1. Global Urbanization

No discussion about the urban spatial structure can take place without an overview of urbanization, which has been one of the dominant trends of economic and social change of the 20th century, especially in the developing world.

Urbanization. The process of transition from a rural to a more urban society. Statistically, urbanization reflects an increasing proportion of the population living in settlements defined as urban, primarily through net rural to urban migration. The level of urbanization is the percentage of the total population living in towns and cities while the rate of urbanization is the rate at which it grows (UNFPA, 2007).

This transition will go on well into the second half of the 21st century. Urban mobility problems have increased proportionally with urbanization, a trend reflected in the growing size of cities and in the increasing proportion of the urbanized population. Since 1950, the world's urban population has more than doubled, to reach nearly 3.16 billion in 2005, about 48.7% of the global population. This is due to two main demographic trends:

• Natural increase. It is simply the outcome of more births than deaths in urban areas, a direct function of the fertility rate as well as the quality of healthcare systems.
• Rural to urban migrations. This has been a strong factor of urbanization, particularly in the developing world where migration accounted between 40 and 60% of the urban growth. Such a process has endured since the beginning of the industrial revolution in the 19th century but has become prevalent in the developing world. The reasons for urban migration are numerous and may involve the expectation to find employment, improved agricultural productivity which frees rural labor or even political and environmental problems where populations are constrained to leave the countryside.

The outcome has been a fundamental change in the socio-economic environment of human activities as urbanization involves new forms of employment, economic activity and lifestyle. Thus, industrialization in the developing world is directly correlated with urbanization, the case of China being particularly eloquent. The industrialization of coastal China has led to the large rural to urban migration in history. According to the United Nations Population Fund, about 18 million people migrate from rural areas to cities each year in China alone. Current global trends indicate a growth of about 50 million urbanites each year, roughly a million a week. More than 90% of that growth occurs in developing countries which places intense pressures on urban infrastructures, particularly transportation, to cope (see Concept 4). By 2050, 6.2 billion people, about two thirds of humanity, are likely to be urban residents.

2. the Urban Form

At the urban level, demographic and mobility growth have been shaped by the capacity and requirements of urban transport infrastructures, be they roads, transit systems or simply walkways. Consequently, there is a wide variety of urban forms, spatial structures and associated urban transportation systems.

Urban form. Refers to the spatial imprint of an urban transport system as well as the adjacent physical infrastructures. Jointly, they confer a level of spatial arrangement to cities.

Urban (spatial) structure. Refers to the set of relationships arising out of the urban form and its underlying interactions of people, freight and information.

Even if the geographical setting of each city varies considerably, the urban form and its spatial structure are articulated by two structural elements:

• Nodes. These are reflected in the centrality of urban activities, which can be related to the spatial accumulation of economic activities or to the accessibility to the transport system. Terminals, such as ports, railyards, and airports, are important nodes around which activities agglomerate at the local or regional level. Nodes have a hierarchy related to their importance and contribution to urban functions, such as production, management, retailing and distribution.
• Linkages. These are the infrastructures supporting flows from, to and between nodes. The lowest level of linkages includes streets, which are the defining elements of the urban spatial structure. There is a hierarchy of linkages moving up to regional roads and railways and international connections by air and maritime transport systems.

Urban transportation is organized in three broad categories of collective, individual and freight transportation. In several instances, they are complementary to one another, but sometimes they may be competing for the usage of available land and/or transport infrastructures:

• Collective Transportation (public transit). The purpose of collective transportation is to provide publicly accessible mobility over specific parts of a city. Its efficiency is based upon transporting large numbers of people and achieving economies of scale. It includes modes such as tramways, buses, trains, subways and ferryboats.
• Individual Transportation. Includes any mode where mobility is the outcome of a personal choice and means such as the automobile, walking, cycling and the motorcycle. The majority of people walk to satisfy their basic mobility, but this number varies according to the city considered. For instance, walking account for 88% of all movements inside Tokyo while this figure is only 3% for Los Angeles.
• Freight Transportation. As cities are dominant centers of production and consumption, urban activities are accompanied by large movements of freight. These movements are mostly characterized by delivery trucks moving between industries, distribution centers, warehouses and retail activities as well as from major terminals such as ports, railyards, distribution centers and airports.

