Defining Urban Systems: Land Use Transportation Interaction In the context of cities, many of the processes discussed in Chapter 1 can be

approached from a systems perspective, from transportation flows, to urban development and population change. Interactions between urban land use and transportation systems are the focus of this research, and are discussed here. The issue of inter-relationships between systems is particularly relevant in an urban context. Cities integrate many systems at varied spatial scales from local to global; and varied temporal scales from day to day processes, to those operating over decades. Table 2.1 lists general urban processes by temporal scale. Slow gradual processes relate to the techno-economic paradigms discussed in Chapter 1, including structural change in the global economy, demographic changes and

communication (which can take seconds in the digital world) and the daily cycles of urban travel (Wegener, 2004).

Table 2.1: Urban Spatial Processes and Temporal Scale. Adapted from Wegener (2004).

Fast Processes

The processes influencing the spatial structure of cities in terms of land use and urban function are generally medium term temporal interactions, principally including the location choices of firms and residents, and cycles of urban development. These are in turn linked to both more rapid dynamic processes, such as travel patterns (which influence accessibility), and to slower urban processes, such as economic change (which influence the dominant forms of economic production in different urban eras). The challenge therefore in the study of urban systems is to incorporate the many inter-relationships between systems at different temporal and spatial scales, whilst bearing in mind the impossibility of „modelling everything‟ and the need for analysis that is comprehensible for research and planning practice.

The earliest urban applications of systems theory were in the transportation field. Classic models based on the four stage transportation model (Figure 2.1) essentially worked by assuming land use (and medium-slow urban dynamic processes) to be fixed. Based on the existing distribution of population and employment in an urban area, transport trips could be generated, then distributed and assigned to transport modes and networks. Transport cost restrictions in terms of congestion operate by feedback mechanisms from the assignment stage to the trip generation and distribution stages.

Trip Generation

Trip Distribution

Modal Split

Traffic Assignment

Figure 2.1: The Four Stage Transportation Model

The four stage transportation approach has proved to be very effective in predicting transportation patterns over relatively short time scales. In the simple form presented above it does not represent the behavioural aspects of travel patterns in any detail, and does not tackle the medium to long term processes of urban dynamics. As explored in Chapter 1, urban spatial structure is created through the co-evolution of land use and transport, and an understanding of urban form needs to be based on feedback cycles between land use and transportation (Wegener, 1994), as illustrated in Figure 2.2.

Activities

Transport Land Use

Accessibility

Figure 2.2: The Land Use Transportation Feedback Cycle. Source: Wegener (1994).

Thus for example, the development of new transport infrastructure such as a motorway will change accessibility and consequently land use patterns, as urban development is attracted to areas with improved accessibility (Hansen, 1959).

Conversely changes in land use such as the development of a major new office

will affect activities and transport flows, changing accessibility through

providing new employment opportunities and potentially increasing congestion.

The feedback cycle presented in Figure 2.2 is highly generalised, and can be expanded to consider in more detail the processes of urban development and land use. In Figure 2.3 the land use transportation feedback relationship is presented as a resolution of supply and demand for real-estate through property markets. In this system the concept of accessibility links between travel network flows (the output of the four stage transportation model) and the location decisions of firms and residents. Physical built-environment change through urban development is a connected process incentivised by rental profit. Urban planning also plays a variable role in land use decisions depending on the degree of state intervention, through policies such as planning permissions and zoning, and the development of transportation and public service infrastructure.

Economy

Buildings Accessibility Planning

Transport

Urban Development

Figure 2.3: Model of Urban Land Use and Development

Urban land use and spatial structure is a major focus of this research and it is worth exploring the assumptions of the system model in Figure 2.3 in more detail. It essentially assumes that the urban economy and population is determined externally or exogenously to the urban land use process. In many senses this is true, as processes of global economic change, trade flows, migration flows, business cycles and so forth strongly affect urban

socio-economic processes and are independent from the spatial structure of cities.

However the urban built-environment and socio-economic structures co-evolve together over long periods of time, and in a successful urban environment there is a close correspondence between them. The lack of correspondence between populations and the built-environment, in situations such as overheating property markets and urban decay, represents urban dysfunction. A feedback mechanism from land use to socio-economic geography is depicted in Figure 2.3, indicating how the ability of a city to facilitate the living and working demands of residential and commercial activities affects future development.

A key issue in the study of urban systems is the degree of disaggregation considered in the analysis of urban actors. Figure 2.3 implies a single system and property market for all types of households and all kinds of firms, which is clearly a gross simplification. In addition to the highly varying requirements of firms compared to residents, many further levels of disaggregation are possible, for instance families with dependent children will have very different locational priorities compared to single households, as will manufacturing firms compared to a business services firms. Urban actors can also be disaggregated along wealth lines, as their ability to pay for property has a strong influence on the location choices available. State intervention typically offsets market forces to a degree, through providing state housing, benefits and rental policies.