Wed 14 Feb 2007
In America for a long while now, the conventional wisdom has been that isolation equals safety. In that viewpoint, one achieves maximum safety in a home that is hidden from public view, located on an isolated cul-de-sac, placed in a walled, gated community, and removed from the center city to the far exurban fringes of the metro area.
But does that model provide the best security in all situations? Or does designing for isolation sometimes provide an illusion of security?
Burglaries and street crime result from a nexus of factors, including demographics, individual psychology, and environment. Design plays a role as well. The conventional assumption is that greater isolation protects against crime, but researchers are finding that long, isolated cul de sacs with homes that are not visible from the street have higher crime rates. Crime researchers recommend breaks in the street network only under certain limited conditions.
As U.S. crime rates began skyrocketing in the 1960s, concerns about public safety led to a greater focus on prevention. In this field as in so many others, Jane Jacobs was a pioneer. Her book The Death and Life of Great American Cities (1961) identified three qualities of successful city neighborhoods: clear demarcations between public and private spaces; “eyes upon the street,” or natural surveillance by inhabitants and proprietors; and a fairly continuous level of sidewalk activity supported by diverse land uses.
The keystone of Jacob’s theory was that ordinary citizens, not the police, were the first line of defense against crime.
The first thing to understand is that the public peace — the sidewalk and street peace — of cities is not kept primarily by the police, as necessary as police are. It is kept by an intricate, almost unconscious network of voluntary controls and standards among the people themselves, and enforced by the people themselves.
— The Death and Life of Great Americian Cities (p. 31)
Crime researchers, architects and planners subsequently developed a concept called Crime Prevention Through Environmental Design (CPTED) that drew from Jacobs’ theories. CPTED was a multidisciplinary, holistic approach to crime deterrence. It incorporated individual psychology, family and community dynamics, sociodemographic factors, and cultural and environmental influences.
Oscar Newman, an architect and researcher of crime in public housing projects, popularized a more limited version of CPTED. In his books Defensible Space: Crime Prevention Through Urban Design (1972) and Design Guidelines for Creating Defensible Space (1976), Newman wrote that architectural design could influence and deter criminal activity.
Newman was especially concerned with public housing projects. Repetto (1976) described the results of applying modernist urban design to low-income housing:
A favorite theme of urban planners of the past few decades was to tear down the slums and place the residents in clean, modern apartment buildings known as public housing. It was also thought necessary to create a park-like atmosphere, for the inhabitants’ recreation, in the place of dirty and dangerous city streets. Thus a familiar pattern is the large interior space surrounded by housing units, to which vehicle access has been denied by simply closing the streets, creating a super block. In theory the recreation area belongs to all, but in practice victory in the struggle for its use goes to the strong, usually young males. For other residents to venture on the area is to risk insult or injury. Since the streets are closed off, patrol cars cannot traverse them, nor can interior areas be seen from the streets.
— Crime Prevention Through Environmental Policy: A Critique
This is what Newman strove to fix, along with other design flaws. His prescriptions stated that residences should look over, and be associated with, adjacent streets and land. Shared spaces should be located so that residents could feel a sense of ownership and influence. The boundaries of properties and shared spaces should be clearly demarcated.
Newman made no overall recommendation to close off streets and paths that had adequate natural surveillance, and indeed, he was frequently in favor of increased circulation. In Defensible Space he wrote, “Large super-blocks, at various densities, have been found to exhibit systematically higher crime rates than projects of comparable size and density that have city streets continuing through them.” In Guidelines, he described his design for Indianapolis public housing, writing, “we have developed a system of streets to penetrate the entire site” where increased circulation would “greatly facilitate” police patrols. His Indianapolis street layout had no cul-de-sacs; rather, it used T-intersections to reduce through traffic.
In some cases, however, Newman favored interventions that installed vehicular barriers in grid street patterns. These prevented vehicular through traffic, while still permitting vehicular access and through travel on foot. This technique was used in high-crime neighborhoods where drive-through illegal drug trade and prostitution were major problems. Police departments throughout the United States tried this technique; it was simple, direct, and, in the short term at least, seemed to be effective.
