What do you think would happen to road casualties and crime rate if you turned off all the street lights at night? You might think that more people would be hit by cars and crime would rocket, but in a study looking into the public health implications of turning the lights out to save local authorities money, researchers from the London School of Hygiene and Tropical Medicine found something quite surprising.

“We’ve got data on something like 20,000 kilometers [12,400 miles] of road, from 62 local authorities. And we have information on road traffic collisions and crime. So our analysis has got a lot of data, and we found no evidence for an increase in road traffic casualties or for crime at an area level – which wasn’t what we were expecting at all,” Dr. Phil Edwards, who co-authored the study, told IFLScience. “I was expecting to see an increase.”

This result seems counterintuitive, but when you look into it and start to pick it apart, there is some logic behind it all. “Because drivers are taking more care, they’re maybe driving into a road with no lights on and they might slow down a bit,” explains Edwards. “[It] may be that most drivers adapt their driving to the conditions. But, equally, maybe the lights going out means that there are fewer people walking down those streets. So maybe there are fewer people who could be injured.”

However, Edwards notes that there is a change that they couldn’t control for. “The other thing that we weren’t able to adjust for was that some local authorities may decide they need to improve the road in other ways,” says Edwards. “If they’re going to reduce the lighting, they might need to repaint the white stripes on the road, say, or improve signage or something.

“So if local authorities were to do those things, at the same time as the lights were changed, then we wouldn’t be able to disentangle the effect of those two things.”

The study, published in the Journal of Epidemiology and Community Health, managed to get data from local authorities on when they started either turning street lights off at midnight, dimming them by 25%, or switching from old-fashioned orange lights to newer white LEDs. They then used publicly available data on road traffic casualties and crime rates to map month by month when and where accidents and crimes occurred. They then looked at whether there was an associated change when the street lights changed. They found no difference.

Edwards does warn, however, that this doesn’t mean that local authorities should all start turning their street lights off to save on money and reduce their carbon footprint; each street will have to be assessed for other risks, such as how fast the traffic is at night on each particular road. They will also need to take into account the views of the local residents, as the study also found that people living in the country were far more open to having street lights turned off compared to those living in cities.