Dense civilisations: the shape of cities in the 21st century

Applh'd Energy 43 (1992) 41-66

Dense Civilisations: the Shape of Cities in the 21st Century

Bill Hillier & Alan Penn

Bartlett School of Architecture, University College, Wares House. 22 Gordon Street, London, WCIA 0QB

I N T R O D U C T I O N AND S U M M A R Y OF THE A R G U M E N T

People living in towns and cities use cars less and walk more than country dwellers. 1 In city centres walking can even be the dominant form of movement. 2 Theoretical models also show that substantial future savings in transportation energy could be made by densifying cities rather than thinning them out and spreading or dispersing them. a These and other arguments suggest that building up densities in existing cities might be a sensible part of a future urban policy.

One aspect of the debate is, however, that a high-density urban policy conflicts with current cultural attitudes built into our planning system, which tends to see density as a bad thing in itself, to be tolerated where necessary but to be minimised as part of an overall policy of bringing order into the 'unplanned chaos' of cities by thinning out, reducing scale, and separating the untidy but continuous city into well-defined zones and identifiable units. New research, however, using a new type of computer- modelling technique to analyse and design city areas, suggests that many of these attitudes are rooted in ignorance of the spatial and functional nature of the organic city. Many positive aspects of urban living can be shown to be associated with high density, good spatial scale and continuity, and much of the pathology of the inner city comes from the very culturally backed policies that are intended to reform the apparent 'unplanned chaos'. At a deeper level, the research shows that the spatial form of the organic city is not disorderly but has a functional logic which is generated by movement - - more precisely, by what we call the 'movement economy' of the city, by

41 Applied Energy 0306-2619/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

42 Bill Hillier, Alan Penn

which we mean the total relation between land-use patterns, movement of all kinds including pedestrian movement, and the structure of the urban grid.

The spatial structure of the grid is the key element. 'Organic' towns which have grown over a long period seem to optimise certain key aspects of movement and land-use patterns by exploiting the structural properties of the urban grid. Many of the positive qualitative judgements we make about ~urbanity" are probably comments on this optimising functional logic of the organic city.

With this theoretical understanding, backed by computer techniques capable of bringing far more of the functional intricacy and spatial complexity of the organic city into the intuition of the creative designer, urban design can become a knowledge-based continuation of the process of growth and change which creates the organic city. This in turn opens the way for a re-consideration of the anti-urban planning culture, which has its origins as much in the nineteenth century belief in the availability of unlimited cheap energy to replace spatial integration with mechanical movement, as in the nineteenth century fear of the rapid growth of high- density cities brought about by industrialisation. With the new models, dense civilisations become a much more attractive possibility, opening the way for a 'movement economy' approach to design in which people, for example, have cars but use them less because they need them less.

OR GANIC CITIES OR U N P L A N N E D CHAOS

Cities are the largest and most complex objects that human beings make. With a few exceptions, they come into existence not through design but through a process of growth and change, spread over tens, hundreds or even thousands of years. Each generation extends, substitutes, re-arranges and adapts what it inherits and passes it on to the next generation, so that even a modestly-sized city is an object of such spatial and functional complexity that it seems to defy rational understanding.

It is common to call cities which grow in this way 'organic'. This is a suggestive term. We might be inclined to think that it expresses little more than our non-comprehension of the less-than-orderly products of this type of process. Figure 1, which shows about 1/550th of the built-up area of London using a convention in which the built blocks are in white and the spaces of public access, the urban grid, is in black, suggests why we might be tempted by the term. Even before we consider the complications introduced by land uses, the plan is complex and irregular enough to defeat our powers of description. We have no linguistic or mathematical concepts for such objects. Locally there may be signs of orderly design intentions, but at the

Dense cirilisations. The shape q/' cities in the 21st century 43

Fig. 1. About 1 550th of tile built up area of London using a convention in u hich the built blocks arc in w hitc and thc spacc of public acccss, thc urban grid, is in black.

level of the whole there are none. The word "organic' expresses our helplessness in the face of these disorderly complexities. But it also seems to express the expectation that the grid, while lacking formal order, has some kind of underlying functional logic to it. This is an interesting possibility. We will see below that we believe this to be correct.

But there may be even more interesting reasons for using the term. Human beings make the city bit by bit, but they do not make the whole. The whole emerges, just as an economy emerges from the actions and transactions of individuals, and, like an economy, the city seems to have laws and dynamics which are independent of individuals. The study of artefacts like economies, languages and societies has familiarised the twentieth century


with the notion that things may be made and used by human beings but not be understood by them. The city is such an entity. We understand how we act on it, but we do not understand it as a whole. The term 'organic' expresses the belief that the emergence of the whole from the bit by bit process is due to something more like natural laws than human will or agency. We will see below that again we believe this proposition may be quite reasonable.