Research on the crime impacts of such street closings has been limited and inconclusive. For example, the US Department of Justice publication Closing Streets and Alleys to Reduce Crime: Should You Go Down This Road? lists several studies that report positive results. However, these studies generally look at short term impacts; they were self-selected for positive results, usually unable to control for related effects such as increased police presence, and applicable only to a few specific locations and situations.
As Cozens (2005) points out, most research about the impact of design on crime has similar methodological flaws. CPTED itself has come under enormous criticism (Clarke, 1998) as a form of environmental determinism that discounts social factors and reduces crime solutions to a facile “one-size-fits-all” approach. As a result, a “second generation CPTED” has been proposed (Saville and Cleveland, 1998) that takes into account unique, site-specific factors, and in particular elevates the importance of social factors, activity in the public realm, and community involvement.
More rigorous studies that specifically focus on connectivity have been performed by teams using Space Syntax techniques. Space Syntax, originated by Bill Hillier, is a system for analyzing the connectivity of street patterns and its relationship to factors like pedestrian activity and crime. It defines connectivity in multiple ways, the most common being the number of corners one must turn to get from one place to another. Space Syntax also measures connectivity with visibility — i.e., how much of a street is visible from any other street segment or intersection.
Space Syntax is in accord with second generation CPTED insofar as it highlights the careful evaluation of specific design techniques and the contexts in which they are applied. Researchers look at the configuration of a cul de sac, the placement of homes within it, and its connection to the larger-area street network. Those are some of the factors that make a difference in crime rates.
In general, the hot spots of crime are the locations with low pedestrian traffic and low visibility of homes and entrances. Hidden or partly visible homes, on long, curvy cul de sacs that are part of a dendritic (i.e., “tree-like”) network of thoroughfares, have the highest crime rates.
Conversely, the safest locations are on well-connected streets with plenty of foot traffic and many highly visible dwellings. The safest cul de sacs are short and straight, with many highly visible dwellings, and connect directly to through streets. An analysis of crime in London found the more residences on a street segment, the lower the burglary rate. As the researchers concluded, “There is safety in numbers!” (Hillier, 2004).
Segregated footpaths that connect cul de sacs — the classic Garden City formula — can be highly vulnerable to crime if they are secluded. Similarly, urban alleyways may also experience high crime if they lack the foot traffic, accessory dwellings and clear sight lines that enable natural surveillance of the public space.
Here’s how Hillier summarizes the findings of Space Syntax analyses in the UK and Australia:
At the level of the overall layout, we can say, fairly unambiguously, that reasonably regular street layouts with fairly large blocks (to structure movement and reduce unnecessary permeability) are best, provided the “flip-over effect” is avoided by the local spatial detail. If such a layout is then interspersed with simple linear cul-de-sacs directly attached to the through street, then the cul-de-sacs may well be the most secure parts of the layout — but only if the street system is there in the first place to keep the cul-de-sacs simple and linear.
So a layout works as a whole. We cannot isolate elements and say that this is good and that is bad. It all depends on how the elements are put together in themselves and how they are combined to form the overall layout. Although pickpockets need busy streets, and muggers locations where integration turns to segregation so that victims are available one at a time, it is clear that what burglars — and to a lesser extent car criminals — need is secluded access. The less we provide it either by breaking up lay outs into poorly used, low visibility public spaces, or creating secluded secondary access to premises, then the more difficult the burglar’s job will be.
— Can Streets Be Made Safe?
And a similar conclusion from another study, phrased slightly differently:
Although footpaths and alleys are known to be crime hazards, this does not mean that high segment connectivity in the street network itself is a hazard. On the contrary, in many instances, high street connectivity in a more grid like layout is associated with low crime where the numbers of dwellings per segment are sufficiently large. If urban blocks are too small, so that there are fewer dwelling per segment — that is between the escape routes — then burglary tends to increase. In design, the issue of permeability must be linked to block size: the over provision of permeability with reduced block size will be hazardous. Overall, the idea that numbers of escape routes facilitate crime does not apply to the street system. Both burglary and robbery occur on average on less connected spaces than average.
— High resolution analysis of crime patterns in urban street networks
Hillier also critiques a specific idea sometimes seen in Defensible Space writings: that household groupings with restricted access are safer from crime. Although this concept has had considerable influence on UK planners, in the Space Syntax analysis it lacks empirical support.
