City_buiding video game essay

2 THE CHALLENGE

The image opposite shows the abrupt halt that urbanism faces when it meets the disurban territory of the distributor road: in this case, in the form of a prosaic roundabout. What happens when the irresistible force of New Urbanism meets the immovable object of ‘road hierarchy’? This is a ques- tion that lies at the heart of the challenge of this book.

On the one hand, we have plenty of ideas, guidance and consensus on what urbanistic grids of streets might look like. On the other hand, we have plenty of principles and conventions on how distributor roads should relate to each other in a road hierarchy. But the two do not necessarily match up.

The mismatch is currently exemplified in the way that the UK design guide Places, Streets and Movement – which is intended to address the design of streets in mixed use areas – is a companion to Design Bulletin 32, which is a guide to the design of roads in residential (i.e. mono use) areas.1 Falling between the cracks, as it were, is a fair amount of urban design territory, not least the traditional main street, which is not comfort- ably addressed by either guide. And yet these guides perhaps represent one of the closest points that the two traditions approach each other – that most closely span between the extremes of streets as architectural ensem- bles and streets as trunk roads.

Accordingly, although there is increasing recognition of the significance of the street to urban design – by engineers as well as urban designers and planners – the streets often seems to somehow ‘float free’ of any clear or consistent conceptual framework. To fit engineering convention the street has to be conceptualised as an urban ‘access road’, that belongs in a ‘road hierarchy’, not a ‘street grid’. In a sense, we have to suspend our belief in2.0 • Pandbury

the existence of streets when submitting design concepts to fit the realm of conventional engineering practice, or suspend belief in the existence of those conventions at the creative stage of envisioning urbanistic grids of ‘streets’, ‘mews’ and ‘boulevards’. And so, although there may be a con- sensus that there is a need for reform, it is not so clear what exactly should be reformed and what should be retained.

For a start, we seem to have lost the art of designing street grids. The street grid lies in a disputed territory between the spheres of influence of the transport and urban design professions. For many years, while Modernism held sway, this territory became a kind of no man’s land in design terms, since streets themselves – and hence street grids – were out of favour. In the ensuing theoretical void, engineers simply got on with designing layouts of ‘roads in urban areas’ (Figure 2.1).

The advent of neo-traditional urbanism and the revival of the street have changed all this. Urban designers and planners are showing renewed inter- est in both streets and street patterns; they may rue the ceding of this design territory, and wish to ‘reclaim the street’, and with it, street pattern. These now represent an overlapping area and a potential area for conflict. The reconciliation of this conflict – or the filling of the ‘void’ with something positive – represents the challenge tackled by this book as a whole.2

Chapter 1 set out in a broad sweep the general challenges of trans- port and urban design. This chapter now focus on the specifics of the challenge of urban layout constituted by streets and patterns, by looking in more detail at key areas of the professional design debate. It examines existing priorities and issues for design guidance, demonstrating the lack of clear articulation of desired street types and patterns from an urban design perspective. In doing this, it sets out the different areas of confu- sion and conflict between different aspects of street pattern, structure and hierarchy. This stimulates the investigation into the nature of street type and pattern that forms the core part of the book.

STREETS

The types of street to be included in a scheme are the key to its overall character.

Urban Design Compendium 3

A street can be seen as a road that happens to have an urban character; or as an urban place, that happens to serve as a right of way.

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22 STREETS & PATTERNS

Route location

Junction design

Transport planner/ engineer

Traffic flow

Highway alignment

Land use distribution

Built form

Urban designer/ planner

Public spaceStreets and

patterns ?

Development control

2.1 • Streets and patterns as overlap or void between design professions.

The first view is typically the starting point of the transport planner or engineer, who would see street design as a specialist aspect of road design – one that has to deal with several urban ‘complicating factors’ that get in the way of designing a good road.

The second view is perhaps more like the starting point of the urban designer, who will conceive of ensembles of buildings, sequences of spaces, and their associated functions, one of which is the specialist concern of movement – which becomes a transport ‘complicating factor’ that gets in the way of designing a good place.

Under the system of Modernism, the view of the street as an urban road prevailed; but increasingly the recognition of the street as a multi- functional urban place has been gaining ground. This book addresses the tension between the two. It does not attempt to deal with all aspects of street design across the spectrum of engineering and urban design concerns, but focuses on the interface between the two points of view.4

Street type The issue of street type is important because it leads from description of type to prescription of type. In other words, the types of street that are systematically recognised in design guidance will influence the types of street that are designed and built. If certain types of street are not recog- nised, and do not feature in design guidance, then these types may disappear, there being no adequate guidance or specifications available for employing them. Even the street itself – as an urban route type in the first place – might be in danger, if the category ‘street’ is denied in official guid- ance geared to the design of ‘urban roads’. Therefore, in order to meet today’s needs for streets-oriented urban design guidance, we need to be aware of what kinds of urban route and street are potentially available.

Street roles and classification Any particular street will tend to have ‘multiple personalities’, that is, have a variety of different characteristics that are present simultaneously.5 For example, Marylebone Road in London is a major traffic route and bus route; it serves as a ring road and bypass to central London; it has the form of a dual carriageway boulevard; it is designated a Red Route and part of the Transport for London network; it is the A501 (Figure 2.2).6

Any particular street is likely to have a variety of official designations, as well as any number of other possible bases for distinction. The Institution of Civil Engineers has noted this as a confusion of different systems of road classification, that are each directed towards different purposes. Those

23THE CHALLENGE

purposes include distinguishing administrative responsibility for routes (e.g. national trunk road), assisting with information (e.g. route signing), or distin- guishing road standard (e.g. dual carriageway) or construction criteria (e.g. based on the design life measured in ‘millions of standard axles’).7

Particularly influential has been the idea of a ‘functional’ classification, in which roads are classified according to their (intended) function, rather than their (present) form.8 An example of conventional ‘functional’ hierarchy is that given in the UK professional manual Transport in the Urban Environment (Table 2.1).

