Urban Micro Scale Tools:A method to Measure the Relationship Between Private and Public Space

Akkelies van Nes, Manuel J. J. López

1 Introduction

Micro scale spatial measurements in urban studies are about the spatial relationship between buildings and street segments. More precisely it is about demonstrating how dwelling openings are connected to the street network, the way buildings’ entrances constitute streets, the degree of topological depth from private space to public space, and inter-visibility of doors and windows across streets.The challenge is how to measure the spatial parameters of the street plinth or how to quantify various degrees of ‘active frontages.’

What Jane Jacobs[1]and Jan Gehl[2]are missing in their research on urban street life is a consistent spatial tool for measuring the street and building relationship. Micro scale measurements indicate the spatial conditions for vital street life and degree of urban safety. The building — street relationship has been applied on a small scale in Hillier and Hanson’s book “The social logic of space” and in Chih-Feng Shu’s PhD thesis[3].

In 2006 we conducted a study on space and crime in the two Dutch towns Alkmaar and Gouda. At that time a systematic research method on the street-building relationship was missing. We registered 25 different spatial features. As turned out,the following four spatial relationship influenced burglary risks; entrance density, topological depth between private and public space, street constitutedness and entrance inter-visibility. Moreover,these four spatial features are influenced by the degree of street network integration[4].

2 Entrance Density

The most traditional way is to measure entrance density per meter. Figure 1 shows entrance density in Pompeii. As can be seen from the map, the centre area in Pompeii has the streets with the highest entrance density.

However, high density of entrances connected to a street does not always imply high inter-visibility.There is a distinction in the way entrances constitute streets and in the way they are inter-visible to each other. The way entrances and windows are positioned to each other influences the probabilities for social control and street life.

3 The Topological Depth between Private and Public Space

There are several ways of analysing the spatial configurative relationships between building entrances and the street network. An easy way is to register the topological depth between private and public space, which is done as follows:

Count the number of semi-private and semi-public spaces from the private space to the public street.If an entrance is directly connected to a public street, it has no spaces between private and public space. Then the value or depth is like zero. If there is a small front garden between the entrance and the public street, it gets the value like one since there is a space between the closed private space and the street. Moreover, if the entrance is located on the side of the house and it has a front garden or covered behind high hedges or fences it has a value like two. Entrances from back paths covered behind a shed have a value like three. It is the topological steps between the street and the private spaces that are counted.

Entrances into flats can be represented in several ways. It all depends on the degree of permeability between the private space and the street. Some flats have upper walkways where the entrances to each apartment are connected to. Visitors can walk all the way up to each apartment’s front door to ring the doorbell. Other flats have a closed main entrance, where visitors have to use a calling system when visiting someone.

In a study on the dispersal of burglaries in Alkmaar and Gouda, the degree of permeability was used as a rule of thumb. In those cases where a flat’s front door or main entrance was permanently locked and they was provided with a doorbell or calling system, it was registered as a private space from thereon. As regards flats with open main entrances,the number of semi-private spaces was counted up to the apartments[4].

The diagram in Figure 2 illustrates various types of relationship between private and public spaces.The black dots illustrate the private spaces, while the white dots illustrate semi-private and semi-public spaces. Each side of a street segment must be registered separately. There exist many streets where entrances are for example directly connected to the street on the one side, while there is a flat on the other side with an upper walk gallery. In a street segment where there are different depth values between private and public spaces on a street’s side, the average value is used.

In most traditional urban areas, housing entrances face directly to the street, while in most post War urban areas one has to walk through several semi-private or semi-public spaces before private spaces are entered. Moreover, in post War areas with detached houses, entrances are not always directly connected to the street. Many houses have the entrance on their side rather than at their front façades facing towards the street.

Figure 3 shows two typical extreme examples of a dwelling entrance in a Pre War housing and a Post War housing area.

Figure 4 shows five different street segments. The first four streets segments are easy to calculate.The average topological depth between private and public space is the same for all entrances for each street segment. The last street segment represents a gated community with several semi public and semi private spaces. The rule is to count the number direction changes for the street segment at issue. If the values are different for each side of the street, then take the average value. In street E the average is 6.5 steps.

The same topological features can be seen in the difference between traditional shopping areas and modern shopping centres. Shopping centres has one entrance to reach several shops. A feature of their façades is, except from the entrance, that the building has no “active frontages” oriented towards the streets. Often shopping centres have a large wall with “blind” windows, or sometimes the wall is covered by advertisements. Conversely, a feature of traditional shopping areas is a high density of shops’ entrances and windows directly connected to the street.

The topological depth from private spaces to public spaces seems to increase in highly segregated urban areas or in areas topologically far away from the main routes[4]. Often entrances in the topologically deep street segments are hidden away from insight from streets and visible neighbours.In urban areas located close or adjoining to main routes, entrances tend to be located directly to public streets. The streets are frequented by visitors as well as by inhabitants.

