James Holloway writes for Ars Technica about new research from Temple University that has found that in urban areas the, “More vegetated areas tend to have less crime.”
One school of thought has it that inner-city trees and shrubs make convenient hiding places and covered escape routes for criminals. Another, supported by an increasing body of evidence, argues that urban foliage may actually reduce crime. Where previous studies have tended to focus on individual housing blocks or, at best, neighborhoods, new research out of Temple University is among the first to examine the issue at the city-scale. TU researchers analyzed the relationship between vegetation concentration and crime for the whole of Philadelphia.
The researchers broke the city down into 363 “tracts” identified from socioeconomic census data, each containing between 100 and 8,000 people. The data, taken from the years 2005 to 2009, was also used to assess poverty and education levels in these tracts. Vegetation coverage was assessed from satellite imagery from 2005, courtesy of NASA’s Landsat 7. Recorded incidents of aggravated assault, robbery, burglary, and theft were drawn from the University of Pennsylvania’s Neighborhood Information System CrimeBase, also for the year 2005.
Holloway notes that the more educated and well-off people lived in the greener areas of Philadelphia. The researchers from Temple controlled for, “Population density, poverty and education,” as well as the, “Control of the spatial contagion of crime,” according to the report.
Researchers offered two “hypothesis” to explain the connection between less crime and greener areas within the city.
By way of explanation, the researchers echo hypotheses from prior studies. There are two arguments. The first is that more vegetation promotes the use of public space, leading to more “social supervision” and surveillance. Social supervision, in essence, is the idea that people are more likely to establish beneficial relationships with positive, crime-deterring role models. Increased surveillance, meanwhile, does not imply the proliferation of CCTV. This is simply a reference to more users of a space bringing more eyes to the street. The theory: ne’er-do-wells don’t like being observed when ne’er-do-welling.
The second argument is that vegetation provides a “mentally restorative effect” that can reduce violent crime. The implication is not so much that a glimpse of hydrangea will deter a violent criminal in the act, but that the presence of vegetation reduces mental fatigue and irritability, which can be the precursors to violent crime. The authors admit that such benefits are difficult to quantify, but their results would seem to support this idea. The negative correlation between vegetation and crime was stronger for incidents of robbery and aggravated assault.
Holloway points out that although the study does show a correlation between “urban vegetation and crime”, it does not prove that crime is caused by lack of vegetation, nor visa versa.
The study does show that urban greenery is important to cities. And, although the study doesn’t address the idea that greenery also provides a way to control water run-off, this has been addressed before in the “City of Brotherly Love”.
National Geographic carried an article in 2006 about Philadelphia’s “innovative program” to clean up “storm water”, including green roofs, porous paving, storm-water planters, rain gardens and rain barrels.
As I recall, the city was faced with the dilemma to either green up the city or replace the inadequate grey water infrastructure. The program that Philadelphia adopted was a much cheeper alternative. Perhaps the Temple University study was inspired by the city’s earlier implementation of greenscaping.