This kind of classification is not just a description of possible types, but it is a prescription for ongoing management and future design, that

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24 STREETS & PATTERNS

2.2 • The multiple personalities of Marylebone Road. Marylebone Road is a boulevard, a traffic route, a ring road, a primary route, among many roles.

implies that these are ideal types, or the only allowable types. It is also significant, because this is not just a linear listing, but is an ordered ‘hier- archy’ that is linked to allowable connections between street types, as well as binding together the distinction of different types of movement and frontage function.

The Transport in the Urban Environment hierarchy has only four types of road which accommodate vehicular traffic, and only one type – the access road – which is dedicated to providing a full multi-functional role for any vehicle type and frontage access. In other words, it only has one type of road that fits the description of a street.

This has meant in practice that a diversity of actual streets has had to be shoehorned into a narrow range of categories apparently dominated by traffic function. An arterial street like Marylebone Road has to be forced into a category either as a distributor road – placing vehicular movement ahead of other considerations, and in denial of its pedestrian and frontage roles – or as an ‘access road’, which is not meant as a strategic traffic conduit (Figure 2.3).

Urban design and street type Clearly, the above situation is at odds with the urban design view of streets. The Urban Design Compendium suggests that ‘A new terminology is required to describe all the roles that streets can play in making successful places.’9 Whatever kind of terminology is advocated, it is likely to involve some recognition of streets that combine through movement and local use

25THE CHALLENGE

Table 2.1 The road hierarchy of Transport in the Urban Environment

Road type Predominant activities

Primary distributor Fast moving long distance through traffic. No pedestrians or frontage access

District distributor Medium distance traffic to primary network. Public transport services. All through traffic between different parts of the urban area

Local distributor Vehicle movements near beginning or end of all journeys

Access road Walking. Use of highway by frontagers. Delivery of goods and servicing of premises. Slow moving vehicles

Pedestrian street Walking. Meeting. Trading

Pedestrian route Walking. Some cycling in shared space

Cycle route Cycling

Source: IHT (1997: 146).

or frontage activity. This wider view of streets is also echoed by the Institution of Civil Engineers and Urban Design Alliance, whose recent Designing Streets for People report suggests the need to balance the tradi- tional ‘right of way’ with a sense of ‘right of place’.10

Urban design approaches to street type often tend to focus on phys- ical form, such as the street cross-section (including width of carriageway and building type), or the kind of streetspace on plan (e.g. square, mews) or perhaps land use function (e.g. residential, commercial). One possible approach is given in the Urban Design Compendium (Table 2.2). However, although this is a step in the right direction from the urban design point of view, it remains to be seen how to reconcile these types with conventional hierarchy, which for better or for worse prevails as a key influence on struc- turing urban layout.

Another approach to street type is suggested by Peter Calthorpe, who advocates the use of ‘connector streets’ in his influential work The Next American Metropolis (Figure 2.4). Connectors are supposed to carry ‘moderate levels of local traffic smoothly, in a way that is compatible with bicycle and foot traffic’.11 While associated with traffic function, the connector itself is significantly defined by its position in a particular kind of connective network. That is, the connector street’s distinctive character is owed significantly to the structure of the road network, rather than to its physical form or its traffic function. While Calthorpe’s ‘connector street’ is clearly demonstrated on plan, the limits of what is or is not a connector street are not defined; this could merit further attention.

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26 STREETS & PATTERNS

2.3 • Denial of the traditional arterial street. Under the current system, traditional arterial streets such as Marylebone Road (Figure 2.2) are not recognised as such; only the separate roles of (a) primary distributor or (b) access road (frontage street).

Table 2.2 Suggested street types from The Urban Design Compendium

Conventional Suggested street types that combine capacity and character

Primary Main road – routes providing distributor connections across the city

District Avenue or boulevard – formal, distributor generous landscaping

Local High street – mixed uses, distributor active frontages

Access road Street or square – mainly residential, building lines encouraging traffic calming

Cul-de-sac Mews/courtyard – shared space for parking and other uses

Source: Llewelyn-Davies (2000: 75)

(a) (b)

Overall, there is a need to address the issue of street type by consid- ering the full variety of possible kinds of form and function: how should street types relate to each other within a given typology (e.g. how a connector street might relate to an arterial or cul-de-sac)? How might street type relate to pattern type (e.g. Calthorpe’s connector street with connec- tive network)? How might different types relate to each other within a ‘hierarchy’ (e.g. how street types are related as a ‘hierarchy’)? Finally, how might these notions of street typology be reconciled with conventional road hierarchy, addressing different modes of movement and traffic functions?

HIGHWAY LAYOUT

The nature of the hierarchy of the elements of the network and the elements themselves are what gives any urban design its character . . . The geometry of the road network . . . and the manner in which the elements are linked together are all the subject of urban design.

Jon Lang12

Just exactly how the urban design of streets and spaces may be recon- ciled with road network structure is a central concern of the design debate addressed in this book. As we have seen earlier, highway engineering layout standards are perceived to be part of the disurban problem. Therefore, we need to explore the kinds of highway layout guidance to see more clearly where the problem lies in terms of street pattern.