Figure 5 shows a map with the topological depth between private and public spaces in Wolverhampton in 1780 based on old maps. The red lines show the streets where the entrances are directly connected to the streets. The green lines have one public space between the private spaces and the street,whereas the blue lines has two spaces between the private and public spaces. Often, it is enough to divide the categories between 0, 1, 2, 3 steps, and more.

4 Constitutedness and Un-constitutedness

A street’s degree of constitutedness depends on how buildings’ entrances are connected to the street. It is about the degree of adjacency and permeability from buildings to public space[5]. If, and only if, a building is directly accessible to a street,then it constitutes the street. Conversely, when a building is adjacent to a street, but its entrance is not accessible directly from the street, the street is un-constituted. Naturally, there must be no other buildings with entrances directly connected to this street.

Figure 6 shows some examples of constituted and un-constituted streets. The two entrances on the top are constituted streets. In both cases the entrances are located only at one side of the street. The two examples below are un-constituted streets with no entrances on ground floor level.

The diagram in Figure 7 illustrates the differences between constituted and un-constituted streets.It is thus a difference between a building located adjacent to a street and being permeable from a street. As results from research have shown, spatial relationships of this kind can have impact on vital street life in urban areas. Few people tend to sit or stand for a long time in un-constituted streets.Often rape and street robbery tend to take place in un-constituted streets[6].

In order to measure various degrees of constitutedness, the number of entrances directly facing a street in comparison with the number of buildings located along it is at issue. There are several ways of visualising the “urban network—building permeability” relationship. Bill Hillier and Julienne Hanson use the concept “the interface map” to illustrate street links directly connected to adjacent buildings[5]. All the entrances of the buildings which are both adjacent and directly connected to the street network are marked. A colour code is given for each street segment directly connected to one or more dwellings. Then the same can be done with street segments where there is one space between the dwelling entrance and the street,where there are two spaces between the entrance and the street etc.

Chih-Feng Shu applied another variant to quantify various degrees of constitutedness. According to Shu, urban space is defined to be constituted when more than 75% of the adjacent dwellings have front doors facing directly to the street. If a street segment consists of for example four buildings and three of them have their entrances directly connected to a street, the street is constituted[3].

A street without no entrance connected to is thus un-constituted. Clear examples of un-constituted streets are subways and highways.

The number and density of entrances are not at issue. Degree of constitutedness is about the number of entrances connected to a street in correlation of the number of buildings located along that street.

As research has shown, there is a correlation between the degree of constitutedness and the dispersal of burglaries. Un-constituted streets are affected more by criminal activities than in the constituted ones. Moreover, entrances covered behind high fences and hedges have little visibility from neighbours. Criminals seem to prefer to operate in spaces of this kind[3-4].

Figure 8 visualises the difference between constituted and un-constituted streets in Gouda. In this research project all streets with no building entrances located towards them are defined to be un-constituted. They are coloured in blue, while the constituted streets are coloured in red. Most intruded homes (presented as circles) are entered from un-constituted streets. The points of entry into dwellings are marked as a line from the street or back path at issue to the circle[4].

5 Inter-visibility and Density of Entrances to Streets

The density of entrances can to some extent indicate a degree of liveliness in a street. As mentioned earlier, the more entrances connected to a street,the higher probability that someone can come out from a private space into the public space. Often streets constituted by many entrances are perceived to be safe to walk through at night[7]. However,high density of entrances connected to a street does not imply high inter-visibility. There is a distinction in the way entrances constitute streets and in the way they are inter-visible from the street and to each other. The way entrances are located opposite of each other in a street gives a high probability in the way people can keep an eye on each other. Naturally, there must be windows next to the doors. It can prevent homes from being burgled, through the natural surveillance mechanism the close relationship between windows and streets provide. Figure 9 shows some diagrammatic principles on the relationship inter-visibility and density of entrances.

There are several way of measuring or registering degrees of inter-visibility. In a study on the relationship on burglars’ home address and the location of his targets, four different degrees of entrances’inter-visibility was used. A street is highly inter-visible if the density of entrances is high and more than 75% of them are inter-visible to one another. An inter-visible street has a low density of entrances, but more than 75% must be inter-visible for one another. A street has little inter-visibility if and only if the density of entrances is high and more than 75% of them are located on one side of the street. A non-visible street has low density of entrances and less than 75% of them are inter-visible for one another. Likewise, streets with no entrances at all or in which the entrances are covered by high hedges and fences are defined as non-visible[8].

In a research project on the dispersal of burglaries in Alkmaar and Gouda, a registration of windows and doors inter-visible to one another and to the street was carried out.

The number of inter-visible houses was divided with the total number of houses in a street segment.The percentage of inter-visibility from windows and doors from houses to streets and between houses was registered separately. The percentages was grouped in 100%, 80%, 60%, 40%, 20% and 0% inter-visibility for each registration. Degree of density of houses and entrances was not taken into account. It was registered separately. When comparing theses results with the dispersal of burglaries, inter-visibility from windows between houses prevent homes from being burgled[4]. Thus,two buildings with two entrances facing towards each other indicate 100% inter-visibility of doors.Conversely, a street with high density of entrances on only one side of a street and no entrances on the other side is defined to be 0% inter-visible. High density of entrances directly facing a street at only one side can be an indicator for street life, but not for preventing homes from burglaries.