~Organic' is, however, not simply a neutral, analytic term. It also implies some degree of approval, and what is approved of is usually a certain spatial diversity and functional vitality which is often thought missing in the more formalised products of conscious design. Today, we are far more sensitive to this than, say, two decades ago, when planners could still complain about the 'unplanned chaos' of the organic city without fear of contradiction, and propose wholesale replacement with more 'up-to-date' designs.

Our experience of such wholesale replacement has been more than a little paradoxical. It has led to the fragmentation of the city into specialised enclaves: estates, precints, shopping centres, cultural centres, and so on, linked by equally specialised ~movement spaces'. In spite of attempts by theorists to rewrite history, such specialised enclaves were rarely found in the organic city. This has reminded us that the organic city was continuous, without any natural or obvious division into parts. At the same time, each area was differentiated and unique. The puzzle of the organic city was that it was a continuity of differences, not an aggregation of parts. It is strange indeed that a piecemeal process should have created continuity and conscious design discontinuity, but this is what has happened, and the experience has put us in a frame of mind where we are fearful of changing anything in our cities for fear of destroying the delicate ecological balance.

Unfortunately, a policy of keeping everything exactly as it is, is just as inimical to the organic city as wholesale demolition and replacement. If organic cities are the product of a constantly shifting process, then to stop the clock is to stop history. You cannot conserve an organic form by stopping the process that creates it at an arbitrary point in time. An overly historicist approach to the city, again paradoxically, is a-historical. It threatens the death of the organic city by stopping the process which led in the first place to what we value.

M O D E L L I N G THE CITY

The only way to avoid both paradoxes is to try to understand the organic city, both as an object and as a process, and to learn to intervene in the process of growth and change on the basis of this understanding. We need a science of the city. Of course, such a science exists, and has a distinguished

Dense civilisations. The shape ~[ cities in the 21st century 45

history. For the past quarter of this century its primary technique has been the urban model. Urban modelling is the general name for computer techniques intended to assist in the process of understanding existing urban systems, and foreseeing the likely consequences of change. The object of urban models is to represent the urban system to the planner in such a way that growth and change decisions can be informed by the true nature and dynamics of the systems that are being modified. Urban modelling is not a science of urbanism, but it could potentially be (see Batty 4 for a comprehensive discussion). To the degree that an urban model was generally applicable to urban systems, and permitted general prediction, then we would be inclined to regard the model as a theory, and the practice of urban modelling as a science.

The urban models we have, however, are of little help to the urban designer, because they are more concerned with the process than the object, and deal with the city at a level of resolution well above the level at which most urban designers must intervene. The difficulty lies in the nature of the models themselves. An urban model must begin by capturing its physical and spatial object in some form of representation. Most urban models do this by making some initial division of the urban system into areas or zones~ perhaps using political or census boundaries, or major dividers like motorways, or even an arbitrary grid. The models then seek to assign quantifiable attributes to these zones and flows between them. What is missing from such models from the point of view of the urban designer is that there is no attempt to represent the detailed physical and spatial structure of the urban object as it exists. But this detailed physical structure is the reality designers have to work with and manipulate.

Against this background~ our group at UCL have for a number of years been trying to develop a new kind of urban model, one which tries to serve designers by addressing the physical reality of cities and urban areas directly and trying to represent the spatial and functional complexity of the city or urban area in the computer. The guiding hope was that if we could learn to use the computer to represent the reality that designers must manipulate directly, and use this also as a basis for analysing spatial and functional complexity, then we could also hope to engage more of the complexity in the intuition of the working designer. Our belief was that in the past designers have been forced to use oversimplified spatial and functional concepts, of what urban systems were like, such as ~zoning' for land-use patterns and ~hierarchy' for spatial patterns, because no better means of grasping the true complexity of cities were at hand. Our hope is that with the new models, designers will design with a much more detailed understanding of the complex nature and dynamics of urban systems.

The first task was, however, the most difficult: how to represent the city as


a physical and spatial object in such a way as to capture properties that were relevant to its functioning. This meant beginning from the fact that cities have no obvious or natural subdivision into ~parts' that would allow us to treat the city as an assemblage of parts, nor obvious 'elements' that would allow us to treat it as a system of "elements and relations'. However, as de Saussure 5 said of language 'When a science has no concrete units that are immediately recognisable, it is because they are not necessary'. The secret is to understand how it is that the city is everywhere differentiated, yet forms a continuous system.

