In the early 2000s, the city of Glasgow, Scotland, changed over to blue street lighting in an effort to improve the appearance of the city, but areas with the newly blue lights saw an unexpected and disproportionate decrease in crime.
No one could say for sure whether there was a causal correlation, but half a world away another city followed suit – Nara, Japan, experimentally began turning their street lamps blue. They also saw a decrease in crime.
In the years that followed, similar deployments were tested, including the addition of over 150 blue lights to an expressway near Tokyo, aimed at reducing accidents.
The Keihin Electric Express Railway Co. even changed the color of eight lights on the ends of platforms at Gumyoji Station in Yokohama, Japan, to curb track-jumping suicides. A subsequent report showed that in the years following, the station saw no suicide attempts (after previously being the site of multiple suicides per year).
There are a number of theories about why the blue light could be causing a decrease in criminal behavior.
Some argue the association of blue with police presence, except: Japanese police use mostly red lights.
Another theory posits that blue light has a calming effect, feeling more pleasant than orange or red. Indeed, studies have shown that blue lights (associated with calm waters and open skies) can reduce stress responses in fish and be used to treat SAD (Seasonal Affective Disorder) in humans.
Of course, it is also possible that the new colored lights are simply different and unfamiliar enough to cause people to feel awkward engaging in illicit or dangerous behavior where they are present. And if that is the case, changing all the public lights in the world to blue might be at best temporary fix, working until people get used to the new blue lighting. Experts have warned not to read too much into the effect, regardless, since the causal mechanism or connection has not yet been definitively determined.