Author: Dr. Jean-Paul Rodrigue
1. The Land Use – Transport System
Urban land use comprises two elements; the nature of land use which relates to which activities are taking place where, and the level of spatial accumulation, which indicates their intensity and concentration. Central areas have a high level of spatial accumulation and corresponding land uses, such as retail, while peripheral areas have lower levels of accumulation. Most economic, social or cultural activities imply a multitude of functions, such as production, consumption and distribution. The urban land use is a highly heterogeneous space and this heterogeneity is in part shaped by the transport system.
Socioeconomic functions take place at specific locations and are part of an activity system. Some are routine activities, because they occur regularly and are thus predictable, such as commuting and shopping. Others are institutional activities that tend to be irregular, and are shaped by lifestyle (e.g. sports and leisure) or by special needs (e.g. healthcare). Others are production activities that are related to manufacturing and distribution, whose linkages may be local, regional or global.
The behavioral patterns of individuals, institutions and firms have an imprint on land use in terms of their locational choice. The representation of this imprint requires a typology of land use, which can be formal or functional:
Formal land use representations are concerned with qualitative attributes of space such as its form, pattern and aspect and are descriptive in nature.
Functional land use representations are concerned with the economic nature of activities such as production, consumption, residence, and transport, and are mainly a socioeconomic description of space.
At the global level, cities consume about 3% of the total land mass. Although figures can vary considerably depending on the city, residential land use is the most common, occupying between 65 and 75% of the surface of a city. Commercial and industrial land uses occupy 5-15% and 15-25% of the surface respectively. Land use, both in formal and functional representations, implies a set of relationships with other land uses. For instance, commercial land use involves relationships with its supplier and customers. While relationships with suppliers will dominantly be related with movements of freight, relationships with customers would include movements of people. Thus, a level of accessibility to both systems of circulation must be present. Since each type of land use has its own specific mobility requirements, transportation is a factor of activity location, and is therefore associated intimately with land use.
Within an urban system each activity occupies a suitable, but not necessarily optimal location, from which it derives rent. Transportation and land use interactions mostly consider the retroactive relationships between activities, which are land use related, and accessibility, which is transportation related. These relationships have often been described as a classic “chicken-and-egg” problem since it is difficult to identify the triggering cause of change; do transportation changes precede land use changes or vice-versa? There is a scale effect at play as large infrastructure projects tend to precede and trigger land use changes while small scale transportation projects tend to complement the existing land use pattern. Further, the expansion of urban land uses takes place over various circumstances such as infilling (near the city center) or sprawl (far from the city center) and where transportation plays a different role in each case.
Urban transportation aims at supporting transport demands generated by the diversity of urban activities in a diversity of urban contexts. A key for understanding urban entities thus lies in the analysis of patterns and processes of the transport / land use system. This system is highly complex and involves several relationships between the transport system, spatial interactions and land use:
- Transport system. Considers the set of transport infrastructures and modes that support urban movements of passengers and freight. It generally expresses the level of accessibility.
- Spatial interactions. Consider the nature, extent, origins and destinations of the urban movements of passengers and freight. They take into consideration the attributes of the transport system as well as the land use factors that are generating and attracting movements.
- Land use. Considers the level of spatial accumulation of activities and their associated levels of mobility requirements. Land use is commonly linked with demographic and economic attributes.
A conundrum concerns the difficulties of linking a specific mode of transportation with specific land use patterns. While public transit systems tend to be associated with higher densities of residential and commercial activities and highways with lower densities, the multiplicity of modes available in urban areas, including freight distribution, conveys an unclear and complex relationship. Further, land use is commonly subject to zoning restrictions in terms of the type of activities that can be built as well as their density. Land use dynamics are therefore influenced by planning restrictions.
2. Urban Land Use Models
The relationships between transportation and land use are rich in theoretical representations that have contributed much to regional sciences. Since transportation is a distance-decay altering technology, spatial organization is assumed to be strongly influenced by the concepts of location and distance. Several descriptive and analytical models of urban land use have been developed over time, with increased levels of complexity. All involve some consideration of transport in the explanations of urban land use structures. The following is a non-exhaustive categorization:
- Central places and concentric land uses. Von Thunen’s regional land use model is the oldest representation based on a central place, the market town, and its concentric impacts on surrounding land uses. It was initially developed in the early 19th century (1826) for the analysis of agricultural land use patterns in Germany. It used the concept of economic rent to explain a spatial organization where different agricultural activities are competing for the usage of land. The underlying principles of this model have been the foundation of many others where economic considerations, namely land rent and distance-decay, are incorporated. The core assumption of the model is that agricultural land use is patterned in the form of concentric circles around a market that consumes all the surplus production, which must be transported. Many concordances of this model with reality have been found, notably in North America.
- Concentric urban land uses. The Burgess concentric model was among the first attempts to investigate spatial patterns at the urban level (1925). Although the purpose of the model was to analyze social classes, it recognized that transportation and mobility were important factors behind the spatial organization of urban areas. The formal land use representation of this model is derived from commuting distance from the central business district, creating concentric circles. Each circle represents a specific socioeconomic urban landscape. This model is conceptually a direct adaptation of the Von Thunen’s model to urban land use since it deals with a concentric representation. Even close to one century after the concentric urban model was designed, spatial changes in Chicago are still reflective of such a process.