We can understand this a little better by looking at a class of urban forms that do possess clear 'part-whole' and ~element-system' relations: the 'ideal towns' that are from time to time proposed as purifications of the organic city. Looking at the plan of an ideal town in Fig. 2 we immediately grasp the whole because a number of similar parts are brought together into similar relations. The regular deployment of elements that results creates a superordinate pattern, for example the pattern of squares, which the mind picks up as quickly as the local geometry of the elements. Unfortunately, the very properties that make the plan easy to understand as a whole make it difficult to understand on the ground. A person moving through the parts is

Fig. 2. The plan of an idcal town dcsigned by Ricardo Bofil. Wc immcdiatcly grasp thc whole because a number of similar parts are brought together into similar relat ions--each set of buildings creates a squarc, groups of buildings and squares create higher-ordcr squares and so on, but it does not take much imagination to sce that, at ground level, the rationality and

repetition leads to unintelligibility.

Dense civilisations. The shape 0/" cities & the 21st centuo' 47

quickly disorientated, precisely because parts and relations are repeated. With a limited local view, we cannot tell where we are in the object as a whole without further information. It seems that the attributes that make for conceptual clarity make for practical unintelligibility. Ideal towns tend to be clear for passing balloonists; but not for the earthbound.

Plans of organic cities display no such order, especially at larger scales. Locally, there may be signs of the visible and geometric regularities by which we recognise the orderly 'designing" activity of the human mind, but at the level of the whole there are none. On the contrary, each area seems to have a distinctive spatial and functional character, with little or no repetition, and the system as a whole appears continuous, with gradual shifts from one kind of areal character to another. However, the 'continuity of differences' that makes life very hard for the analyst, is practically very useful to those moving within the city. We recognise where we are by the local differences of configuration, and we recognise where we might go by the continuity of differences. The organic city has easy intelligibility on the ground because it lacks repetition of parts and relations. The organic and ideal city are, it seems, not simply unlike each other. In important senses they are opposites.

However, the organic city and the ideal city have one thing in common: both are grids, in the sense that they are made up of blocks of outward-facing buildings, lined by a continuous system of space, which everywhere forms rings around them. In the ideal town, the grid is regularised and repeated. In the organic town, the grid is deformed and differentiated. The secret of the organic city as an object must, it seems, lie, at least in part, in how the grid is deformed.

The concept of the 'deformed grid' is at the heart of the new models. We believe that the deformed gr id-- the very irregularity that makes us label the city as organic--has an interior spatial and functional logic to it which can be retrieved by what we call configurational computer analysis, and tested by direct functional observation of the urban system at work. By modelling the deformed grid, we model the continuous entity that holds the city together. We also model one of the fundamental elements that the urban designer manipulates. Research and design could both be based on the same model.

REPRESENTING AND Q U A N T I F Y I N G D E F O R M E D GRIDS

If the deformed grid has no natural "parts' or ~elements', what does it have that can be modelled? The answer is that as we move about the urban grid, it exhibits everywhere local properties which continually change, but which can be captured by representations. For example, wherever we are in a


deformed grid, we are in some maximal convex element of space defined by the surfaces of building blocks. The property of convexity means that any two points that can be seen from a point can also see each other.

Figure 3 is a hypothetical arrangement of built blocks in which ~Space syntax' software has first identified all such convex elements, allowing them to overlap as much as necessary, then analysed their inter-relationships in terms of how "deep' each is from all the others, that is how many convex elements intervene between each convex element and every other convex element in the system: the darker the fewer, and the less the mean 'depth'. Figure 4 then takes the same analysis, selects a point x, and draws all the convex fields that overlap at that point, thus defining the total convex visual field at that point; it then calculates the mean relative-depth of the convex spaces at that point.

Similarly, at every point in a deformed grid we are aware of the lines of sight from that point defined by the disposition of building blocks. The maximal set of such lines will be those that are tangent to the vertices of the blocks. Figure 5 takes the same hypothetical block arrangement and finds all such lines tangent to vertices, then carries out the same analysis of

Fig. 3. Hypothetical arrangement of built blocks in which ~Space syntax' software has first identified all convex elements of space, allowing them to overlap as much as necessary, then analysed their inter-relationships in terms of how 'deep" each is from all the others, that is how many convex elements intervene between each convex element and every other convex

element in the system: the darker and thc fewer steps, and the less the mean "depth'.

Dense civil&ations. The shape ~ff cities in the 21st centu O' 49

Fig. 4. T h e s a m e a n a l y s i s as Fig. 3, se lects a p o i n t x, a n d d r a w s all the fields t h a t o v e r l a p at

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generalised depths, assigning colours as before: the darker, the less linear depth the system has from that line, the lighter, the more.

Each of these figures thus represents a different view of the system as a whole. Each says: seen in terms of this type of local element, the global system looks like this. This is the essence of 'space syntax' modelling. It is not a single technique, but always poses two questions: how is the spatial system of interest to be represented as (relatively localised) elements? And how are the inter-relationships to be analysed to identify global patterns?