Highway layout guidance It is conventional for highway design guidance to specify road standards in terms of design components such as link alignment, carriageway layout, junction type and construction standards. For example, when we look at the aspects of streets to which regulation applies in the USA, we find that these include the relation of proposed streets to adjoining street systems, rules for street intersections and rules for dead-end streets.13 However, whatever else highway layout standards control, there is little that speci- fies actual street pattern.

This is a general trend among transport engineering and policy texts, where street or road network pattern structure may be alluded to here or there, but only mentioned in passing, or not treated at all. For example, some texts refer to very general kinds of pattern (such as radial or gridiron)

27THE CHALLENGE

2.4 • Calthorpe’s connector streets.

but these relate more to the description of typical forms already existing, than to target patterns intended for network design application. When we look for any detailed guidance on specification of pattern, we do not find any explicit prescription.14

For example, in the UK, the general reference work Transport in the Urban Environment says little explicitly on the matter of network pattern. In the USA, the American Association of State Highway and Transportation Officials’ ‘Green Book’ has no guidance on street pattern. In its opening chapter, indicative street layouts are shown, for the purposes of demon- strating the role of typical street types, but, throughout the rest of this comprehensive and authoritative volume, there is no suggestion of what a desirable or optimal street pattern should be.15

The UK design guide for residential roads and footpaths, Design Bulletin 32, alludes to a variety of kinds of layout, including ‘hierarchical layouts’ and ‘network configurations’, but it is not possible to infer exactly what is meant by these, as there are no accompanying diagrammatic examples. Therefore, although this kind of layout guidance has been accused of giving rise to ‘a plethora of standardised housing layouts’, in DB32 itself there is virtually no explicit guidance on overall street pattern. In other words, any ‘monoton- ous’ effect is surely the result of lack of imagination on the parts of the designers (of whatever profession). This hints that the highway engineering rules per se do not necessarily preclude good design, nor inevitably create ‘bad urbanism’.16

In the USA and elsewhere, the New Urbanist movement and associ- ated neo-traditional approaches have brought much creative effort to bear on the issue of street pattern. The principle of having interconnected streets is central to traditional neighbourhood design (TND).17 However, even the Institute of Transportation Engineers’ Traditional Neighborhood Develop- ment Street Design Guidelines have no explicit guidance on the overall form of street pattern – other than the general suggestion that ‘TND streets are interconnected’.18

The elusive issue Despite criticism of engineering standards in general, we look in vain for the actual traffic or highway engineering specification indicating what overall form street pattern should take. This is curious, since it implies that there is – if anything – a lack of guidance on the highways approach to pattern, rather than a straitjacket of authoritarian prescription.

In practice, highway and traffic engineering tends to deal with com- ponent parts rather than wholes. That is, they deal with the design of links,

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28 STREETS & PATTERNS

junctions and their relationships (e.g. junction spacing). They do not prescribe the resulting pattern (Figure 2.5).

The road network is effectively like a kit of parts, chiefly conceptualised as links and nodes (junctions), joined together like an engineering structure. The road links and junctions – like structural members and joints in a building structure – are designed to carry so much capacity, in the fulfilment of such and such a function. Individual junction types might be recommended or discouraged. But as for overall pattern, this appears to be left open.

This lack of prescription and indeterminacy actually allows a certain amount of flexibility. However, it may also result in a feeling of lack of control; or may leave a creative vacuum. Given the lack of explicit guidance on pattern, there may be a de facto tendency to use ‘hierarchical’ systems to generate only ‘tree-like’ or ‘loop and cul-de-sac’ road patterns on the ground. Yet, although commonly associated, ‘hierarchical’ and ‘tree-like’ are, as we shall see, not necessarily the same thing.

To get a fuller grasp of the nature of the challenge, we turn to look at the elusive issue of street pattern from the point of view of those advo- cating alternatives to the status quo, and expressing a variety of desired patterns.

DESIRED PATTERNS

What are those qualities? Easier to recognise than describe, and easier to describe than prescribe with any precision . . .

Urban Villages19

Such guidance as exists on overall street pattern tends to come not from highways or traffic engineering conventions, but from the urban design and planning traditions. Unlike the definite, often quantified standards of highway engineering, urban designers’ and planners’ expressions of desired patterns are often couched in terms of verbal descriptions of properties, or demonstrated by means of illustrative plans.

The quest for clarity

I would give each path an identifiable character and make the network memorable as a system of clear and coherent sequences . . . Each road could be given a coherent form, and the intersections with other paths made clear.

Kevin Lynch20 [emphasis added]

29THE CHALLENGE

Allowable spacing

Allowable junction type 1

Allowable junction type 2Allowable

route type A

Allowable route type B

2.5 • Highway guidance for layout structure. Despite criticism of ‘standardised’ layouts, highway guidance tends to address individual components, not prescribe overall pattern.

Terms such as coherence, clarity and legibility permeate planning literature and recur frequently in works of design aspiration and guidance. They are often used liberally to describe the desired qualities of urban structural features such as street patterns and networks of spaces (Figure 2.6).

The selection of examples – listed fully in Appendix 2 – is not exhaust- ive, and one could surely point to more cases in a similar vein. Nevertheless, the haul of concepts from even a preliminary trawl can be illuminating, in this case with regard to the recurrence of the terms: ‘legible’ appears five times, ‘coherent’ appears ten times, and ‘clear’ or ‘clearly’ appear over twenty times (Appendix 2).

It is not always obvious, however, what is meant by these terms, espe- cially since, more often than not, the statements are not accompanied by diagrams to illustrate patterns with (or without) these qualities. For example, Lewis Keeble is one of many advocates of a ‘clear pattern’, but warns against ‘artificial symmetry or factitious order’.21 The lack of a definition or demonstration of the clear pattern makes it difficult to know at which point clarity might become factitious. Meanwhile, the idea of a ‘connected network’ is almost tautological, since any given network should be, by

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30 STREETS & PATTERNS

…a clear street

pattern….

