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Urbanization has been one of the dominant contemporary processes as a growing share of the global population lives in cities. Considering this trend, urban transportation issues are of foremost importance to support the passengers and freight mobility requirements of large urban agglomerations. Transportation in urban areas is highly complex because of the modes involved, the multitude of origins and destinations, and the amount and variety of traffic. Traditionally, the focus of urban transportation has been on passengers as cities were viewed as locations of utmost human interactions with intricate traffic patterns linked to commuting, commercial transactions and leisure/cultural activities. However, cities are also locations of production, consumption and distribution, activities linked to movements of freight. Conceptually, the urban transport system is intricately linked with urban form and spatial structure. Urban transit is an important dimension of urban transportation, notably in high density areas. To understand the complex relationships between transportation and land use and to help the urban planning process, several models have been developed.
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 motor-cycle. The majority of people walk to satisfy their basic mobility, but this number varies according to the city considered. For instance, walking accounts for 88 percent of all movements inside Tokyo while this figure is only 3 percent 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.
The amount of urban land allocated to transportation is often correlated with the level of mobility. In the pre-automobile era, about 10 percent of urban land was devoted to transportation. 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 – the amount of space devoted to walking. This space is often shared with roads as sidewalks may use between 10 and 20 percent 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 are for servicing people’s access to parked automobiles;
• roads and parking areas – the amount of space devoted to road transportation, which has two states of activity: moving or parked. In a motorized city, on average 30 percent of the surface is devoted to roads while another 20 percent is required for off-street parking. This implies for each car about two off-street and two on-street parking spaces. In North American cities, roads and parking lots account for between 30 and 60 percent of the total surface;
• cycling areas – in a disorganized form, cycling simply shares access to 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 for buses. Other transport systems such as subways and rail have their own infrastructures and, consequently, their own rights of way;
• transport terminals – the amount of space devoted to terminal facilities such as ports, airports, transit stations, railyards and distribution centers. Globalization has increased the amount of people and freight circulation 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 percent 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. In Western Europe, roads account for between 15 and 20 percent of the urban surface while for developing countries, this figure is about 10 percent (6 percent on average for Chinese cities).
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