However we choose to answer these two questions, the analysis invariably produces a set of ' located parameters', that is values assigned to well-defined individual locations that express some aspect of how that element fits into the local or global pattern of space. Once we have these 'located parameters', then they can be statistically correlated with any functional parameter that can be expressed in numerical form and located. Obviously, these would include movement rates of different kinds, densities of different types of land use, even crime rates. The degree to which statistical correlations were found between spatial and functional parameters would indicate the degree to which that particular function was sensitive to configuration, looked at in that particular way. If such correlations are found in deformed grids, then we would conclude that the organic city did indeed have an underlying functional logic.

Virtually everything we have tried to look at in this way turns out to be sensitive to some aspect of configuration. Because this is the case, and because the models are actually constructed of the same material that the designer manipulates, spaces and physical objects, the models have found rapid application in a number of urban design projects, and also in research areas inside and outside buildings from laboratory design to urban crime pat terns--anywhere spatial design appears to be a variable that matters from the point of view of some kind of function.

Our purpose is however not to present the use to which these techniques are now being put, but to sketch the theory of urban form--and of the organic city in part icular-- that is now emerging from a growing family of research and application studies using ~space syntax' models. The relevance is that the theories have implications not only for the design of urban areas but also for strategic thinking about what future cities might be like, and how we might come to think about cities in future as energy-using systems.

The argument centres around the relation between the structure of the urban grid and movement, and the relation of both to the pattern of land uses. All three taken together, we call the 'movement economy', and they seem to be related as follows. The structure of the deformed grid, although quite different in different cases (and especially in different culturesl is the prime determinant of the pattern of movement, especially pedestrian

Dense cieilisations. The shape t?] cities in the 21st century 51

movement. The proportion of movement determined by the structure of the grid we call natural movement. Through its relation to natural movement, the grid also influences the evolution of land use patterns, in that different functions are optimally located in parts of the grid with different spatial characteristics and different relations to the whole. The organic city, with its characteristic deformations of the grid, is actually all about optimising certain aspects of the movement economy, and this relates back to the fundamental nature of cities as forms of spatial engineering designed to optimise the flow of movement related energy through society.

THE F U N C T I O N A L LOGIC OF THE GRID: A CASE STUDY

We may best proceed by looking at an example: the application of space syntax techniques to the design of the Fosters/LRC masterplan for the King's Cross development in London. We were brought in first by the Communi ty Group to make a study of the structure and functioning of the area and to suggest how the development could be designed to optimise integration into its urban context, and ensure that natural pedestrian movement permeated the site, and then by Fosters/LRC as consultants on the masterplan, working with the design team with the same objectives.

The procedure used in this, and other similar studies, is to make a computer representation of the existing urban area around the site, make space syntax analyses to understand the structure of the area, observe patterns of space use and movement in the surrounding area and correlate them with the spatial analysis to verify the representation and analysis being used, then test candidate designs by inserting them into the verified area model and re-analysing, and then suggesting new possibilities to the designers where appropriate.

Figure 6 shows the King's Cross site. The scale of area chosen for a study focused on pedestrian movement is decided by several calculations. First, for the purposes of investigating the potential effect of a masterplan on pedestrian movement into and through the site, the area study must include the probable pedestrian catchment area for the site. In fact the model is made at both levels, with the smaller area then being checked against the larger for general agreement about the area surrounding the site. Strong agreement between the two levels of model can assure the investigator that his results are not unduly influenced by the selection of a boundary for the area modelled. In fact, it is always advisable to study a larger area wherever possible, since any spatial representation which cuts out part of a larger urban grid must suffer from ~edge effects', that is inaccuracies towards the edge of the representation, because these areas are


Fig. 6. The King's Cross site embedded in about 15 km 2 of its surrounding deformed urban grid using the black on white representation. The small scale broken-up spatial structures surrounding the site in the centre of the area are recent (postwar) public and very

recent private-housing estates.

Dense cicilisations. The shape ~/]" cities in the 21st century 5 3

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close to the place where the urban grid has been cut. In the large area selected, we can be confident that inaccuracies due to the edge effect are well removed from the catchment area for the site.

Figure 6 is already interesting because it highlights a fact that is far from obvious: that modern housing estates radically down-scale ground level


Fig. 8. Ten observation routes covering a sample of 239 segments of street in ten areas surrounding the King's Cross site.

Dense civilisations. The shape q] cities in the 21st century 55

space. The very much smaller scale, seemingly 'labyrinthine' clumps of space close to the site are all housing estates. This kind of down-scaling is true for nearly all estates, from high-rise modern to more recent low-rise neo- vernacular, and does not seem to be correlated with the height of the buildings. We will return to this phenomenon when we discuss urban dysfunction.