UK DTER

Raymond Unwin

…clear and legible structure…

Lewis Keeble

Kevin Lynch

Urban Task Force

Charter of the New Urbanism

Western Australia

…high degree of street

connect- ivity…

…a clear hierarchy of spaces…

…interconnected network of streets…

…a clear structure of accessible routes…

…clear hierarchy…

…clear and coherent system of

sequences…

2.6 • Examples of desired properties of urban structure. A fuller list of desired properties and sources is given in Appendix 2.

definition, connected up. And when the Urban Task Force calls for ‘a clear structure of accessible routes . . . which lead from one destination point to another’, we are left none the wiser as to what is envisioned.22 These state- ments could apply to almost any pattern, from a rural patchwork of pathways to the spatial blueprint for a modernist megastructure.

Yet there is clearly something of significance that is trying to be articu- lated. Although there may be a confusing profusion of terms, there must be something ‘there’ that is being referred to. This reflects the fact that patterns cannot be concisely described with words alone.

It seems that if ‘legible’ and ‘clear’ and ‘coherent’ are to be treated seriously as design qualities, then they must be capable of adequate specifi- cation. Instances of street patterns that are clear and unclear, coherent and incoherent, and legible and illegible should be distinguishable. This suggests that they should be demonstrable on plan.

Pattern demonstrations A desired pattern may be depicted on plan, which can be more structurally demonstrative than a verbal description. That said, a single diagram on its own may not necessarily isolate the key ‘active ingredients’ of a desired design. A more effective method is to contrast ‘preferred’ and ‘discour- aged’ diagrams, to help demonstrate the key properties (Figure 2.7).23

Current urban design guidance typically depicts a grid-like pattern as the preferred case and a tributary pattern of loops and culs-de-sac as the dis- couraged one – although in the past the reverse was the case.24

However, in presenting a simple polarisation, even the use of paired diagrams may only be able to demonstrate rather crudely the difference between extreme types, such as between grid and cul-de-sac systems. In reality, there will be a range of types spanning between these extremes; although as yet design guidance tends not to draw or define finer distinctions.

A second problem is that illustrative diagrams in general tend to bind together different connotations in a single layout depiction – and it may not be known which connotations are essential and explicitly intended, or which are incidental features that are not supposed to be a definitive part of the demonstration. For example, a ‘preferred grid’ may be depicted as orthogonal (right-angled) and rectilinear (straight-lined), whereas a simple topological connectivity might be all that was intended. Conversely, there is no way of knowing if a ‘preferred grid’ that happens to depict a loose ‘organic’ pattern expressly implies that rigid rectilinearity is to be rejected or not.

31THE CHALLENGE

This tells us that while the depiction of a plan is in one sense useful by being structurally explicit, in another sense, it may also be open to mis- interpretation, since it is not necessarily clear which parts of the plan are essential properties, intended to be taken literally as generalised principles. In other words, the use of plans has advantages from the point of view of description, but may have disadvantages from the point of view of prescription.

Problems of prescription There appear, therefore, to be some problems with the prescription of patterns for design guidance. First, we have seen verbal descriptions of properties which do not clearly specify geometric patterns that could be followed (Figure 2.6). Second, we have seen explicit graphical presenta- tions of ‘preferred’ and ‘discouraged’ patterns (Figure 2.7), which might be ambiguous from another point of view. These might be too specific, by including incidental detail that is not explicitly intended as part of the prescription. This detail may inadvertently suppress other possible variants, inhibiting creativity.25

There is also a further problem, relating to how desired exemplars may be followed by practitioners. For example, the design guide Places, Streets

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32 STREETS & PATTERNS

2.7 • Examples of ‘preferred’ and ‘discouraged’ layouts.

and Movement depicts the layout of Poundbury as an exemplar for good design (Figure 2.8). The development is laid out around ‘a network of spaces’ where ‘primacy is given to the creation of coherent, attractive neigh- bourhoods’. The Poundbury example includes a typology of streets and squares (Figure 2.9).

While the Poundbury example is an interesting illustration of a ‘hier- archy’ of street types, there is no explicit theory of hierarchy accompanying it, nor generalised guidance on overall patterns, that might allow one to repeat the exercise to achieve the desired ‘coherence’. Similarly, the Essex Design Guide demonstrates graphically that areas should be focused on ‘cores’ from which the street system should radiate – but it is not so clear how a general ‘theory of cores’ might directly translate into an actual design approach.26

Indeed, Better Places to Live asserts that there is ‘no standard formula’ for designing the well-connected layouts it advocates. The Urban Task Force recommends that design guidance should provide good practice examples, rather than the ‘prescription’ associated with conventional roads-oriented design guides such as DB32.27

While these approaches may be welcomed for getting away from ‘rigid prescription’ and ‘standard formulae’, they also both seem to suggest aban- doning an attempt at generalised principles that may easily be adopted by design practitioners up and down the country. Both imply looking at good practice exemplars; but this on its own might yet have less flexibility, and

33THE CHALLENGE

2.8 • Poundbury as a ‘good practice’ exemplar.

2.9 • Poundbury’s typology of streets and spaces.

could in itself be seen as a kind of standardisation – the ‘typecasting’ of pattern, perhaps.

That is, there could be an over-reliance on employing specific master designers to weave their design magic in the creation of their own ‘signa- ture’ patterns, which could generate their own coherence internally, but without any transparent means of transferability. Or, taken to another extreme, it could lead to a plethora of ‘Poundburys’ marching across the countryside, as the present exemplar is replicated mechanistically by others, within the existing framework of engineering convention (Figure 2.10).