First, there are no burglaries on the first lines of sight into cul de sac complexes from the through roads. The burglaries are dispersed, but are usually found in the deep parts of cul-de-sacs, and most of all at the end of the cul-de-sac. If this pattern was repeated — as it has been in all the studies we have done — then it in itself casts doubt on Newman’s core argument, since by his reasoning the safest places should be at the ends of cul-de-sacs where small groups of neighbours can conjointly survey the sole space from which their dwellings are accessible, and higher in the more anonymous entry lines where strangers pass through on their ways to the deeper parts of the cul-de-sac complex.
— Can Streets Be Made Safe?
Conclusions about dwelling types, while not directly related to connectivity, are nonetheless instructive:
At the level of the dwelling, there are pretty unambiguous results. The British Crime Survey (Budd, 1999) shows that if you control for social and economic variables — meaning, in effect, that you take the same family and put them in different types of dwelling — then the most secure type is the flat [apartment], then the mid-terrace house [townhouse], then the end-terrace house [townhouse end unit], then the semi-detached house and finally the detached house is the least secure. In other words, the fewer sides on which your dwelling is exposed to the public realm the safer you are likely to be. … [Space Syntax researchers] have found exactly the same thing, in exactly the same order, with the additional vital fact that not only are flats the safest kind of dwelling and detached houses the least safe, but also that in flats you are a good deal safer off the ground than on the ground.
— Can Streets Be Made Safe?
As a result of an Australian study — the largest urban crime study in the southern hemisphere — the city of Gosnells in Western Australia changed its urban design policy. From Australia’s Local Government Focus, July, 2000:
Applied to Gosnells, Space Syntax research suggests that crime was highest where pedestrian and vehicle movement was low and visibility to onlookers negligible. “The cul de sac developments, that characterised residential development over the last 30 years, appear to be an invitation to crime in that they reduced pedestrian movement and accessibility to facilities,” [city designer] Stephen Thorne said.
Gates and Enclaves
In parts of the world where deficient law enforcement is combined with wide disparities in income, such as urban and suburban areas of Brazil, Argentina, Chile, Columbia, Saudi Arabia, and South Africa, the pattern of residential isolation and restricted access is assumed to be necessary. It’s taken a step further by surrounding each house with high walls topped with barbed wire or broken glass. Kidnapping for ransom money is a very real fear in those developing countries.
The ultimate question is, are the developed countries headed in that direction? Is that the type of society we desire and wish to build for ourselves? Many in the developed nations have already chosen gated, restricted residences for themselves; many others view it as a form of dystopia. In the United States there is a constant background hysteria about child abductions, promoted by a sensationalistic media, while in reality there is no increasing trend of “stranger danger.”
The theorists of second generation CPTED share the view that a universal strategy of barriers is unsustainable in the long run. They highlight the importance of social factors, and the folly of ignoring community involvement.
We are suggesting that if environmental influences are only one simple step in the community-building process, then barriers may only reinforce feelings of fear of the surrounding neighborhood outside the barriers. There is nothing inherently safe about an internally gated community, except that it may be occasionally more difficult for the simple rational offender to victimize the simplest target. In the public realm, building a ten-foot wall around a school yard to keep out guns and dealers may help, but without true community-building strategies both inside and outside those walls, will this environmental modification serve anyone but the construction company? Walls such as these are, in the parlance of our time, a Y2K virus for future development. For a sustainable, ecological, approach to crime prevention, we must address the social Y2K virus.
— 2nd Generation CPTED
Alain Chiaradia, a Space Syntax researcher, weighed in on this issue in Gates and the Danger of Some Strangers. Chiaradia noted that much of the debate over gated communities revolves around a fear of strangers, while at the same time many writers and researchers have found benefits associated with the presence of strangers: “Specifically, Jacobs and others have stressed that it is interactions between strangers that is valuable for urban life: certainly for commerce and for the intangible buzz that is the hallmark of a great city, but very much for security and socialisation as well.”
Chiaradia recognized that gating is an important strategy in crime control. However, as he wrote, “Gates should be used for remediation, as a short or medium-term solution to a problem that ultimately needs a much more comprehensive solution — large-scale redesign that stitches these areas back into their urban context.”
