How do ambient light levels influence criminal activities?

As budget cuts force more local authorities turn off the streetlights, are you more likely to get mugged or burgled? Two researchers in the US shed light on the stats.

Do light levels influence criminal activity? With streetlights being turned off in a bid to save money, economists Jennifer L. Doleac of the University of Virginia, and Nicholas J. Sanders of Cornell University have investigated the impact.

“Should policy-makers have expected crime to change as a result? How would this affect the cost-effectiveness of their budget-cutting strategy? Those are tough questions to answer,” they told Significance, the magazine of both the Royal Statistical Society (RSS) and the American Statistical Association (ASA). “A lot of factors influence how well-lit a particular area might be, such as local funding, demand from residents, and local crime trends. So understanding the effect of light on criminal activity requires more than a simple comparison of areas with and without good lighting.”

Fortunately, the US government had provided them a natural experiment to work with. In 2007, daylight savings time (DST) was extended by four weeks: clocks rolled forward three weeks earlier in spring, and rolled back one week later in fall. By looking at crime data for that three-week period in spring, Doleac and Sanders were able to compare the number of recorded incidents in hours that were darker pre-2007 against hours that were lighter post-2007.

They found that “in the weeks after the start of DST, the overall daily robbery rate (in terms of robberies per 100,000 population) decreased by around 7%”.

Their research into data on crime across the day showed a correlation between light and street crime – crime rates are higher at night than they are during the day. “However, there are clearly other differences between the day and night, and even gradual change in sunrise and sunset times over the year correlate with other seasonal trends. Simple correlations don’t tell us that less light means more crime,” they said.

 

In the weeks after the start of DST, the overall daily robbery rate (in terms of robberies per 100,000 population) decreased by around 7%”

Doleac and Sanders

Effect of streetlights

Much of the previous literature focused on streetlights, but effects were typically not well-identified. For instance, researchers looked at the difference in crime before and after neighborhoods added streetlights. They usually found crime fell at night, but also found crime fell during the day, when street lighting shouldn’t play a role. “This suggests it wasn’t the lighting that made a difference, but that streetlights are a signal of investment in a neighborhood, or the timing of installation correlated with underlying crime trends,” said Doleac and Sanders.

The only earlier research on daylight was a report following a year-round DST extension in 1974, comparing crime in Washington, DC, from one year to the next. It found violent crime was 10-13% lower during the year with extended DST than in years without. “This provided suggestive evidence about the crime-reducing effects of evening daylight, but of course many things change over the course of years so the study was far from conclusive,” they said.

 

Crime and time

Doleac and Sanders used data from the National Incident-Based Reporting System (NIBRS) because it contained detailed information on the time that crimes occurred. By using specific hour of the day (rather than just the date) to classify crime timing, they were able to investigate how shifts in daylight changed crime rates during different hours of the day. “If the mechanism was really about changes in ambient light, we expected effects primarily during the hours of sunrise and sunset (which were affected by the time change). We didn't expect anything different before and after the start of DST for hours of the day that were always light (eg 1 pm) or always dark (eg 11 pm).”

Similar logic led the pair to focus on robbery. It is a face-to-face crime where the danger of observation and recognition can play a larger role in the criminal decision, and it is a common street crime. For example, the data on robberies include muggings. It is also a crime where the hour of occurrence and the hour of reporting likely line up. Other crimes, while important and interesting to study, are less well suited for our particular source of variation and identification.

“When we looked at hourly crime rates, we found the only times with statistically and economically significant changes in crime rates were right around sunset. In the weeks after the start of DST, the overall daily robbery rate (in terms of robberies per 100,000 population) decreased by around 7%. Going along with our hypothesis about the mechanism being shifts in ambient light, that change is a product of a 27% decrease in robberies during the hours right around sunset,” they said.

 

Our research suggests that areas should think twice about reducing lighting, even when budgets are tight. Cutting down on a power bill might look like a good idea on paper, but the harder-to-observe social costs could easily outweigh the savings.”

Doleac and Sanders

What’s the cost?

The pair used work by McCollister, French, and Fang (2010), to estimate the economic cost saving generated by DST, including tangible costs, such as medical expenses, lost wages, and costs to the legal system, as well as intangible costs such as victims’ pain-and-suffering. “Those all add up to about $42,310 per robbery. By reducing the number of robberies, we avoid those social costs. Since our paper considers a three-week extension of DST in 2007, we multiplied our estimated reduction in robberies (0.215 per million residents per day) by the US population (roughly 310 million) by 21 days by $42,310. That comes to a $59 million social cost savings for those three weeks."

The research also found suggestive evidence that DST reduces rape, where each reported rape has a social cost of $240,776; if that effect is included, the annual social cost savings total $246 million over those three weeks.

So what are the implications for policy? “The main policy implication is that ambient light plays a role in crime rates, and shifting daylight from the morning to early evening hours appears to reduce street crime,” said Doleac and Sanders. “We find criminal activity isn’t simply shifted to the (dark) morning hours when DST is in effect; this is what makes DST good at reducing crime rates overall, and suggests that extending DST could be a simple way to reduce street crime.

“Regarding streetlights, our research suggests that areas should think twice about reducing lighting, even when budgets are tight. Cutting down on a power bill might look like a good idea on paper, but the harder-to-observe social costs could easily outweigh the savings.”

Read the original article in Significance

Read the full paper: 'Under the Cover of Darkness: How Ambient Light Influences Criminal Activity'

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