Figure 7 is the next stage of analysis, an 'axial map' of the area shown in Fig. 6. An axial map is the least set of longest straight lines that cover the whole system of public space and pass through all its convex elements, that is, it is a reduction of the line analysis shown in Fig. 5 to the least set that covers the system. The axial map has been analysed to assign an 'integration value' to each line, which is, in effect, the relative depth of each line from every other line in the system, as shown in the earlier figures. Lines which are shallowest to the system as a whole, that is most 'integrating', are dark, and progressive lightening means less integration and more segregation and depth.

Now, extensive research has shown 6-'~ that the distribution of the 'located parameter' of integration in the axial map of a settlement is a powerful predictor of the pattern of pedestrian movement. So it is in this case. Figure 8 shows ten observation routes covering a sample of 239 segments of streets in ten areas surrounding the King's Cross site, including three of the housing estates. Each route was observed by a walking observer a total of 30 times at different times of day and under varying weather conditions. (Details of the technique are given in the earlier cited references.i This method gives a relatively rapid statistical picture of how the urban grid is used. Densities of a pedestrian use can then be set against various located parameters of the urban model for each observation space.

Figure 9 is a scattergram plotting the relationship between the integration value of a street segment (vertically) and the observed density of pedestrian movement in people per 100 m (on a logarithmic scale horizontally) for that segment. The correlation is strong and, obviously, very highly statistically significant, and shows, as other studies have, that the physical configuration of the urban grid is the most important single factor determining how people move through urban spaces.

Now, this finding runs counter to received wisdom in the field. The commonly accepted view is that patterns of movement are generated by flows to and from 'attractors', most generally shops. But here we find that a model, constructed without reference to the location of shops or other land uses, can give an account of a large proportion of the observed movement pattern. This result is all the more remarkable in that the area studied does include a number of very powerful 'attractors', which have strong local effects on movement, including King's Cross and St Pancras railway

5 6 Bill Hillier, Alan Penn

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stations, and Camden High Street and Caledonian Road, both important local shopping streets.

No less remarkably, the correlations hold at similar levels for each of the ten sub-areas studied taken independently (see Hillier & Penn 9 for a more extensive discussion). But there is another twist to the tale. Figure 10a is a scattergram of one of the sub-areas again plotting the relationship between integration and moving people on a logarithmic scale. Figure 10b plots the same relation, but this time with moving people on a linear scale rather than a logarithmic one. This seems to show two regression lines: one with the bulk of spaces, the other with the few with most movement. The latter, it turns out, are the shopping streets. Figure 10c is the same as 10b but with the shopping spaces removed. The relation of integration to movement for spaces without shops is clearly linear.

This pattern is repeated in all of the sub-areas with shopping streets with more than one or two shops. In all cases, the correlation with moving people is strong and logarithmic if the shopping spaces are included, but equally strongly linear if the shopping spaces are excluded. In the housing estates, however, and in the areas without significant shopping streets, the correlation is linear and not logarithmic. This result has been repeated in several other studies, and it is now standard practice with space syntax research to assume that in the presence of significant shopping streets, the relation between integration and movement is likely to be logarithmic, but that in their absence, it is more likely to be linear.

There is an unavoidable inference from these results: that in the 'growth

Dense eivilisations. The shape q[ cities in the 21st century 57

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people on a linear scale; (c) same as (b) but with shopping spaces removed.

and change' process, which includes the opening and closing of shops in whatever locations are possible or suitable, shops tend to cluster in locations according to their integration value in such numbers as to increase the linear relation between integration and movement to a logarithmic one. In other words, although it appears at any time as though the people are in certain locations because the shops are there, in fact (and it cannot logically be otherwise) the shops are there in the first place because the structure of the grid has brought enough natural movement of people through those


locations to make them viable for a certain number of shops. The grid itself thus becomes a primary influence on both pedestrian movement and the distribution of at least certain kinds of land uses within the grid. This kind of process is, we believe, the stuff of the organic city and the functional logic of its deformed grid.

The same general technique of spatial analysis, observation and correlation can be used to detect other processes. The most obvious next line of study is vehicular movement, and its relation to patterns of pedestrian movement. A pilot study using space syntax to explore the relation between vehicular movement and grid structures is now under way. Pathological processes can also be studied. For example, a recent study of one of the areas around King's Cross showed that the dwellings where burglary had been reported to the police during a twelve-month period were on average in significantly less integrated locations than the average for all dwellings, and that in general there were higher rates of burglary in less integrated locations. Similar results have been found over a range of studies (too few unfortunately to generalise reliably), though often with local vulnera- bility factors playing as important a role as the global 'grid' variable of integration.

U R B A N D Y S F U N C T I O N

A more general understanding of urban dysfunction (we use the word in full awareness of the value judgement that is being made, but we believe it to be unavoidable) can be had by considering the results of the movement study for the housing estates surrounding the King's Cross site. It is a remarkable fact that average rates of movement within each of the estates is, at about 0.27 people per minute, more than an order of magnitude less than the 'street average' of 2"98 per minute (discounting main shopping and near-station streets).

