Researchers looking for displacement have sometimes found precisely its reverse. Rather than finding that crime has been pushed to some other place or time, they have found that crime has been reduced more widely than expected, beyond the intended focus of the measures. This is a relatively recent discovery, but already many examples exist:

• As expected, electronic tagging of books in a University of Wisconsin library resulted in reduced book thefts. However, thefts of videocassettes and other materials that had not been tagged also declined.

• When a New Jersey discount electronic retailer introduced a regime of daily counting of valuable merchandise in the warehouse, employee thefts of these items plummeted - but thefts also plummeted of items not repeatedly counted.

• When LoJack vehicle tracking systems were introduced in six large cities, rates of theft declined citywide, not just for car owners who purchased the devices.

• Simon Hakim and his colleagues at Temple University have shown that widespread ownership of burglar alarms in an affluent community near Philadelphia resulted in reduced burglary rates for the community at large.

• When red light cameras were installed at certain junctions in Strathclyde, a large city in Scotland, not only did fewer people run the lights at these locations, but also at other traffic lights nearby. (In a smaller city, with more local traffic, this effect might be short-lived as people learned exactly which junctions had cameras.)

• The implementation of added security for houses that had been repeatedly burgled on a U.K. public housing estate in Kirkholt reduced burglaries for the whole of the estate, not just for those houses given additional protection.

These are all examples of the "diffusion of benefits" resulting from crime prevention measures. It seems that potential offenders may be aware that new prevention measures have been introduced, but they are often unsure of their precise scope. They may believe the measures have been implemented more widely than they really have, and that the effort needed to commit crime, or the risks incurred, have been increased for a wider range of places, times, or targets than really is the case. This means that diffusion can take several forms, paralleling the different kinds of displacement (see table).

Diffusion of benefits is a windfall that greatly increases the practical appeal of situational crime prevention, but we do not yet know how to deliberately enhance it. One important method may be through publicity. A publicity campaign helped to spread the benefits of video surveillance cameras across an entire fleet of 80 buses in the North of England, although these were installed on just a few of the buses. One of the buses with the cameras was taken around to schools in the area to show students they could be caught if they misbehaved and vandalized the bus, and the first arrests resulting from the cameras were given wide publicity in the news media.

We should expect the diffusion of benefits to decay when offenders discover that the risks and effort of committing crime have not increased as much as they had thought. Research has shown that this occurred in the early days of the breathalyzer in the U.K., which had a much greater immediate impact on drunk driving than expected, given the actual increase in the risk of getting caught. However, as drivers learned that the risks of being stopped were still quite small, drunk driving began to increase again. This may mean that ways will have to be found of keeping offenders guessing about the precise levels of threat, or about how much extra effort is needed if they are to continue with crime.

At a practical level, diffusion is important as a counterargument to displacement from those resisting the introduction of preventive measures. And you will certainly encounter many of those! Second, it is important that you plan your evaluation to take account of diffusion. Ways to do this are discussed in Step 51, by using two sets of control areas, both near and more distant. Otherwise, you might find that people question the effectiveness of the preventive initiative on grounds that crime fell across a broader area than was targeted.

Next Step

Diffusion of Benefits and Video Surveillance in a University's Parking Lots

A new head of security at the University of Surrey in the U.K. decided to deal with a plague of thefts in the university's parking lots by introducing video surveillance (or CCTV - closed-circuit television). He installed a CCTV camera on a mast to provide surveillance of the parking lots. As the diagram shows, the camera could not provide surveillance equally for all four parking lots because buildings obscured its view of parking lot 1.

It might have been expected, therefore, that if the camera had any value in preventing crime this would only be for the parking lots it covered adequately. It might also have been expected that crime would be displaced by the camera from these parking lots to the one not given proper surveillance. In fact, in the year following the introduction of the camera, incidents of theft and vandalism in the lots were cut in half, from 138 in the year prior to 65 in the year after. Incidents declined just as much in parking lot 1, not covered by the cameras, as in the other three lots. This diffusion of the benefits of the video surveillance probably resulted from potential offenders being aware that it had been introduced at the university, but not knowing its limitations. Many probably decided that it was no longer worth the risk and effort of going to the university parking lots to commit crime.

Source: Poyner, Barry (1997). "Situational Prevention in Two Parking Facilities". Situational Crime Prevention: Successful Case Studies, edited by Ronald V. Clarke. Monsey, NY: Criminal Justice Press.