In traditional urban areas, the density of entrances are in general high in contrast to the sub-urban detached houses areas and post War urban areas with flats.

Figure 10 shows how to visualise the degree of inter-visibility in the excavated town Pompeii. The streets with 100% inter-visibility are those street segments where the archaeologist presumed where the shops was located[9].

6 Combinations of Micro and Macro Spatial Measurements

A combination of various micro spatial measurements makes possible to describe in a systematic and quantitative manner the local spatial features of neighbourhoods. These features are, however,not always present in studies focusing on the macro spatial analyses. For example, a street with few connections to its vicinity can still be full of social activities if a high density of entrances constitutes the street and when there is high visibility between public and private spaces. The reverse can be seen in, for example, in highly integrated but distributed un-constituted streets with a low number of entrances and low inter-visibility. Thus, an urban area’s degree of liveliness depends on its spatial conditions on a macro level as well as on a micro level. Therefore, analyses of both scale levels contributes at least to a throughout spatial configurative description of urban areas, independent of cultures and architectural styles.

Since no software development is current available for analysing micro scale spatial relationships, the Statistical Package for the Social Sciences (SPSS)software is useful. In the Gouda and Alkmaar study, 1,168 street segments were registered manually for the micro scale analyses part and compared with the street network integration values from the space syntax analyses. As the results from the SPSS show, both micro and macro spatial variables are highly inter-dependent[4]. Most micro spatial variables turn out to be strongly correlated to the macro scale variable Angular choice with a high metrical radius. This variable identifies the main routes through cities.

The following results were obtained (Figure 11).The higher number of direction change a street segment is situated away from a neighbourhood’s main routes, the greater the topological depth between private and public space (Figure 11 top,left side). Along the main routes through urban areas, most entrances are directly connected to the street. When changing direction two times from the main routes, the average topological depth for entrances is 2 while it is 3 in all street segments that are located more than six times direction changes from the main routes.

The further a street segment is away from the main routes, the lower percent of inter-visibility of entrances and doors on the ground floor level(Figure 11, middle). Seemingly, homes located in segregated streets tend to hide their entrances from the streets. Likewise, the further a street segment is away from the main routes, the more the streets tend to be un-constituted (Figure 11, right side).

As research has shown, the further a street segment is located away from the main routes, the more mono-functional the adjacent buildings tend to be.Offices, shops and public buildings tend to locate themselves along the main routes. The semi-private segments are among the most segregated streets[4].

7 Conclusion

The micro scale tool has been tested on several research projects since it was introduced in 2007.Regards street safety and social reputation of the neighbourhood, the micro scale method is applied in a research project on 40 deprived neighbourhoods in the Netherlands. As it turned out, most of these neighbourhoods have a high number of un-constituted streets with low degree of inter-visibility and many topological steps between private and public space[10]and[11]. In two research projects on behaviour patterns of various ethnical groups[12]and safety perception of dwellers in their own neighbourhood[13]in Rotterdam, people avoid to stay or move in segregated and un-consti-tuted streets with low degree of inter-visibility of entrances and windows. These kinds of streets are perceived to be unsafe. Likewise, as research has shown, women tend to avoid frequenting un-constituted segregated streets with low inter-visibility of entrances[14].

In a research project on space and protests during the Arab spring in Egypt in 2011, the degree of visibility of entrances in Tahrir and Rabaa Al-Adawiya squares played a role for the protesters to succeed. Protesters seem seek spaces with a high degree of accessibility on a local scale as well as on a city-wide scale and a high degree of symbolic value. Furthermore, high visibility of entrances and windows helped the protesters to escape at Tahir Square when tanks and police tried to stop the demonstrations[15].

The micro scale method is also applied on the 2000 years old excavated town Pompeii. Shops were located in streets with high density of entrances and high degree of inter-visibility[9]. Likewise,the location of shops in informal areas in Cairo in Egypt takes place along highly inter-visible locally integrated streets[16].

In a project on densification strategies in Bergen in Norway, the topological depth between private and public space influence degree of walkability.The more the entrances are directly connected to streets, the more people tend to choose to walk instead of using the private car[17]. In a research project on planning regulations and its impact on urban form in Recife in Brazil, building types with parking garages on the ground floor contributes to streets with no inter-visibility and no natural surveillance[18].

Micro scale conditions are often neglected in the contemporary planning and design of urban areas.In particular, urban renewal projects, modern housing areas and new large-scale urban development projects often tend to lack adjacency, permeability and inter-visibility between buildings and streets.This has negative effects both on the quality and quantity of the street life and the safety of these urban areas.At present urban project developers tend to build with high density or high floor-space-index and propose large, variations of urban functions (dwellings, offices, etc.) in these areas. However, the degree of inter-connectivity and the topological shallow public-private interface is often forgotten.All these activities depend on how the spatial configuration is on the plinth or built up street sides.Therefore, there is a need to bring micro scale spatial relationships on the research, policy making as well as on the design agenda in the urbanism discipline.