If one takes the average length of lines of sight in the urban grid as a whole, then one can use the average movement rate to predict for what proportion of time a pedestrian moving at average speed will be in visual contact with, say, more than one other person. For most areas of the urban grid, and for most of the time, the efl'ects of scale, integration and density of movement are such that the pedestrian is rarely out of sight of others. Figures of being in contact for over 90% of the time are normal.

In housing estates, however, the figures are more or less reversed. The combinat ion of very much lower densities of natural movement with the radical reduction in spatial scale means than for a very high percentage of the time you are on your own in space. Again, figures of being out of contact

Dense civilisations. The shape of cities in the 21st centuo' 59

Fig. ! 1. The area with the urban grid proposed by the masterplan.

with other people for over 90% of the time are normal. The reversal is complete. It is only when one spends time in such a human desert that one realises how much the natural presence of people and through movement contribute to the sense of well-being and safety in 'normal' urban space.

Once again, this pathology can be shown to be the result of how space is


structured, and specifically of the loss of adequate scale and spatial integration in the design. The effect is simple, and is found in almost all UK public housing estates, regardless of their built form or geometry. As the eye picks up by looking at Fig. 6, the estates, unlike the remaining urban area, have no internal pattern of integration which relates the heart of the estate to its edges, and brings natural movement through on well-defined routes. Analysis shows, in fact, that the strongest integration is always at the edges, and segregation increases linearly with steps of depth (the number of turnings) into the estate. Natural movement also falls linearly with increasing depth. The implication is simple. Because there is no natural movement structure within the estate, people take the shortest route from their dwelling to the surrounding streets and back again.

It is not part of our aim here to discuss the effect these analyses had on the evolution of the King's Cross masterplan. For information, Fig. 11 shows the area with the urban grid proposed by the masterplan added. Analysis shows that the masterplan draws integration into the site from all directions, around the central element of the park. Unlike most urban interventions on this scale which impose a geometry on the site and let function fend for itself, the geometry of the masterplan is built on the basis of an organic continuation of key elements of the area into and through the site. The plan in no sense 'reproduces' an existing urban grid. It is an entirely original creation, unlike any other comparable piece of urban grid, yet it is faithful to the inner functional logic of the deformed grid of London. It follows the genotypes, not the phenotypes, of urban structure.

M O V E M E N T ECONOMY

The aim of this paper is to draw theoretical lessons from the results of research using space syntax techniques, and to offer some tentative suggestions as to what they imply for future urban strategy and for cities as energy-using systems. The most important and consistent findings are about movement. Many aspects of urban function, from the evolution of land-use patterns to the distribution of crime seem to be related to movement. The fact that the structure of the grid so consistently predicts the pattern of movement is also very striking. We are now confident that the structure of the deformed grid in the organic city is fundamentally about movement, and that the integration structure of the axial map is not only a model for the urban grid but also a theory of the urban grid, and, as such, part of a theory of the organic city.

If our results are as general as we think they are, then the implication is that the structure of the urban grid, which has been thought epiphenomenal

Dense civilisations. The shape of cities in the 21st century 61

to urban processes for over a century, i~the key not only to movement but what we call the whole movement economy. The argument runs something like this. Seen as a physical system, a city is two things: a continuous network of spaces, usually in the form of a not very regular street grid of some kind; and an assignment of land uses, or functions, to locations in the network. Urban movement is a set of origin-destination pairs between functions in the network. Evidently, the total amount of movement in an urban system will be some function of the distances from location to location and the necessity of trips between functions.

However, that is not all that movement is. Every trip is not only an origin-destination pair. It also has a by-product in the form of the sequence of spaces that are passed through on the way from origin to destination. The distribution of this by-product is an effect of the configuration of the network of spaces, since it is this that determines what routes are available and natural from everywhere to everywhere else.

Now it is an easily observable fact that in most urban spaces for most of the time, most pedestrian movement, and a Jbrtiori probably most other movement, is a by-product. The people that you see in any space are more likely to be on the way to somewhere else than to be coming from an origin or seeking a destination in that space. It is hard to generalise from existing data, but as far as we can see the split is usually about two thirds by-product, one third origin or destination, regardless of actual movement densities.l°

Now if movement in urban spaces is dominated by the by-product of trips between located functions, then this is the same as saying that most of the observable movement density that you see in urban spaces is determined by the spatial configuration of the urban grid. Since the by-product effect is so strong, it itself becomes over time a powerful dynamic factor affecting the distribution of land uses and functions. Retail functions, for example, become biased towards spaces where the by-product is strongest. The presence of shops then has a multiplier effect on the movement rates, making established locations even more attractive for retail. In this way spaces that are important in the configuration for movement by-product also become important destinations, and vice versa. Put another way, to-movement and through-movement come to prioritise the same spaces.