And aside from this issue of standardardisation – as the form of Figure 2.10 hints – there is still no resolution of the issue of what happens when the exemplary urbanism of Poundbury hits the distributor road network.

COMPLEXITY AND CONTRADICTION While we have an emerging impression of desirable features such as connective, walkable street grids, and apparently undesirable properties such as road hierarchy, it is not wholly clear what kind of connectivity or hierarchy is meant. This section deconstructs five key areas of confusion which this book as a whole aims to clarify:

1. street type and hierarchy; 2. ‘bad’ versus ‘good’ hierarchy; 3. hierarchy versus pattern; 4. preferred versus discouraged patterns; 5. pattern versus process.

Street type and hierarchy Systems of road hierarchy which specify a limited typology or ‘hierarchy’ of allowable street types – typically expressed as road types – have been criticised for being apparently based on traffic flow or road capacity.28 The classification may appear to be a rather narrow ranking, which appears to prioritise vehicular traffic over other modes.

In fact, the basis for specification of type is not clear or consistent. Although conventional road hierarchy is based on ‘functional’ classification, there is ambiguity about whether the function refers to what a street ‘func- tions as’ (e.g. a rat run) or refers solely to its intended function is (e.g. street for local traffic). There is also ambiguity as to whether the functional basis for the classification is ‘trip function’, ‘traffic function’, ‘road function’ or ‘network function’. A variety of trip, traffic, road or network-related para- meters could be used to specify the function of any given road or street, but the definitions are not necessarily clear or consistent. Ray Brindle comments

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34 STREETS & PATTERNS

2.10 • A plethora of Poundburys.

on the ambivalence as to ‘whether we are specifying design characteristics to help define road types, or vice versa’. Meanwhile, Phil Goodwin has suggested that a functional hierarchy of roads is simply a ‘fantasy’.29

Exploring the characteristics of classification and hierarchy will require consideration of the meaning of ‘function’, how function is allocated, how street type relates to position in the street pattern, and to hierarchical struc- ture. Getting to the bottom of the issue of street type will be a full analysis in its own right (Chapter 3).

‘Bad’ versus ‘good’ hierarchy Although conventional road hierarchy is often criticised, sometimes urban designers and planners themselves appreciate some kind of hierarchy, based on distinguishing different kinds of street type. Design guides will call for a network that ‘clearly distinguishes between arterial routes and local streets’, a ‘clearly recognisable hierarchy of streets’, a ‘hierarchy of clear connections’ or a ‘hierarchy of routes and places’. Earlier, we saw the case of Poundbury (Figure 2.9) where a hierarchy of street types was presented. 30

So, some kind of hierarchy can be ‘good’ from an urban design point of view – although it is not necessarily clear or consistent what this ‘good hierarchy’ entails. It might be contrasted with ‘bad’ hierarchy of conventional engineering approaches, but even here, the distinction is not necessarily clear.

For example, in the USA, the ITE’s Traditional Neighborhood Develop- ment Street Design Guidelines suggest that ‘While TND street networks do not follow the same rigid functional classification of conventional neigh- borhoods with local, collector, arterial, and other streets, TND streets are hierarchical to facilitate necessary movements.’31

The Essex Design Guide is confusing in a slightly different way. It expli- citly rejects having a ‘hierarchy of road types’, but this appears to be contradicted by the depiction and listing of road types which appear to be controlled in a way indistinguishable from conventional road hierarchy32

(Figure 2.11). There is surely something going on here. There is evidently something

‘good’ about a hierarchy of different kinds of street that may be desirable from the urban designers’ and planners’ point of view. Yet this is not clearly articulated in the examples discussed. Nevertheless, the urban designers seem fairly sure that it is not the conventional engineers’ hierarchy that is desired. What is needed is, in effect, a clearer deconstruction of hierarchy and how this relates street type to street pattern (Chapter 7).

35THE CHALLENGE

2.11 • Diagram demonstrating layout of three types of route (redrawn from the current Essex Design Guide).

Hierarchy versus pattern Hierarchy and pattern are often confused: the terms are often used ambigu- ously and apparently interchangeably. Table 2.3 gives some examples of the mix of uses of the terms.

In effect, the term ‘hierarchical’ is commonly used to denote a tribu- tary (‘loop and cul-de-sac’) layout, in contradistinction to traditional grid layouts, even though grid layouts may also be hierarchical in nature, either by design or emergence. In other words, the adoption of a particular layout (e.g. grid) does not preclude the retention of a particular kind of hierarchy. Indeed, the kind of ‘integrated grid network’ envisaged by advocates of neo-traditional urbanism may well have greater differentiation of route types than conventional ‘hierarchical’ road networks (Chapter 6).

Design guidance may inadvertently reinforce the confusion of the ver- bal descriptions by use of graphic depictions. The expression of a desired hierarchy may suggest a diagram depicting a particular pattern. As Ray Brindle points out, conventional road classifications typically ‘depict roads as forming a tributary (i.e. tree) system, each road picking up traffic from less important roads and channelling it to more important roads’.33 While this is indeed often the case, the depicted pattern does not necessarily follow from the underlying rules of hierarchy (Figure 2.12).

This gives us an inkling of the kind of richness of possibility that we need not lose: we can have layouts that are both ‘hierarchical’ and ‘grids’ – such as the case of the Craig Plan of Edinburgh (Figure 2.13)

Part of the challenge of this book, therefore, is to sort out the different meanings and implications of different kinds of tree or grid patterns and different kinds of hierarchy, ‘tree-like’ or otherwise (Chapter 7).