- Polycentric and zonal land uses. Sector and multiple nuclei land use models were developed to take into account numerous factors overlooked by concentric models, namely the influence of transport axis (Hoyt, 1939) and multiple nuclei (Harris and Ullman, 1945) on land use and growth. Both representations consider the emerging impacts of motorization on the urban spatial structure. Such representations also considered that transportation infrastructures, particularly terminals such as rail stations or ports, occupy specific locations and can be considered as land uses.
- Hybrid land uses. Hybrid models are an attempt to include the concentric, sector and nuclei behavior of different processes in explaining urban land use. They try to integrate the strengths of each approach since none of these appear to provide a completely satisfactory explanation. Thus, hybrid models, such as that developed by Isard (1955), consider the concentric effect of central locations (CBDs and sub-centers) and the radial effect of transport axis, all overlaid to form a land use pattern. Also, hybrid representations are suitable to explain the evolution of the urban spatial structure as they combine different spatial impacts of transportation on urban land use, let them be concentric or radial, and this at different points in time.
- Land use market. Land rent theory was also developed to explain land use as a market where different urban activities are competing for land usage at a location. It is strongly based in the market principle of spatial competition where actors are bidding to secure and maintain their presence at a specific location. The more desirable a location is, the higher its rent value. Transportation, through accessibility and distance-decay, is a strong explanatory factor on the land rent and its impacts on land use. However, conventional representations of land rent leaning on the concentric paradigm are being challenged by structural modifications of contemporary cities.
- Cellular automata are dynamic land use models developed on the principle that space can be represented as a grid where each cell is a discrete land use unit. Cell states thus symbolize land uses and transition rules express the likelihood of a change from one land use state to another. Because cells are symbolically connected and interrelated (e.g. adjacency), models can be used to investigate the dynamics, evolution, and self-organization of cellar automata land use systems. The cellular approach enables to achieve a high level of spatial detail (resolution) and realism, as well as to link the simulation directly to visible outcomes on the regional spatial structure. They are also readily implementable since Geographic Information Systems are designed to work effectively with grid-based spatial representations. Cellular automata improves upon most transportation – land use models that are essentially static as they explain land use patterns, but they do not explicitly consider the processes that are creating or changing them.
The applicability and dynamics of land use models is related to issues such as the age, size and the locational setting of a city. For instance, concentric cities are generally older and of smaller size, while polycentric cities are larger and relate to urban development processes that took place more recently. While most of the conceptual approaches related to transportation and land use relationships have been developed using empirical evidence related to North America and Western Europe, this perspective does not necessarily apply to other parts of the world. A dualism in land uses has been observed in cities in developing countries where on one hand processes such as economic development and motorization are creating an urban landscape which is common in advanced economies. On the other hand, an informal landscape of shantytowns represents a land use structure that is not effectively captured by conventional land use models. It remains to be seen to what extent globalization will favor a convergence of land use patterns across the world’s cities.
3. Transportation and Urban Dynamics
Both land use and transportation are part of a dynamic system that is subject to external influences. Each component of the system is constantly evolving due to changes in technology, policy, economics, demographics and even culture or values. As a result, the interactions between land use and transportation are played out as the outcome of the many decisions made by residents, businesses and governments. The field of urban dynamics has expanded the scope of conventional land use models, which tended to be descriptive, by trying to consider relationships behind the evolution of the urban spatial structure. This has led to a complex modeling framework including a wide variety of components. Among the concepts supporting urban dynamics representations are retroactions, whereby one component changes it influences others. The changes will influence the initial component back, either positively or negatively. The most significant components of urban dynamics are:
- Land use. This is the most stable component of urban dynamics, as changes are likely to modify the land use structure over a rather long period of time. This comes as little surprise since most real estate is built to last at least several decades. The main impact of land use on urban dynamics is its function of a generator and attractor of movements.
- Transport network. This is also considered to be a rather stable component of urban dynamics, as transport infrastructures are built for the long term. This is particularly the case for large transport terminals and subway systems that can operate for a very long period of time. For instance, many railway stations are more than one hundred years old. The main contribution of the transport network to urban dynamics is the provision of accessibility. Changes in the transport network will impact accessibility and movements.
- Movements. The most dynamic component of the system since movements of passengers or freight reflect almost immediately changes. Movements thus tend more to be an outcome of urban dynamics than a factor shaping them.
- Employment and workplaces. They account for significant inducement effects over urban dynamics since many models often consider employment as an exogenous factor. This is specifically the case for employment that is categorized as basic, or export oriented, which is linked with specific economic sectors such as manufacturing. Commuting is a direct outcome of the number of jobs and the location of workplaces.
- Population and housing. They act as the generators of movements, because residential areas are the sources of commuting. Since there is a wide array of incomes, standards of living, preferences and ethnicity, this diversity is reflected in the urban spatial structure.
The issue about how to articulate these relations remains, particularly in the current context of interdependency between local, regional and global processes. Globalization has substantially blurred the relationships between transportation and land use as well as its dynamics. The main paradigm is concerned with some factors once endogenous to a regional setting have become exogenous. Consequently, many economic activities that provide employment and multiplying effects, such as manufacturing, are driven by forces that are global in scope and may have little to do with regional dynamics. For instance, capital investment could come from external sources and the bulk of the output could be bound to international markets. In such a context it would be difficult to explain urban development processes taking place in coastal Chinese cities, or in a region such as the Pearl River Delta, since export oriented strategies are among the most significant driving forces. Looking at the urban dynamics of such a system from an endogenous perspective would fail to capture driving forces that are dominantly exogenous.