Author: Dr. Jean-Paul Rodrigue

Transportation has an influence on the urban spatial structure and is shaping urbanization.

1. Global Urbanization

Urbanization. The transition from a rural to an 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 urbanization is the rate at which it grows.

(UNFPA, 2007).

Urbanization has been one of the dominant economic and social changes of the 20th century, especially in the developing world. Although cities played a significant role throughout human history, it was not until the Industrial Revolution that a network of large cities started to emerge in the most economically advanced parts of the world. Innovation diffused through this interconnected network of cities articulating economic development. Since 1950, the world’s urban population has more than doubled, reaching nearly 4.4 billion in 2021, about 56.6% of the global population. This transition is expected to go on well into the second half of the 21st century, a trend reflected in the growing size of cities and the increasing proportion of the urbanized population. By 2050, 70% of the global population could be urbanized, representing 6.4 billion urban residents. Cities also dominate the national economic output, accounting for the bulk of production, distribution, and consumption.

Global urbanization is the outcome of three main demographic trends:

• Natural increase. The outcome of more births than deaths in urban areas is a direct function of the fertility rate as well as the quality of healthcare systems (lower mortality rates, particularly for infants). Phases in the demographic transition are commonly linked with urbanization rates, with peak growth years corresponding to large differences between birth and death rates. Although natural increase played an essential role in the past, it is of much lesser importance today as fertility rates in many developed economies have dropped significantly. In some cases, like Western Europe, Japan, and South Korea, fertility is below the replacement rate. Further, fertility rates in cities are usually lower than rural areas, underlining that cities can be seen as engines of demographic decline.
• Rural to urban migrations. This has been a dominant urbanization factor, particularly in the developing world, where migration accounted for between 40 and 60% of urban growth. Rural-to-urban migration endured through the Industrial Revolution in the 19th century. It surged in the first half of the 20th century in the developed world and then in the developing world in the second half of the 20th century. The factors behind rural-to-urban migrations may involve the expectation to find employment, gains in agricultural productivity, which frees rural labor, or even political and environmental problems where populations are constrained to leave the countryside. The industrialization of coastal China and its integration into the global trade system since the 1980s has led to the largest rural-to-urban migration in history, with estimates above 375 million migrants.
• International migration. The growth in international migration has been an important factor in the urbanization of major gateway cities, such as Los Angeles, Miami, New York, London, and Paris. This process tends to occur in the largest cities, but there is a trickle-down in smaller-sized cities.

Through urbanization, fundamental changes in the socio-economic environment of human activities have been observed, with new forms of employment, economic activity, lifestyle, and mobility. What drives urbanization is a complex mix of economic, demographic, and technological factors. The growth in GDP per capita is a dominant driver of urbanization, which is supported by corresponding developments in transportation systems. More recently, the diffusion of air conditioning allowed for settlements in high-temperature areas such as in the southwest of the United States or the Middle East (e.g. Dubai).

Urban mobility problems have increased proportionally with urbanization, which is associated with two outcomes. First is the emergence of a network of megacities that account for the most salient urban mobility challenges. Second, mobility demands tend to be concentrated over specific urban areas, such as central business districts and main circulation corridors.

Global trends indicate about 50 million new urbanites each year, roughly a million a week. More than 90% of that growth occurs in developing economies, which puts pressure on urban infrastructures, particularly transportation. What is considered urban includes a whole continuum of urban spatial structures, ranging from small towns to large urban agglomerations. This also brings the question of optimal city size since technical limitations (road, utilities) are not much of an impediment to building large cities. Many of the world’s largest cities are challenged because rising operational and infrastructure complexities are not effectively coped with managerial expertise as city size increases. Still, urbanization remains the dominant socioeconomic paradigm associated with improvements in economic development.

2. The Urban Form

Urbanization has been shaped by transport infrastructures, such as roads, transit systems, or simply walkways. Since each city has a different temporal process of accumulation and development of transport infrastructures, there is a wide variety of urban forms, spatial structures, and associated urban transportation systems.

Urban form. Refers to the spatial footprint of an urban transport system, which defines the spatial arrangement of cities.

Urban spatial structure. Refers to the set of relationships arising out of the urban form and its underlying mobility of passengers and freight. Specific urban structures can be achieved with specific transport systems.

Considering transport developments, the urban spatial structure can be categorized by its level of centralization and clustering:

• Centralization. Refers to the locational setting of activities in relation to the whole urban area. A centralized city has a significant share of its activities within a defined center, while a decentralized city does to a lesser extent. Large employers such as financial institutions are the main drivers of centralization.
• Clustering. Refers to the locational setting of activities in relation to a specific part of the urban area. A cluster of activities is, therefore, a concentration around a specific focal point, such as a highway interchange, a transit terminal, or a smaller town that has been absorbed by the expansion of the metropolis.