Preferred versus discouraged patterns The ongoing debate on street pattern often boils down to a simple polarity between ‘grid’ network forms versus ‘tributary’ (or ‘loop and cul-de-sac’) forms. But justification of these preferences is a complex and to some extent contradictory issue.

1. What kind of pattern? A pattern of streets may comprise any or all of: a pattern of roads, a pattern of paths, and a pattern of blocks or plots of land. While in some cases these may be coincident, in other cases there may be different patterns serving different purposes (Figure 2.14).34

2. Clarity, identity and legibility. Conventional tributary (loop and cul-de-sac) networks may be criticised on the basis that they lack a ‘clear structure and identity’. However, a grid is not necessarily ‘clearer’ or more ‘structured’ than a tree pattern. Some research results suggest that grids may have fewer

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36 STREETS & PATTERNS

Table 2.3 Contradistinctions of hierarchy and pattern

Conventional Neo-traditional (problem) (solution)

‘hierarchy of roads’ ‘network of spaces’

‘hierarchical street ‘reduced street networks’ hierarchy’

‘sparse hierarchy’ ‘dense network’

‘hierarchical street ‘highly connected networks’ gridded streets’

‘a highly differentiated ‘an integrated grid street hierarchy’ network’

Sources: Llewelyn-Davies (2000: 76); Kulash (1990); Kulash (in Ewing, 1996: 16); McNally and Ryan (1993); Banai (1996: 183)

37THE CHALLENGE

2.12 • Hierarchical layout prescription. A descending hierarchy of route types (1, 2, 3 etc.) is here depicted in a tributary pattern of loops and culs-de-sac. But the ‘permissible types of access’ do not themselves preclude more grid-like configurations.

2.14 • Superimposed patterns. The pedestrian network is a grid but the vehicular network is discontinuous.

2.13 • Craig Plan, Edinburgh. This traditional street grid could meet the caricature of ‘good hierarchy’ admired by urban designers. It is a planned layout with a clear hierarchy of streets and lanes that connect together in a systematic manner. It is therefore ‘hierarchical’, but in a way unlike Figure 2.12.

‘identifiable’ districts than curvilinear suburban patterns. And the Urban Design Compendium promotes a ‘hierarchy of routes and places’ to assist the objective of legibility. Perhaps it is more the differentiation or regularity of pattern, rather than whether this is a grid or a tree, that is at stake.35

3. ‘Traditional’ versus ‘neo-traditional’. Although neo-traditional design may favour permeable street networks, it is not uncommon to see neo-traditional designs with liberal use of culs-de-sac. Indeed, Michael Southworth has demonstrated that some neo-traditional designs may resemble conventional ‘auto-oriented’ suburbs as much as traditional grid-based neighbourhoods, in terms of proportions of crossroads and culs-de-sac.36

4. ‘Concentration’ versus ‘dispersal’. One of the neo-traditionalists’ arguments in favour of ‘permeable’ street networks is that the route options offered by the grid ‘disperse’ traffic, ‘rather than forcing all traffic onto increasingly crowded collector and arterial streets’.37 However, these oversimplify the dynamics of traffic networks. Ray Brindle observes that ‘Recently there has been much talk of the “permeability” of networks which has, in part, generated mischievous theories about the ideal structure of local networks.’ He warns of the dangers of transport planners and traffic engineers being seduced by the rhetoric of neo-traditionalist planners with their preference for the grid.38

5. ‘Geometric’ versus ‘organic’. Finally, even the merits of the grid from an ‘urbanist’ point of view cannot be taken for granted, and have for long enough been subject to criticism from a variety of quarters. Camillo Sitte, perhaps one of the most famous pioneers of a ‘neo-traditional’ approach, was certainly against rigid engineering solutions. However, he also strongly argued against the use of crossroads, grid planning and monumental symmetrical layouts – three of the characteristic hallmarks of many of today’s New Urbanist designs. We could say that Sitte’s brand of neo- traditional design was ‘organic’ rather than ‘geometric’.

Ironically, the systematic purge of crossroads by late twentieth-century traffic engineers has often been supported by the use of a much propa- gated diagram, depicting the disproportionate number of crossing conflicts involved in crossroads layouts, which appears to have descended from Sitte (Figure 2.15).39

The last two cases (points 4 and 5 above) share another irony: in both cases the neo-traditionalist viewpoint is ostensibly arguing for better traffic circulation. Sitte was using the claim that crossroads impede vehicular flow to help to back up his main argument, which was for more irregular street patterns (which would be based on T-junctions). The Urban Task Force is using the argument that tree-like patterns are supposedly inferior to grids – on the grounds of traffic flow and congestion – to support its (convincing enough) arguments against tree-like patterns for pedestrians and public transport.40

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38 STREETS & PATTERNS

2.15 • Camillo Sitte’s layout considerations. Sitte favoured T-junctions (a) over crossroads (b) primarily for urban reasons.

(a)

(b)

As a whole, the arguments presented above point to the lack of a consistent theoretical justification for ‘grids’ over ‘culs-de-sac’. Part of the reason for their inconsistency is that there is an underlying problem of in- adequate specification of the alternative patterns being debated. This illustrates the need to be explicit in description, in order to be clear about what aspects of structure we are arguing about. What would be useful, therefore, is the separate articulation of these kinds of structural issue. Better description can in turn be used to support whatever kind of justifi- cation is being made for different kinds of urban space or network pattern. In other words, clear prescription first requires adequate description of pattern (Chapters 4 and 6).