Bill Hillier put street and public space activity into a historic context, and took a firm position against isolation as a crime-fighting strategy:
A residential culture, it might be conjectured, is first a culture of civilised co-presence, and only second, and after due time, a culture of community formation. This, perhaps, is what made historic cities, which always brought heterogeneous population into dense patterns of contact, the civilised places they seemed to be. As both Jane Jacobs and Oscar Newman observed, a society which does not civilise its streets cannot be civilised. … It has always been, and remains, unclear how breaking the link between residence and the street, as implied by the universalisation of the residential enclave, can lead to anything but an increasingly insecure public realm of our cities.
— High resolution analysis of crime patterns in urban street networks
These voices should be heard and their messages should be heeded. A free and vital civic life requires a free and vital civic realm, based on open access to fully public spaces in cities, towns and neighborhoods.
European Designing Out Crime Association — Publications List
The Institute for Community Design Analysis, devoted to the works and methodology of Oscar Newman
International CPTED Association — A Canada-based association of CPTED practitioners
Aurbach, Laurence, Correcting the “Crimeogenic” Crowd, February 14, 2005.
Chiaradia, Alain, Gates and the Danger of Some Strangers. SpaceSyntax.com, posted 24 September 2004.
Clarke, Ronald V., The Theory of Crime Prevention Through Environmental Design. European Designing Out Crime Association website. 1998.
Cozens, Paul M., Pascoe, T. and Hillier, D., Critically Reviewing the Theory and Practice of Secured-by-design for Residential New-build Housing in Britain. Crime Prevention and Community Safety: An International Journal, Vol. 6, No. 1, pp. 13-29, 2004.
Cozens, Paul M., Greg Saville and David Hillier, Crime prevention through environmental design (CPTED). Property Management, Vol. 23 No. 5, 2005, pp. 328-356
Cozens, Paul M., Designing Out Crime: From Evidence to Action. Conference paper presented at: Delivering crime prevention : making the evidence work. 21-22 November 2005.
Hillier, Bill and Shu, Simon C.F., Do Burglars Understand Defensible Space? New evidence on the relation between crime and space. Space Syntax website, 1999.
Hillier, Bill, Can streets be made safe? Urban Design International, Vol. 9, No. 1, pp. 31-45, 2004. Similar material was published in: Hillier, Bill, Designing safer streets: an evidence-based approach, Planning in London, No. 48, pp. 45-49, 2004.
Hillier, Bill and Ozlem Sahbaz, High resolution analysis of crime patterns in urban street networks: an initial statistical sketch from an ongoing study of a London borough. In Fifth International Space Syntax Symposium, Vol. I, pp. 451-478, 2005.
Jacobs, Jane, The Death and Life of Great American Cities. New York: Random House, 1961.
Lloyd, Richard E., “Book Review: Creating Defensible Space.” Journal of the American Planning Association, Vol. 63, 1997.
Newman, Oscar, Defensible Space: Crime Prevention Through Urban Design. Macmillan Publishing Company, October 1973.
Newman, Oscar, Design Guidelines for Creating Defensible Space, U.S. Department of Justice, National Institute of Law Enforcement and Criminal Justice, Law Enforcement Assistance Administration, 1976.
Newman, Oscar, Creating Defensible Space, U.S. Department of Housing and Urban Development, April 1996.
Repetto, Thomas A., “Crime Prevention Through Environmental Policy: A Critique.” American Behavioural Scientist, 20, 1976, pp. 275-288.
Saville, Greg and Gerry Cleveland, 2nd Generation CPTED: An Antidote to the Social Y2K Virus of Urban Design, Paper presented at the 3rd Annual International CPTED Conference, Washington, DC, December 14-16, 1998.
Shu, Simon C.F. and Jason N.H. Huang, Spatial configuration and vulnerability of residential burglary: A case study of a city in Taiwan. Proceedings, 4th International Space Syntax Symposium, London, 2003.
SpaceSyntax.com, Housing and Crime (a summary of Space Syntax housing layout/crime research and publications), including Shu and Hillier, “Crime and Urban Layout: the need for evidence” published in Secure foundations: Key issues in crime prevention, crime reduction and community safety by V. MacLaren, S. Ballintyne and K. Pease, eds., 2000, London, IPPR, pp. 224-248.
Steventon, Graham, “Defensible space: a critical review of the theory and practice of a crime prevention strategy.” Urban Design International, Vol. 1, No. 3, 1 September 1996, pp. 235-245(11).