their area is cared for and worth taking pride in, so they watch over it more attentively throughout the whole day. That explanation covers daytime and night-time alike.

Source: https://prohic.nl/wp-content/uploads/2022/05/389-StreetLightingEffectivenessPreventingCrimeSystematicReview.pdf

 

The Other Explanation That Cannot Be Ruled Out

What makes this review trustworthy is that the reviewers do not stop at the explanation they prefer. They warn that another explanation cannot be excluded, and it reverses the order of cause and effect entirely.

The first explanation runs: the lights go in, the community takes pride and watches over the place, and crime falls.

The second runs: communities that are already strong and already care about their surroundings are the ones that organise, lobby and get the lighting installed, and such communities already have lower crime for other reasons. If that is what happened, the lighting did not cause the fall in crime. It was a by-product of a community that was already strong.

The reviewers state that, based on most of the studies included in the review, it is difficult to exclude the second explanation.

Another limitation they consider more pressing concerns displacement, meaning cases where crime does not actually disappear but moves to the next street along, which is still dark. Where that happens, the figures for the lit area look better while the city as a whole is no better off at all.

The difficulty is that only three of the eight newer studies examined this, which leaves it unanswered whether the crime that fell truly went away, or simply moved.