Pattern versus process Finally, there is the issue of pattern versus process. Neo-traditional design, which has been one the main stimuli behind this book, is to a significant extent associated with the desire to replicate traditional street patterns. As alluded to earlier, these neo-traditional patterns are not necessarily struc- turally faithful to actual traditional patterns.41 Moreover, these neo-traditional patterns are not necessarily created in a ‘traditional’ manner – in other words, how those original patterns themselves developed – but may be imposed in a rigid manner of formal design.

For example, the design of Poundbury was inspired by neo-traditionalist and, indeed, organic ideas. The master plan for Poundbury forms a kind of pseudo-organic pattern, and was partly aiming to ‘complement’ or emulate the historic core of Dorchester.42 However, on inspection, the layout appears not exactly similar to the centre of the town, which contains a grid of streets.

Moreover, the layout appears to be simply imposed on the existing site, oblivious to the existing pattern of fields and farms. The master plan for Poundbury – perhaps ironically for a development so closely associated with the traditionalist Prince of Wales – has boldly proposed a full build-out that would obliterate an existing Roman road that runs through the site. Where once there was a dead straight, historic route running out from the town, there would now be a ‘village’ of houses arranged around a street pattern of contrived irregularity. Despite its success in integrating architec- tural and highway forms internally, Poundbury is hardly an ‘organic’ extension of the town of Dorchester. It is a design conceived elsewhere, and parachuted down, as it were, from the drawing board.

Therefore, when considering what is an ‘organic’ or ‘geometric’ pattern, it is necessary to consider also process, and how different generative processes may give rise to different patterns (Chapter 9).

39THE CHALLENGE

CONCLUSIONS This chapter has demonstrated some of the current confusion and incon- sistency regarding the specification of desirable street types, patterns and hierarchies that influence the form and structure of urban layout. This effect- ively expresses the design debate as a research agenda for the remainder of the book (Box 2).

The first key issue is that of street type: there is no place in conven- tional road hierarchy for the traditional urban street. Then, there is no clear or consistent basis for reconciling the street of the current urban design agenda with the ‘link’ in conventional transport analysis, or road type in conventional hierarchy. The basis for conventional hierarchy itself seems somewhat unclear – whether regarded as ‘good’ or ‘bad’ hierarchy by engin- eers or urban designers.

We have also seen that there is a lack of clear and consistent guid- ance on the overall form that a street pattern should take. On the one hand, highway engineering approaches are criticised for being too rigid, and yet these do not seem to prescribe any particular pattern; on the other hand, there is a rather diverse range of urban design and planning aspirations, which are either expressed too unclearly – for the purposes of description – or perhaps too specifically, for the purposes of prescription. For those approaches departing from conventional engineering principles, it is some- times difficult to grasp what kind of theory might be underlying the rhetoric. Where a certain kind of desired pattern is clearly graspable – the simple rectangular grid, for example – its theoretical justification is not at all clear or consistent.

In general, design guidance has to maintain a careful balance between being too nebulous and being over-prescriptive. Bill Hillier points out that the drawback of many normative theories of design is that they are over-

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40 STREETS & PATTERNS

BOX 2. KEY ISSUES FOR RESOLUTION 1. The basis for street type. 2. The connection between street type and hierarchy. 3. The identification and justification of ‘good’ and ‘bad’ hierarchy. 4. The distinction between hierarchy and pattern. 5. The identification of ‘preferred’ and ‘discouraged’ patterns. 6. The relationship between pattern and process of generation.

specific where they should be permissive, and vague where they should be precise.43

In the case of street pattern design guidance, the problem is partly caused by the fact that patterns are difficult to describe verbally. As we have seen, a verbal description typically runs the risk of being too vague, whereas an illustrated pattern may appear too specific, too limiting. The result tends to be a polarisation – between grids and culs-de-sac – which is unable to handle the possibility of alternatives in between. Without descriptors of urban structure of sufficiently high resolution, the debate over street pattern is likely to remain entrenched as a battle between the extrema of endlessly permeable grids and terminally tributary culs-de-sac.

Effectively, what we need is a better specification of street type, pattern and hierarchy in order to have a firmer and more consistent basis upon which to suggest ‘preferred’ options. The issue of street type will be investi- gated in Chapters 3 and 5, while the characterisation of patterns will be investigated in Chapters 4 and 6. Ultimately, these will be drawn together in a single framework in Chapter 7, from which recommendations for onward design processes will subsequently flow.

Overall, this chapter has to some extent ‘softened up’ what might before have been perceived as firm, clear-cut matters of unquestioned orthodoxy. We can now proceed to the main assault on the topic.

NOTES 1 DoE/DoT (1992); DETR (1998a). 2 On reclaiming the street for people, see, for example, Jacobs (1961), Rudofsky

(1969), Appleyard (1981), Moudon (1987), Engwicht (1993, 1999), Chorlton (2003), Hazel (2003), Rook (2003). On issues of inter-disciplinary territory, see also Scully (1994: 225); Cowan (1995, 1997); Campbell and Cowan (2002: 24). Dunnett also describes the street as a ‘battleground’ (2000: 78).

3 Llewelyn-Davies (2000). 4 Rykwert (1978) and Rook (2003) discuss the different meanings of ‘roads’ and

‘streets’. Streets may be analysed from a variety of historical, anthropological, sociological and cultural perspectives (Anderson, 1978; Moudon, 1987; Kostof, 1992; Fyfe, 1998; Gehl, 1998); from urban design perspectives (Moughtin, 1992; Jacobs, 1993; Southworth and Ben-Joseph, 1997; Jacobs et al., 2002) and from the perspective of sustainability (Jefferson et al., 2001).

5 ICE (1996: 8). 6 Marshall (2002a). 7 ICE (1994: 22). For further discussion of street classification from the point of

view of road classification, see Morrison (1966); Jones (1986); ICE (1996); AASHTO (2001); Bartlett (2003a, 2003b).