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 clusters are reflected in the centrality of urban activities, which can be related to the spatial accumulation of economic activities or accessibility to the transport system. Terminals, such as ports, train stations, 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, with high-order nodes such as management and retailing and lower-order nodes such as production and distribution.
• Linkages. These are the infrastructures supporting mobility 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 from local streets, regional highways, and railways to international connections by air and maritime transport systems.

Depending on their nature, urban nodes and linkages provide functional connectivity, implying interdependent urban functions related to trade, management, and production. Thus, urban transportation is associated with a spatial form that varies according to the modes used. Grid street patterns have endured throughout history, which was the case for many Roman cities built in the 1st century and American cities built in the 20th century. The reasons behind this permanence are relatively simple; a grid pattern jointly optimizes accessibility and available real estate. Still, many cities are not as organized as a grid. They correspond to cities that grew from a constrained location such as a bay, an island, a hill, or a river junction. Local geographical and historical characteristics remain important influences on the urban form.

In the 20th century, cities developed a unique spatial structure relying on motorized transportation, particularly the privately owned automobile. This incited a shift from a grid pattern toward curvilinear and cul-de-sac patterns commonly found in suburban areas. 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 Mexico City, to those experiencing recent and fast urban growth, such as Mumbai, Jakarta, and Lagos. Contemporary urban expansion is strongly shaped by road transportation as the support for mobility with its hierarchy of local streets, connectors, boulevards, and expressways. Therefore, there are significant differences in the density of cities across the world, in addition to a variety of density gradients observed within cities. The differences are particularly prevalent between North American and European cities.

3. Evolution of Transportation and Urban Form

Urbanization is occurring following the development of urban transport systems, particularly in capacity and efficiency. Historically, movements within cities tended to be mainly restricted to walking, making urban mobility inefficient and time-consuming. Thus, activity nodes tended to be agglomerated, and urban forms were compact with mixed uses. 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 and East Asian 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 more recently, encourage automobile dependency and are linked with high levels of mobility. Still, Chinese cities have experienced a high level of motorization, implying the potential for convergence toward more uniform urban forms. Many cities are also port cities, with trade playing an enduring role in economic vitality and urban spatial structure, with the port district being an important node. Airport terminals have also been playing a growing role in the urban spatial structure as they can be considered cities within cities.

The evolution of transportation has generally led to changes in urban form. The more radical the changes in transport technology, the more the alterations in the urban form. Among the most fundamental changes in the urban form is the emergence of new clusters in peripheral areas expressing new urban activities and new relationships between elements of the urban system. Many cities are assuming a polycentric form, a change associated with new mobility patterns. The central business district (CBD), once the primary destination of commuters and serviced by public transportation, has been transformed 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 offshore locations, expanding the reach and connectivity of cities. 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, implying new forms of urban development and new connections to regional and global economic processes.

Initially, suburban growth mainly occurred adjacent to major road corridors, leaving vacant plots or farmland 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 to attract economic activities. As a result, many new job opportunities shifted to the suburbs, and the activity system of cities has been considerably modified. Depending on the economic sectors they specialize in, cities and even different parts of a metropolitan area can be experiencing development at entirely different rates (or even decline), leading to a highly heterogeneous urban landscape. These changes have occurred according to a variety of geographical and economic contexts, notably in North America and Europe, as each subsequent phase of urban transportation developments led to different spatial structures. Sometimes, particularly when new modern urban road infrastructures are built, the subsequent changes in the urban form can be significant. Two processes had a substantial impact on contemporary urban forms:

• Urban sprawl. Dominant in North America since the end of World War II, where land was abundant, transportation costs were low, and the economy became dominated by tertiary and quaternary activities. Under such circumstances, a strong negative relationship between urban density and automobile use emerged, involving energy use for urban mobility. In the context of cities with high automobile dependency, their built-up areas have grown at a faster rate than their populations, resulting in declining densities. In addition, commuting became relatively inexpensive compared with land costs, so households were incentivized to buy lower-priced housing at the urban periphery. Wherever there was motorization, a pattern of sprawl took shape.
• Decentralization. Resulting in two opposite effects. First, commuting time has remained relatively stable in duration, in the range of one hour per day. Second, commuting increasingly tends to be longer in terms of distance 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 are willing to travel between 30-40 minutes in one direction, known as Marchetti’s constant. Globally, people spend about 1.2 hours per day commuting, wherever this takes place in a low or high-mobility setting. Different transport technologies, however, are associated with different travel speeds and capacities. As a result, cities that rely primarily on non-motorized transport tend to be more compact than automobile-dependent cities. Transport technology thus plays a significant role in defining urban form and the spatial pattern of various activities. Still, the evolution of the urban form is path-dependent, implying that the current spatial structure is the outcome of past developments but that those developments were strongly related to local conditions involving the setting, physical constraints, and investments in transportation infrastructures. It takes substantial effort and long-term commitment to change the spatial structure of a city noticeably.

