What Do Scientific Evaluations Show?

The discussion above shows just how complicated it can be to evaluate the effects of improved street lighting. The evaluation must consider the effects of improved lighting on crimes in daylight hours as well as in darkness. It must look for both increases and reductions in crime; and not just for the relit area, but also for a comparable control area where the lighting has not been improved. It must examine the effect of better lighting on different kinds of crime, because its effect is not consistent for all types of crime. And it must examine not just the displacement of crime to nearby areas but also the possible diffusion of benefits. Finally, the evaluation should consider other possible benefits of improved lighting, such as reduced fear.

If this were not enough, the most recent review of lighting studies5 has also noted that:

The effects of improved street lighting are likely to vary in different conditions. In particular, they are likely to be greater if the existing lighting is poor and if the improvement in lighting is considerable. They may vary according to characteristics of the area or the residents, the design of the area, the design of the lighting, and the places that are illuminated. For example, improved lighting may increase community confidence only in relatively stable homogeneous communities, not in areas with a heterogeneous population mix and high residential mobility. The effects of improved lighting may also interact with other environmental improvements, such as closed circuit television (CCTV) cameras or security patrols.

This means that studies should clearly describe the nature and intensity of the improvements in lighting, the general neighborhood conditions, and any other contemporaneous crime prevention measures. Indeed, a consistent finding of problem-oriented policing projects is that a smart mix of responses, tailored to the situation, produces the best results.

Few if any published studies meet all these evaluation requirements; indeed, it would be very difficult to do so. The principal question examined in most published evaluations is whether street lighting reduces crime at night. This was the focus of eight studies undertaken in the United States, seven of them during the 1970s (see Table 1).

Table 1: Eight Street Lighting Evaluations in the United States

* Dept. of Intergovernmental Fiscal Liaison

Table adapted from Welsh and Farrington (2007).

Although four of these studies found desirable effects from improved lighting, the others did not; a review published by the U.S. Department of Justice of the seven studies undertaken in the 1970s concluded that improved lighting was not an effective means of preventing crime. However, three more recent studies published in the United Kingdom (see Table 2) found significant reductions in crime both in daylight and at nighttime, with no apparent displacement and in one case, some diffusion of benefits.

The report is titled Street Lighting Projects: National Evaluation Program. Phase 1 Report (Tien et al. 1979).

Table 2: Three Street Lighting Evaluations in the United Kingdom

Washington, D.C.: U.S. Department of Justice, National Institute of Justice. Table adapted from Welsh and Farrington (2007).

A recent authoritative review, which used a well-established methodology to combine the results of all the studies from the United States and the United Kingdom, concluded that improved street lighting led to a "21 percent decrease in crime compared with comparable control areas." Reductions in crime of this amount are worthwhile but, of course, there is no guarantee that better lighting will reduce crime in your neighborhood.

Welsh and Farrington (2007, page 8). This percentage improvement might overstate the effect of improved street lighting because their meta-analysis included two studies undertaken in the United Kingdom (not reported in Table 2) that examined the effect of improved lighting in a parking lot and a center city market.

The review could not determine whether these improvements were the result of situational deterrence or improved community pride and cohesion. The review concluded that improved street lighting had a larger effect on property crimes than on violent crimes, but offered no explanation for this result. More detailed research showing the effect on specific types of property crime and violent offenses is needed.