It is because of this process that urban forms that have evolved over a long period appear to have their different functions located in the right type of place--residential accommodat ion is concentrated in areas with less by- product movement, local facilities tend to be in spaces with strong local by- product movement, and more globally available facilities in spaces with more globalised by-product movement, and so on. Even an otherwise symmetrical 'ideal' city evolves through time to prioritise the streets leading to its gates for retail use (Fig. 12).


Fig. 12. The "ideal" city of Palmanova, showing the effect of the "movement economy' on urban growth and changc.

This means that cities are about movement in a far more profound sense than is usually thought: their actual physical and spatial form and land-use distribution both result from and generate a pattern of movement, primarily through the way they distribute the by-product of origin-destination pair trips. We suggest that this permits a tentative new definition of the city as a total m o v e m e n t e c o n o m y linking the spatial and functional structure. By movement economy, we mean a grid configuration and a land-use assignment linked generatively through the by-product effect.

A 'successful' urban movement economy is, we suggest, one which tends, through the process of growth and change, towards optimisation of the usefulness of the by-product of movement, by making every journey from everywhere to everywhere else as useful as possible in terms of the human and material resources that are encountered on the way, and therefore available if necessary. Thus shopping in the supermarket on the way home is

Dense civil&ations. The shape o['eities & the 21st century 63

an event in the movement economy, just as impulse buying a pack of After- Eight Mints at the check-out counter is an exploitation of it. Such optimisations are, we believe, the source of most of the positive qualitative judgements we make about cities, since these are invariably about the peculiar 'life' of the city, and depend on the natural presence of people.

The notion of movement economy may be summarised as follows. The distribution of land uses in an urban grid creates a system of origins and destinations which are related in two ways: metrically, in terms of appropriate proximities and physical distances; and configurationally, in terms of how they lead to the urban grid being used. An urban movement economy arises when land uses are, or become, assigned according to the movement properties of the grid in conditions of sufficient density, proximity and spatial scale to create the necessary densities in by-product movement without which the process will be neither activated nor sustained.

One way this can be done is by using the different faces of the urban block for different types or grades of use, or, as in the past, for different grades of housing. This can produce a pattern which we call 'marginal separation by linear integration' in which land uses change only slowly as you move along a street al ignment--say shifting from residential to shopping along the same line--but change quite sharply when ninety-degree corners are turned and a line is taken down another face of the block.

The dependence of an urban movement economy on density, proximity and scale can be clarified by a thought-experiment working out the effects of, for example, dispersion. As you change from a dense and nucleated system to one which is dispersed and fragmentary, it is obvious that the mean length of journeys will, ceterisparibus, increase. It is less obvious that the by- product effect will also be diminished. As dispersion increases, the movement system becomes more like a pure origin--destination system. Undoubtedly, this is the reason why people travel 40% farther in the country, and most of this extra travel is in private cars. Country dwellers also walk less often and less far, although taking town and country together a surprisingly high proportion of all journeys are still on foot, and even more are partly on foot. It is difficult to work out exact figures, but between 40% of trips overall and up to 75% in urban centres are pedestrian. Obviously the proportion of journeys that are pedestrian is a function of where the things are that you need to go to- - in effect how land uses are distributed in the urban grid. But there will also be some effect here of the loss by-product.

What is even less obvious is that an urban-design policy of replacing continuous urban structure with specialised enclaves will also tend to eliminate by-product. Enclaves are, almost by definition, destinations which are not available for natural movement. They are discontinuities in the


urban grid, comparable in their effects with the physical dispersion of the urban fabric, and similarly disruptive of the movement economy.

It can now become clear that it is the culturally-sanctioned values that are embedded in current attitudes towards urban design--lowering densities wherever possible, breaking up urban continuity into well-defined and specialised enclaves, reducing spatial scale, separating and restricting different forms of movement, even restricting the ability to stop moving and take advantage of the by-product effect--that are inimical to the natural functioning of the city and its movement economy. It is not density that undermines the sense of well-being and safety in urban spaces, but sparseness, not large spatial scale, but its insensitive reduction, not lack-of- order but its superficial imposition, not the 'unplanned chaos' of the deformed grid, but its planned fragmentation. Urban rationality without urban understanding is in danger of eliminating all the properties of density, good spatial scale, controlled juxtaposition of uses, continuity and integration of the urban grid on which the well-ordering and well- functioning of the city depends.

REFLECTIONS ON THE ORIGINS OF URBANISM A N D THE T R A N S F O R M A T I O N OF THE CITY

The value system according to which we have been transforming our cities appears as a kind of urban rationality, but it is not based on urban understanding. Where does it come from? And what should we do about it? Let us begin by reflecting a little on the nature and origins of cities, why we have them and what made them possible.

