High Resolution Analysis of Crime Patterns in Urban Street Networks: an initial statistical sketch from an ongoing study of a London borough

In this paper we report some initial statistical results in a comparison of residential burglary and street robbery in the street network of a London borough, using space syntax to analyse the street network. The results are presented against a background of current issues in urban design under discussion between the New Urbanism movement and the design against crime community: streets or cul-de-sacs, mixed use, permeability, and density. A method of primary risk band analysis is proposed as the basis of a simple, repeatable methodology for the analysis of crime patterns in street networks. 1. New urbanism and defensible space An increasingly animated debate is taking place between two schools of thought on how to design cities to minimise crime. The points at issue are aptly summarised by (Town & O’Toole 2005) in a table of six points where the ‘new urbanist’ position, as set out by Zelinka & Brennan in their 2001 book ‘Safescape’, is contrasted to the ‘defensible space’ position, originating in Oscar Newman’s 1972 book ‘Defensible Space’ (Newman 1972). On some of the issues, for example the dangers of rear or courtyard parking, or the risks introduced by footpaths and alleys, there is good recent evidence (including from space syntax studies) that the critics of New Urbanism are right. But on the main strategic issues of grid versus tree-like layouts, public versus private space, developmental scale, permeability, mixed use and residential density evidence of the appropriate precision is sporadic and inconclusive (for a review see Shu ‘Crime in Urban Layouts’, PhD thesis Figure 214: Summary of design and crime issues (Town & O’Toole 2005) 452 B. Hillier and O. Sahbaz University of London 2000). What is needed is high resolution analysis of crime patterns in urban street networks in which these issues are variables to begin to build the body of evidence needed to decide these issues. In this paper we propose such a high resolution technique. The unit of analysis is the segment of a street between junctions, and the technique combines space syntax analysis with Geographical Information Systems (GIS). In contrast to most available crime analysis packages, which focus on the clustering of crime in particular locations, space syntax research has pointed to the importance of looking at non-clustered patterns, that is the kinds of locations in which certain kinds of crime occurred, but which are dispersed throughout the grid and so do not appear as clusters or ‘hot-spots’. By combining space syntax with the data handling capabilities of GIS, a flexible method of analysis can be developed which works at the high resolution level required, but can also work at higher levels of aggregation such as the Output Area of the 2001 Census or the administrative divisions into wards. Previous space syntax studies have confirmed some of the principles of the prevailing defensible space orthodoxy, but has also opened up a positive window on certain new urbanist propositions. For example, syntax studies of residential burglary have confirmed that when embedded in an urban street network, simple linear cul-de-sacs can often be the safest places, but that when the cul-de-sac is generalised into a design principle so that whole areas take the form of a hierarchy of cul-de-sacs, the effects can be quite the opposite (Hillier and Shu 2000, Hillier 2004). It has also shown that in residential areas, the streets that are most integrated and therefore with more natural movementare often safer than the more broken up spaces, but that where integrated streets have ‘local vulnerability’; for example through ‘secondary exposure’ through alleys or adjacent open areas, or basement access, then integrated streets can become singularly vulnerable. We call these switches ‘flip over effects’ to express the idea that design variables do not act independently, but interact, so that all must be got right together if there is to be a genuine reduction in vulnerability. Equally complex results have come from looking at crimes against the person in urban areas, with patterns of crime shifting with time of day and thus with the presence of greater or lesser degrees of movement (Alford 1996). But space syntax has not so far studied patterns of crime either at the much larger scales of the city and its multiplicity of socially and spatially differentiated areas, or with a full analysis at the high resolution level of the street segment. In this paper we seek remedy this deficiency by reporting the first results of a space syntax study of crime in the street network of an entire London borough, with 99,991 households and 263,464 people within its boundaries (according to Census 2001). We have been given access to a 5 year crime data base in which every crime has been located and by linking this to a space syntax analysis of the whole borough (in the context of its urban surroundings), and linking both to social and demographics data available from the 2001 census, as well as the Borough’s own property database, we have perhaps the largest and best body of spatial, sociodemographics and crime data yet brought together in a single study. As an extra bonus, the fact that the London borough in question is largely made up of traditional housing on a street network, makes this data base ideal for posing the key current questions as to how street networks and their functioning shape the pattern of crime. But we should also issue a ‘health warning’. The London borough in question is socially and ethnically highly varied, and differentiated physically between its inner, more ‘inner city’, and outer, more suburban, areas. Some of its centres are already known as high crime areas. We should be careful not to generalise its results prematurely, before a range of such studies have been made in different cities. High Resolution Analysis of Crime Patterns in Urban Street Networks: an initial statistical sketch from an ongoing study of a London borough 453 Figure 215: Newman’s Hierarchy image. 2. Theoretical background: the other side of Newman The key issue in the debate between new urbanism and defensible space is the relation between the dwelling and the public realm. Throughout the history of urbanism, this relation has been more or less direct, and it would not be an exaggeration to say that the very basis of urban form historically is dwellings opening on to linear spaces linked into network: the street network. In modern times, this relation has been broken twice: first by architectural modernism, especially in its social housing programmes (for a review see Hillier 1988); and then by Oscar Newman’s hugely influential ‘defensible space’ concept, as summarised in figure 215, redrawn from page 9 of his book: But in spite of the fact that his concept radically disrupts the dwelling-street relation, throughout his book, when talking about real cities, Newman again and again acknowledges the security benefits of well-designed streets with dwellings opening on to them: ‘. streets provide security in the form of prominent paths for concentrated pedestrian and vehicular movement’ (op. cit p. 25). ‘the street pattern , with its constant flow of vehicular and pedestrian traffic, does provide an element of safety for every dwelling unit’ (p. 103). He criticises the modernist practice of closing off streets the make larger urban blocks, noting that ‘An inhabitant returning home must leave the public street and wind his way through the undefined and anonymous grounds of the project to reach the front door of the building. He would have been much safer had he been able to go directly from street to front door, and safer still if the front door and lobby of his building faced directly onto the street’(p. 25). Later he adds: ‘The interior of project buildings and grounds suffer the unique distinction of being public in nature and yet hidden from public view’ (p. 32) and later still: ‘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’. There is also the reverse effect, that Newman acknowledges: that dwelling entrances on the street benefits the street and makes it more secure: ‘The street comes under surveillance from the building, the building entries and lobbies under the surveillance of the street’ (p. 15) ‘The positioning of front entrances along the street provides them with continuous natural supervision by passers by; the residents within their houses, in turn, provide these passer-by with protective surveillance’. There is also a policing benefit: ‘Well lit front door paths, with individual lights over the 454 B. Hillier and O. Sahbaz entrances, allow cruising police to spot at a glance any peculiar activity taking place on a row-street house’. And more generally: ‘The street, without the continued presence of the citizen, will never be made to function safely for him’ (p. 15). One might then ask why Newman to universalise his defensible space concept. Several factors seem to have been involved. First, Newman’s object of concern was not the city as a whole but the socially disastrous social housing estates that had been inserted in decades following the two post-second world war. Second, the idea of hierarchically ordered space was at the time the prevailing architectural orthodox, taught in architecture schools under the rubric of the need for a ‘hierarchy of space from public to private’, which seemed to be confirmed by the then fashionable, though now largely discarded, ‘theory of human territoriality’ (Hillier & Hanson 1984). Newman also thought we might need to move towards a city of hierarchically ordered enclaves as a kind of spatial counterweight to what he saw as the increasing openness, liberality and moral heterogeneity of modern societies, which he thought had diminished our powers to act as communities and inhibit c