4. The Spatial Constraints 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 was simply roads for pedestrian traffic and common uses. As the mobility of people and freight increased, a growing share of urban areas was allocated to transport and the infrastructures supporting it. Large variations in the footprint of urban transportation are observed between different cities and parts of a city, such as central and peripheral areas. The major components of the footprint 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 pedestrian areas serve 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 two off-street and two on-street parking spaces are available. In North American cities, roads and parking lots account for between 30 and 60% of the total surface, a share which is lower elsewhere.
• Micromobility areas. In a disorganized form, forms of micromobility such as cycling share access to pedestrian and road space. However, many attempts have been made to create spaces specifically for bicycles, with reserved lanes and parking facilities. The Netherlands has been particularly proactive over this issue, with biking paths and parking areas as active components of the urban transport system; 27% of the total amount of commuting is accounted for by cycling. The diffusion of electric bicycles has given an additional impetus to using bicycles in urban mobility.
• 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 created road lanes reserved for buses permanently or temporarily (during rush hour). Other transport systems, such as subways and rail, have their infrastructures and, consequently, their rights of way.
• Transport terminals. Refer to the space devoted to terminal facilities such as ports, airports, transit stations, railyards, and distribution centers. Globalization has increased the mobility of people and freight, and, consequently, the footprint required to support those activities. Many major terminals are located in the peripheral areas of cities, the only locations where sufficient amounts of land are available.

The spatial importance of each transport mode varies according to several factors, density being the most important. 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. In large urban agglomerations, close to all the available street parking space in areas of average density and above is occupied throughout the day. At an aggregate level, measures reveal a significant footprint of road transportation among developed countries. In the United States, more land is thus used for the automobile than housing. In Western Europe, roads account for between 15% and 20% of the urban surface, while for developing economies, this figure is about 10% but rising fast due to motorization.

5. Transportation and the Urban Structure

Urbanization involves an increased number of trips occurring in urban areas. Cities have traditionally responded to the growth in mobility by expanding the transportation supply by building new highways and transit lines. This has mainly meant building more roads to accommodate an ever-growing number of vehicles. 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:

• Type I. Completely Motorized Network. Representing an automobile-dependent city with limited centrality and dispersed activities.
• Type II. Weak Center. Representing a spatial structure where many activities are located in the periphery.
• Type III. Strong Center. Representing high-density urban centers with well-developed public transit systems.
• 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.

Another aspect of the relationship between transportation and the urban spatial structure involves the orientation of the networks and their level of entropy. Around the world, cities tend to have a cardinal orientation, underlining the influence of a frame of reference dominated by the north, allowing setting grids or streets. Over time, entropy (disorder) has declined as contemporary cities or neighborhoods tend to have car-oriented grids instead of the organic distribution of streets and alleys of pre-industrial cities. 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, a process that occurs in every major metropolitan area worldwide. Although many other factors are important in its development, including low land costs, available land (large lots), environmental considerations (clean and quiet), safety, and car-oriented services (shopping malls), the footprint of the automobile is dominant. The automobile is also linked with changes in street layouts. While older parts of cities tend to have a conventional grid layout, from the 1930s, new suburbs started to be designed in a curvilinear fashion, including some cul-de-sacs (dead ends). By the 1950s, the prevailing design for new suburbs was privileging cul-de-sacs. Although the aim was to create a more private and safe environment, particularly in cul-de-sac sections, the outcome was also a growing sense of isolation and car use.

With the expansion of urban areas, congestion, and the increasing importance of inter-urban movements, the existing structure of urban roads was considered inadequate. Several ring roads have been built around major cities and have become an important attribute of their spatial structures. Highway interchanges in suburban areas are notable examples of clusters of urban development that have shaped the multicentric character of many cities. 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 within reasonable commuting distances; the term “edge cities” has been used to label a cluster of urban development in suburban settings.

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Related Topics

• 8.2 – Urban Land Use and Transportation
• 8.3 – Urban Mobility
• 8.4 – Urban Transport Challenges
• 2.2 – Transport and Spatial Organization
• 2.3 – Transport and Location
• 6.1 – The Function of Transport Terminals
• 4.3 – Transportation, Land Use and the Environment

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