Evolution of the CBD
As the geographical concentration of commercial activity in the city, various CBD forms have been recognizable since at least the end of the eighteenth century. However, the manner in which the CBD functions is medieval as the linkages within the core area often need to be accessible by foot, which places an upper limit on the distance between functions that benefit from being linked together. This is particularly the case in smaller low rise urban centers like San Jose where city officials only need to walk one block to purchase spare car parts. However, in cities such as New York and London commercial pressure on space over time eventually led to the exclusion of (most) other central business functions exemplified by emergence of global financial districts in Lower Manhattan and Canary Wharf, respectively. Research in smaller US cities including Philadelphia show that, as long as this pressure remains constant then the evolution of the CBD will always be in flux rather than some static element on the urban landscape. While scholars support this general thesis, there is less agreement about the precise sequence of driving forces behind CBD evolution. In European cities like Utrecht the CBD evolved from the Industrial Revolution, whereas in Latin American and African cities rural to urban migration was the most important factor. To expose this lacuna the literature has focused on a number of trophy case studies to launch more general ideas about CBD evolution.
The Case of Boston
While scholars rightly continue to identify various local factors as driving forces behind CBD evolution, the case of Boston contributes something much more significant to our understanding of the process. It is the evolution of Boston's CBD from 1840 to 1920 that established a connection between the ways in which the CBD evolves internally 'and' expands physically. The evolution of Boston's CBD also fits with the various urban land rent models which suggest that those land uses least able to pay the highest rent will be relegated toward the edge of the city in zones of discard (discussed below).
The evolution of Boston's CBD from 1840 to 1920 is based around the emergence of 'modern' functional areas that sprouted from three specialized nuclei (Figure 2).
In 1840 food markets, warehouse, and financial functions were largely restricted to central locations to gain access to Boston's large European immigrant population. By 1880 Boston's commercial activities had expanded to meet the demands of regional distribution that in turn led to the expansion of business premises. As a consequence many central residential areas suffered the social disorganization created by the problems of eviction and residential relocation. By 1920 Boston's CBD had split into more specialized functions than had initially spurted its growth. Finance and administration coalesced and expanded their accommodations into premium locations within the existing limits of the CBD, rather than at the expense of peripheral residential locations. By contrast the food industry reorganized their facilities by decentralizing into adjacent residential areas based upon the differing threshold populations required to support different types of retail function. This is because on the supply side different retail functions have different conditions of entry (thresholds), and thus demand minimum trade areas for their support. On the demand side, consumers spend different proportions of their income on different goods and services and purchase them with differing degrees of frequency. This produces the hierarchy of retail functions found in most American and European cities.
A CBD in Seven Stages
Other types of approaches are more schematic in explaining CBD evolution. A number of generic evolutionary stages are proposed to account for the ways in which the CBD can be characterized by various moments of evolution ranging from 'inception' to the 'city of realms'. While there can be overlap between any of the individual stages, schematic approaches assume that there is a 'normal' trajectory that CBD evolution will follow. North American and Canadian cities are thought to be shaped (to different degrees of intensity) by the forces outlined below ad seriatim:
The original setting of a town or city contains pervasive influences on the future of the city and its citizens. Canadian settlement frontiers were linked to the building of railroads that nurtured the growth of prairie towns that grew through phases of urban redevelopment characterized by various inceptions.
Once a town has been founded, growth occurs through the mechanism of the rent gradient (discussed below) that delimits the CBD from its surrounding area. This is evident in US cities such as Chicago where the CBD is the first of five zones radiating outward to form distinct concentric circles. The 'Chicago model' has greatly enhanced our understanding of how a CBD evolves by demonstrating that certain activities are 'pushed' outward due to the limited amount of central land coupled with the rent paying abilities of particular functions. Retailing is often used as a primary indicator of exclusion as higher order services including department stores can afford a central location while smaller shops and neighborhood centers cannot. Exclusion also extends to non-CBD uses such as residential, industrial, wholesale, and vacant land that characterize the zone of discard. The spatial patterns associated with exclusion are directly related to the physical direction of CBD growth along the zone of assimilation.
While exclusion drives certain uses 'out' of the CBD, segregation drives certain land uses apart 'within' the CBD. As the clustering of activities grows in the CBD, the likelihood of functional segregation increases because greater amounts of one type of activity tend to ultimately exclude other CBD functions. New York's financial district in lower Manhatttan and Vienna's Museum Quarter are cases in point. This functional segregation is a result of previous land uses becoming so self contained that they lack any significant external linkages or economies of scale.
As the CBD continues to evolve, distinctions are made between different types of extensions. In the nineteenth century the typical CBD was a compact unit at the focus of crosscutting routes and the typical residential area was a 'bead on a string' shaped by suburban railway lines. After World War I, CBDs generally began to grow outward in thin alignments along arterial streets in the zone of assimilation, although cities like Chicago arguably extended into its hinterland as concentric circles. However, it is not the physical direction of the extension from the CBD but rather, the potential trade losses to outlying locations that have come to dominate the literature.
Replication and readjustment
Replication and the dynamic of adjustment are major concerns for the economy of the CBD. This is part of a more general trend of decentralization whereby the CBD experiences a trade deficit due to the rise of comparative services in suburbia and other outlying areas. The rise of regional shopping malls in the UK and technopoles in the US are indicators of this decentralization. The realization that business and trade can be successfully located outside the CBD is not just a consequence of limited central land supply, but also due to greater individual mobility and wider consumer choices. After World War II, increasing decentralization and capital flight suggested that CBDs lacked the relative urban weight that they once had. The failure of various attempts to reshuffle the hierarchy back in favor of the CBD prompted a number of critical scholars to raise concerns about an 'urban crisis' or the 'crisis of contemporary urbanity'. This doom and gloom thesis became particularly influential in the 1960s and 1970s when the slowdown of manufacturing and heavy industry gave rise to a number of subsequent problems including high levels of unemployment and inflation.
As a result of decades of urban decay in the CBD, redevelopment and physical planning have taken on more teleological roles. At first these redevelopment initiatives attempted to stamp out urban decay only to realize that social problems cannot be solved by physical planning solutions alone. Since the 1980s redevelopment of the CBD has shifted to improving urban livability for conditionally more prosperous residents. This is part of a widespread willingness of municipal authorities to accept change 'only if ' it is 'for the better'. Thus, inner city slums are not replaced with similar low income housing but rather upmarket postmodern flats bankrolled by the private sector under the aegis of the neoliberal local state. The selectivity associated with the redevelopment agenda remains one of the most debated areas in human geography and right across the social sciences.
A city of realms
Despite continuing decentralization, the final stage in the evolution suggests that the CBD will remain the location of highly specialized functions. While there is general acceptance that some activities have moved out of the CBD, there is no universal agreement that this decentralization spells euthanasia for the CBD. This optimism is partly related to the purported success of various forms of civic boosterism, the exploitations of location specific assets, and cultural regeneration programs collectively referred to as 'selling places'. Appellations including the European City of Culture and UK City of Architecture of Design and events like the Olympic Games reinforce the symbolic importance of the CBD as an integral component of urban civic consciousness. The ethos behind such initiatives is to (re)bolster the economic and cultural values of the CBD as the prime location for business and social life.