Historically, movements within cities tended to be restricted to walking, which made medium and long distance urban linkages rather inefficient and time-consuming. Thus, activity nodes tended to be agglomerated and urban forms compact. Many modern cities have inherited an urban form created under such circumstances, even though they are no longer prevailing. The dense urban cores of many European, Japanese and Chinese cities, for example, enable residents to make between one third and two thirds of all trips by walking and cycling. At the other end of the spectrum, the dispersed urban forms of most Australian, Canadian and American cities, which were built recently, encourages automobile dependency and are linked with high levels of mobility. Many major cities are also port cities with maritime accessibility playing an enduring role not only for the economic vitality but also in the urban spatial structure with the port district being an important node.

Urban transportation is thus associated with a spatial form which varies according to the modes being used. What has not changed much is that cities tend to opt for a grid street pattern. This was the case for many Roman cities as it is for American cities. The reasons behind this permanence are relatively simple; a grid pattern jointly optimize accessibility and available real estate. In an age of motorization and personal mobility, an increasing number of cities are developing a spatial structure that increases reliance on motorized transportation, particularly the privately owned automobile. Dispersion, or urban sprawl, is taking place in many different types of cities, from dense, centralized European metropolises such as Madrid, Paris, and London, to rapidly industrializing metropolises such as Seoul, Shanghai, and Buenos Aires, to those experiencing recent, fast and uncontrolled urban growth, such as Bombay and Lagos.

3. Evolution of Transportation and Urban Form

The evolution of transportation has generally led to changes in urban form. The more radical the changes in transport technology, the more the urban form has been altered. Among the most fundamental changes in the urban form is the emergence of new clusters expressing new urban activities and new relationships between elements of the urban system. In many cities, the central business district (CBD), once the primary destination of commuters and serviced by public transportation, has been changed by new manufacturing, retailing and management practices. Whereas traditional manufacturing depended on centralized workplaces and transportation, technological and transportation developments rendered modern industry more flexible. In many cases, manufacturing relocated in a suburban setting, if not altogether to entirely new low costs locations. Retail and office activities are also suburbanizing, producing changes in the urban form. Concomitantly, many important transport terminals, namely port facilities and railyards, have emerged in suburban areas following new requirements in modern freight distribution brought in part by containerization. The urban spatial structure shifted from a nodal to a multi-nodal character.

Initially, suburban growth was mainly taking place adjacent to major road corridors, leaving a lot of vacant or farm land in between. Later, intermediate spaces were gradually filled up, more or less coherently. Highways and ring roads, which circled and radiated from cities, favored the development of suburbs and the emergence of important sub-centers that compete with the central business district for the attraction of economic activities. As a result, many new job opportunities have shifted to the suburbs (if not to entirely new locations abroad) and the activity system of cities has been considerably modified. Different parts of a city have different dynamism depending on its spatial pattern. These changes have occurred according to a variety of geographical and historical contexts, notably in North America and Europe [Muller, 1995]. In addition, North American and European cities have seen different changes in urban density. Two processes have a substantial impact on contemporary urban forms:

• Dispersed urban land development patterns have been dominant in North America over the last 50 years, where land is abundant, transportation costs were low, and where the economy became dominated by service and technology industries. Under such circumstances, it is not surprising to find that there is a strong relationship between urban density and automobile use. For many cities their built up areas have grown at a faster rate than their populations. In addition, commuting became relatively inexpensive compared with land costs, so households had an incentive to buy lower-priced housing at the urban periphery. Similar patterns can be found in many European cities, but this change is occurring at a lower pace and involving a smaller range.
• The decentralization of activities resulted in two opposite effects. First, commuting time has remained relatively stable in duration. Second, commuting increasingly tends to be longer and made by using the automobile rather than by public transit. Most transit and road systems were developed to facilitate suburb-to-city, rather than suburb-to-suburb, commuting. As a result, suburban highways are often as congested as urban highways.

Although transportation systems and travel patterns have changed considerably over time, one enduring feature remains that most people travel between 30-40 minutes in one direction. Globally, people are spending about 1.2 hours per day commuting, wherever this takes place in a low or a high mobility setting [Schafer, 2000]. Different transport technologies, however, are associated with different travel speeds and capacity. As a result, cities that rely primarily on non-motorized transport tend to be different than auto-dependent cities. Transport technology thus plays a very important role in defining urban form and the spatial pattern of various activities.

