Urban Archetypes, Network Logics and Path Dependence The previous evolutionary discussion has implied that urban growth has been a

continuous process towards decentralised cities. Of course the reality is more complex in terms of the historical continuity of urban spatial structure, and the diversity of how particular cities and regions have developed. This discussion focuses on two particular concepts related to how cities resist dramatic physical change. These linked concepts are transport network logics and path

dependence.

In the study of urban geography it is a truism that „history matters‟- that continuity and persistence are key characteristics of urban development (Batty, 2001b). A city that rose to prominence in the 18^th or 19^th centuries will be significantly different in its contemporary form to a city where the majority of growth occurred in the mid-twentieth century. Such a phenomenon has been termed path dependence (Arthur et al., 1987), where a path dependent system is one in which the initial conditions play a key role in determining future

structure. It is important to understand why such continuity is found in urban systems, particularly when urban economies are dynamic and constantly transforming¹. Urban path dependence is related to several characteristics of

1Note that we refer mainly to the path dependence in terms of urban physical structure. Urban function is generally more dynamic, shifting with changing economic systems. Contemporary cities are full of examples of buildings transformed from their original function, from centrally located residential districts used as offices, to inner-city warehouses converted to studio flats. On the other hand, there are also many examples of continuity in function too, particularly at the urban core of monocentric cities.

urban systems, including the durable fixed-capital nature of the built

infrastructure; and socio-cultural factors relating to urban practices and identity.

Buildings and transport infrastructure generally exist for the long term. They are significant capital investments in terms of materials and construction, and their durability means that value is retained over time. Typically it is cheaper to reuse and renovate than to rebuild. The value of property is both in the physical building and in the value of the location and urban area. Value becomes highly interdependent between properties, with urban streets and districts becoming massive collective investments of capital. The urban street network itself is argued to be the most consistent urban feature across time (Conzen, 1960), and certainly London confirms this view with the routes of many medieval roads and even some Roman roads remaining to the present day. That is not to say large scale physical urban reconstruction is impossible (examples exist from Haussmann‟s Paris to British post-war urban development) but is expensive and requires significant political willpower. Resistance to change is also bound up in socio-cultural and political processes. Cities are living environments, with collective civic identity attached to the built-environment and public space, from major symbolic civic spaces, to local residential communities. Thus social capital can also resist changes in spatial structure.

Urban transportation networks are closely connected to these processes of path dependence. This is due to the infrastructure costs of the transportation

networks themselves, and to the central link between transportation, the built-environment and the functioning of cities. Furthermore different transportation modes are not necessarily compatible with each other; they display different network logics in the accessibility they provide and the complementary built-environment forms they encourage. Dimensions of various motorised transport modes, such as speed, capacity and flexibility, are summarised in very basic form in Table 1.1. Rail transport can carry very high densities of passengers at high speeds, whilst being inflexible, and focussed around major hubs, as well as requiring large initial capital outlays for development. Private automobile transport can also provide high speed travel, but contrasts with rail in having much greater flexibility and carrying lower capacities of passengers. The total

costs of private transport are similar to public transport (and often higher when environmental factors are considered), yet the initial infrastructure costs for conventional roads are lower, as the costs of purchasing and running vehicles are paid for by car owners. High speed restricted access road networks (i.e.

motorways) on the other hand are similar to rail networks in requiring very large capital outlays.

Table 1.1: Characteristics of Motorised Transport Modes.

Source: World Bank Reports, cited in Tolley and Turton, (1995).

Mode of transport Maximum Capacity (persons per hour)

Average speed (km per hour)

Interval between access points (km)

Urban Railways 50000-60000 32-48 1.6

Light Rapid Transport System 40000 26-38 0.5-1.3

Bus on conventional road

network 9000-10000 16-24 0.2-0.5

Bus using reserved lane on

express highways 20000 56 0.8-1.6

Private car on conventional

road* 1000 19-40 -

Private car on urban

motorway network* 3000 72-80 -

*Assuming vehicle occupancy of 1.5.

As a result of these various transport mode characteristics, the dominance of a particular transport system encourages complementary built-environment development, pulling urban development towards certain spatial structures.

Thomson (1977) proposed urban archetypes that fully embrace the complementary spatial structure for particular transport modes. For rail

dominated cities, radial structures are very common as these link all sections of a city together whilst minimising investment in expensive rail infrastructure (Figure 1.7.i). This is the monocentric form, providing high accessibility to and from the city centre, whilst orbital accessibility is poor. This limits the

competitiveness of sub-centres as commercial locations. Radial networks are closely tied to the form of monocentric cities with a single commercial core. In complete contrast to the monocentric archetype, the full motorisation archetype has a very high degree of mobility and flexibility in travel patterns, with high accessibility across the entire city. The flexibility and low capacity (with resultant congestion) of private transport discourages a single dominant centre

network grid. Travel patterns occur from multiple origins to multiple destinations using multiple routes, i.e. a complete contrast to the constrained radial journeys in the strong centre archetype. This structure is effectively modular and can expand more easily than the monocentric archetype. Road infrastructure and land costs are high, and congestion is likely to persist due to the high spatial inefficiency of the automobile.

Figure 1.7: Urban Transport Archetypes. Source: Thomson (1977).

Cities are of course in reality hybrids of these archetypes. Even highly automobile orientated cities such as Los Angeles have a downtown, whilst public transport orientated European capitals such as Paris and London have car-dominated peripheries. Nevertheless the periods in which these cities experienced their most rapid growth remain highly influential on their current structure, and attempts to develop cities serving as both public transport and private car archetypes are expensive and face significant hurdles in overcoming legacies in urban spatial structure.

1.1.6 Summary

The economic and social success of cities depends on their ability to enable contact and communication, and transportation is therefore a key technology for cities to function. Cycles in the development of capitalism have produced techno-economic paradigms that define urban functions and transportation technologies in different eras. There are close links between the types of accessibility that particular transport modes provide and urban form. A critical

change has occurred between the industrial public transport era that is

associated with monocentric structures, and the decentralised forms enabled by the speed and flexibility of the automobile.

There is a critical tension between the dynamism of urban economies and the continuity of urban structure through the phenomenon of path dependence.

Harvey (2001) expresses this tension between the dynamics of capital, constantly seeking new markets and technological change to increase profits, and the fixed infrastructure of capitalism (of which cities are the prime

example) which are essential for capitalism to function. The continuity of urban spatial structure depends on the ability of cities to be flexible and adapt to new economic phases of capitalism, and to associated urban cultures and lifestyles.