Towns, as physical objects, are clearly specialised forms of spatial engineering which permit large numbers of people to live in dense concentrations without getting on each others nerves, and minimise the effort and energy needed for face-to-face contact with each other and providers of needs. Towns, we suggest, were made functionally possible in the first instance by a transmutation in the way energy flowed through society. It is most easily explained through a distinction made by the geographer Richard Wagner between two kinds of energy-related artefacts: implements which transmit or accelerate kinetic energy; and facilities which store up potential energy and slow down its transfer. For example, a flint knife is an implement, whereas a dam is a facility. Whatever else made towns possible, there is no doubt that they were usually marked by a radical increase in facilities, most especially irrigation systems and food-storage facilities.

What made towns possible socially was an invention we are so familiar

Dense civilisations. The shape q['cities in the 21st century 65

with that we tend to take it for granted and forget it's there. We mean the urban grid. The urban grid is the idea that space can be organised by having groups of contiguous buildings in outward facing, fairly regular clumps, which between them define a continuous system of space in the form of intersecting rings, with a greater or lesser degree of overall regularity. Grids were not inevitable, many proto-towns exist in the archeological record with quite different morphologies.

The urban grid, however, was a first powerful theorem of urban spatial engineering. The whole point about it was that it was itself a faci l i ty--one that took the potential movement of the system and made it as efficient and useful as possible. The grid was the means to the end of the town as a ~mechanism for generating contact', and it did it by ensuring that origin-destination trips took you past outward-facing building blocks on your rou te - - tha t is they allowed the by-product effect to maximise contact over and above that for which the trip was originally intended.

Now, where we are at seems to us to be this. In the nineteenth century, under the impact of industrialisation and rapid urban-expansion two things happened. First, to cope with sheer scale we began to think of the urban spatial grid as more of an implement than a facility. That is, we saw it as a means of acceleration of movement necessary to overcome size. Alongside this, we saw it as a set of point-to-point origins and destinations, rather than as an 'all points to all points' gr id- - the product of an urban-movement economy. The points which were selected became privileged sites of urban expans ion- -and were still trying to fill in the gaps.

Second, we began to see the city not as a grid-based civilisation, but as the overheated epicentre of focal movement into and out of the city, and as such the most undesirable of locations. We also saw a social problem in the disorderly accumulat ion of people in and around the centres of the cities, brought in to serve the new forms of production. Big became synonymous with bad, and density became synonymous with moral depravity and political disorder. The answer to both was the dispersion and/or ruralisation of the city.

But there was a huge assumption: that the movement energy required was more or less limitless and without significant cost. Although this gave rise to much of the value system of modern planning, learning that it is not the case has not so far generated an alternative value system when it comes to physical planning policy. We suggest that when it does it will redefine both the desirable product of urban design as some new version of the high- density organic city, and the process of urban design as the intervention in the process of growth and change on the basis of a body of knowledge of the city itself.


R E F E R E N C E S

1. Department of Transport, National Transport Surw, y: 1978/79 Report. HM SO, Norwich, 1983, Table 10,4, p. 71. (See also NTS: 1975/76 Report, Table 3.17, p. 37.)

2. Pushkarev, B. & Zupan, J., Urban Space for Pedestrians." A Report to the Regional Planning Association. M|T Press, Cambridge, MA, 1975.

3. de la Barra, T., Integrated Land Use and Transport Modelling: Decision Chains and Hierarchies. Cambridge University Press, Cambridge, 1989, Ch. 8.5, pp. 163-7.

4. Batty, M., Urban modelling and planning: reflections, retrodictions and prescriptions. In Remodelling Geography, Basil Blackwell, Oxford, 1989, pp, 147-69.

5. de Saussure, F., Course in General Linguistics, Part 2, 1916. Fontana/Collins, Glasgow, 1974, p. 107.

6. Hillier, B., Burdett, R., Peponis, J. & Penn, A., Creating L(fe: Or does Architecture Determine Anything? Architecture et Comportement/Architecture and Behaviour, Lausanne, Vol. 3, No. 3, 1987.

7. Hillier, B., The architecture of the urban object, Ekisticks, 56(334/5)(1989) 5-21, 8. Peponis, J., Hadjinikolaou, E., Livieratos, C. & Fatouros, D., The spatial core of

urban culture. Ekisticks, 56(334/5) (1989) 43-55. 9. Hillier, B. & Penn, A., Natural movement or, configuration and attraction in

urban pedestrian movement, paper given at the European Conference on managing urban growth and change, Cambridge, 1990. Forthcoming Environment and Planning, B (1992).

10. Hanson, J., Order and structure in urban space: a morphological history of the City of London. Doctoral Thesis in the University of London, 1989.

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Dense civilisations: the shape of cities in the 21st century