41THE CHALLENGE

8 Some would stress the importance of this classification by function, e.g. Tripp (1950); ICE (1996: 1, 2, 9, 11); AASHTO (2001: 1, 4, 13). See Chapter 3.

9 Llewelyn-Davies (2000: 75). 10 ICE (2002: 11). 11 Calthorpe (1993: 99). 12 Lang (1994: 201). 13 Southworth and Ben-Joseph (1997). 14 O’Flaherty (1986); Banks (1998: 286) and Bell and Iida (1997) refer to patterns

but not in a prescriptive sense. Explicit reference to or guidance on pattern is also absent in texts by Oglesby and Hicks (1982); Macpherson (1993); DoE (1994); Wright (1996). These comments are intended as a demonstration of the extent to which the specific issue of pattern guidance is absent from mainstream transport and highway engineering literature; it is not otherwise a criticism of these particular works, which serve their own purposes satisfactorily. (In any case, see Chapter 9.)

15 IHT (1997); AASHTO (1995, 2001). 16 DB32: DoE/DoT (1992). This guide does give some graphic examples of street

grids – including several incorporating crossroads – but with the caveat that they are not necessarily supposed to be interpreted literally. ‘Plethora of standardised layouts’ – from Local Transport Today (LTT, 24 September 1998). The lack of imagination is suggested by the need for the creation of Places, Streets and Movement (DETR, 1998a), which principally contains guidance on creative inter- pretation of the existing layout rules, rather than being a new set of rules.

17 Crane (1996); ITE (1999: 6). 18 ITE (1999: 6). The only street layouts shown are referring to identity of streets

and use of T-intersections, not to actual pattern. 19 Aldous (1992: 17). 20 Lynch (1990: 93–94). 21 Keeble (1983: 25). 22 DETR (2000: 26); Urban Task Force (1999: 90). 23 Similar kinds of ‘preferred’ and ‘discouraged’ examples are found in Bentley et

al. (1985); Katz (1994); Leccese and McCormick (2000), Carmona et al. (2002: 31).

24 Burdett (1998) has drawn attention to the fact that previously these roles were reversed, juxtaposing two plans explaining the Abercrombie plan for London: one showing the problematic ‘jumble’ of the old street pattern, the other the orderly solution of Modernism (Carter and Goldfinger, 1945: 72–73). Burdett interprets this juxtaposition as the fearless assertion of ‘wrong’ and ‘right’. Today, of course, these labels would be reversed. See also reference to ‘bad’ gridirons and ‘good’ loop and cul-de-sac systems recommended by the US Federal Housing Administration (Southworth and Ben-Joseph, 1997: 85).

25 This is echoed by Cowan (2002: 21) of design guidance in general. 26 Essex Planning Officers’ Association (1997: 12); DETR (1998a: 30).

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42 STREETS & PATTERNS

27 DTLR and CABE (2001); Urban Task Force (1999). 28 For example, The Urban Design Compendium suggests that the UK street clas-

sification of distributors and access roads is based ‘solely on vehicle capacity’ (Llewelyn-Davies, 2000: 75; also Thorne and Filmer-Sankey, 2003: 29).

29 Goodwin (1995: 7); Brindle (1996: 69). 30 Ross (1997: 23); Western Australia (1997: 20); DTLR and CABE (2001: 25). 31 ITE (1999: 21). 32 Essex Planning Officers’ Association (1997: 55). 33 Brindle (1996: 55). 34 This kind of arrangement has also been suggested by the Essex Design Guide

(Essex Planning Officers’ Association, 1997: 11) and Southworth and Ben- Joseph (1997: 126).

35 Southworth and Ben-Joseph (1997: 122) refer to the lack of legibility of cul-de- sac layouts but also point out that grid patterns may be monotonous. Research results on lack of identity of grids are reported by Rapoport (1977). The Urban Design Compendium suggests that permeability assists the urban design object- ive of circulation, while hierarchy assists the urban design objective of legibility (Llewelyn-Davies, 2000).

36 Southworth (1997); Southworth and Ben-Joseph (1997: 105–107); Southworth (2003).

37 Rosenkrantz and Abraham (1995). Similar arguments are proffered by Morris and Kaufman (1998), the Urban Task Force (1999) and DTLR and CABE (2001: 25).

38 Brindle (1995: 9.8). 39 In presenting his diagrams, Sitte remarks that the issue of conflicting paths at

crossroads ‘seems to have been overlooked heretofore’ (Sitte, [1898] 1945: 60). Similar ‘conflict’ diagrams recur in traffic and planning literature, for example, Ritter (1964: 79); MoT (1966: 57); Alexander et al. (1977: 264); Keeble (1983: 43); McCluskey (1992: 62); Leleur (1995: 38); Behrens and Watson (1996: 127); O’Flaherty (1997: 453); Ben-Joseph and Gordon (2000: 250); Bird (2001: 401); also alluded to by Campbell and Cowan (2002: 40).

40 The Urban Task Force claims that ‘tree-like networks’ are unfavourable to cars, since they ‘concentrate congestion’ (1999: 91). This begs the question as to why such layouts – by this logic disadvantageous to all transport modes – were ever built. In fact, the same layouts are elsewhere criticised for being car- oriented. The technical issues of safety of crossroads and congestion versus dispersion themselves lie outside the scope of this book.

41 Also Chapter 2, following from the work of Michael Southworth (2003). 42 Thompson-Fawcett (2000); DETR (1998a); Krier (1993a; 1993b). 43 Hillier (1996: 67).

43THE CHALLENGE