4. The Spatial Imprint of Urban Transportation

The amount of urban land allocated to transportation is often correlated with the level of mobility. In the pre-automobile era, about 10% of the urban land was devoted to transportation which were simply roads for a traffic that was dominantly pedestrian. As the mobility of people and freight increased, a growing share of urban areas is allocated to transport and the infrastructures supporting it. Large variations in the spatial imprint of urban transportation are observed between different cities as well as between different parts of a city, such as between central and peripheral areas. The major components of the spatial imprint of urban transportation are:

• Pedestrian areas. Refer to the amount of space devoted to walking. This space is often shared with roads as sidewalks may use between 10% and 20% of a road's right of way. In central areas, pedestrian areas tend to use a greater share of the right of way and in some instances, whole areas are reserved for pedestrians. However, in a motorized context, most of pedestrian areas are for servicing people's access to transport modes such as parked automobiles.
• Roads and parking areas. Refer to the amount of space devoted to road transportation, which has two states of activity; moving or parked. In a motorized city, on average 30% of the surface is devoted to roads while another 20% is required for off-street parking. This implies for each car about 2 off-street and 2 on-street parking spaces. In North American cities, roads and parking lots account between 30 to 60% of the total surface.
• Cycling areas. In a disorganized form, cycling simply shares access to pedestrian and road space. However, many attempts have been made to create spaces specifically for bicycles in urban areas, with reserved lanes and parking facilities.
• Transit systems. Many transit systems, such as buses and tramways, share road space with automobiles, which often impairs their respective efficiency. Attempts to mitigate congestion have resulted in the creation of road lanes reserved to buses either on a permanent or temporary (during rush hour) basis. Other transport systems such as subways and rail have their own infrastructures and, consequently, their own rights of way.
• Transport terminals. Refer to the amount of space devoted to terminal facilities such as ports, airports, transit stations, railyards and distribution centers. Globalization has increased the mobility of people and freight, both in relative and absolute terms, and consequently the amount of urban space required to support those activities. Many major terminals are located in the peripheral areas of cities, which are the only locations where sufficient amounts of land are available.

The spatial importance of each transport mode varies according to a number of factors, density being the most important. If density is considered as a gradient, rings of mobility represent variations in the spatial importance of each mode at providing urban mobility. Further, each transport mode has unique performance and space consumption characteristics. The most relevant example is the automobile. It requires space to move around (roads) but it also spends 98% of its existence stationary in a parking space. Consequently, a significant amount of urban space must be allocated to accommodate the automobile, especially when it does not move and is thus economically and socially useless. At an aggregate level, measures reveal a significant spatial imprint of road transportation among developed countries. In the United States, more land is thus used by the automobile than for housing [Kauffman, 2001]. In Western Europe, roads account for between 15% and 20% of the urban surface while for developing countries, this figure is about 10% (6% on average for Chinese cities).

5. Transportation and Urban Structure

Rapid and expanded urbanization occurring around the world involves an increased numbers of trips in urban areas. Cities have traditionally responded to growth in mobility by expanding the transportation supply, by building new highways and/or transit lines. In the developed world, that has mainly meant building more roads to accommodate an ever-growing number of vehicles, therefore creating new urban structures. Several urban spatial structures have accordingly emerged, with the reliance on the automobile being the most important discriminatory factor. Four major types can be identified at the metropolitan scale [Thomson, 1977]:

• Type I - Completely Motorized Network: Representing an automobile-dependent city with a limited centrality.
• Type II - Weak Center: Representing the spatial structure of many American cities where many activities are located in the periphery.
• Type III - Strong Center: Representing high density urban centers with well developed public transit systems, particularly in Europe and Asia.
• Type IV - Traffic Limitation. Representing urban areas that have implemented traffic control and modal preference in their spatial structure. Commonly, the central area is dominated by public transit.

There are different scales where transportation systems influence the structure of communities, districts and the whole metropolitan area. For instance, one of the most significant impacts of transportation on the urban structure has been the clustering of activities near areas of high accessibility. The impact of transport on the spatial structure is particularly evident in the emergence of suburbia. Although many other factors are important in the development of suburbia, including low land costs, available land (large lots), the environment (clean and quiet), safety, and car-oriented services (shopping malls), the spatial imprint of the automobile is dominant. Although it can be argued that roads and the automobile have limited impacts on the extent of urban sprawl itself, they are a required condition for sprawl to take place. Initially an American invention, suburban developments have occurred in many cities worldwide, although no other places have achieved such a low density and automobile dependency than in the United States.

Facing the expansion of urban areas, congestion problems and the increasing importance of inter-urban movements, several ring roads have been built around major cities. They became an important attribute of the spatial structures of cities, notably in North America. Highway interchanges in suburban areas are notable examples of new clusters of urban development. The extension (and the over-extension) of urban areas have created what may be called peri-urban areas. They are located well outside the urban core and the suburbs, but are within reasonable commuting distances.