4.3 – The Environmental Footprint of Transportation

Author: Dr. Jean-Paul Rodrigue

Transportation activities have an environmental footprint related to infrastructures such as terminals and roads and their flows.

1. Land Requirement and Consumption

Historically, several environmental aspects impacted the organization and regulation of the footprint taken by transportation activities. Although various forms of pollution were noted since Antiquity, by the 19th century, environmental considerations started to become regulations at the onset of the Industrial Revolution. Zoning restrictions in central business districts forbidding polluting industrial uses were among the first to be implemented. Then, in the 20th-century, land uses judged to be incompatible were separated. The most prevalent were heavy industries and residential areas, which led to a series of zoning definitions of urban areas. Transportation infrastructures, particularly roads, began to have a growing footprint on urban land uses. However, this development is paradoxical since the construction of highways was initially seen as a local benefit, providing mobility and accessibility. It is only later, from the 1970s, that the perspective changed. As providers of mobility, highways were also seen as generators of environmental externalities such as land take, noise, and air pollution.

From the 1950s, urbanization has rapidly seen the expansion of urban land uses, which means that a large city of 5 million inhabitants may stretch over 100 km (including suburbs and satellite cities) and may use an amount of land exceeding 5,000 square km. Such large cities obviously cannot be supported without a vast and complex transport system. Also, modal choice has an important impact on land consumption. The preference for road transportation has led to massive consumption of space with 1.5 to 2.0% of the world’s total land surface devoted to road transportation, mainly for roads rights of way and parking lots. The footprint of transportation has reached a point where 30 to 60% of urban areas are taken by road transportation infrastructure alone. In more extreme cases of road transportation dependency, such as Los Angeles, this figure can reach 70%. Yet, for many developing countries such as China and India, motorization is still in its early stages. For China to have a level of motorization similar to those of Western Europe would imply a fleet of vehicle superior to the current global fleet. From a land requirement perspective, full motorization would generate a massive footprint.

Cities consume large quantities of land, and their growth leads to the notion of metropolitan areas and, further, urban regions oriented along corridors. With urbanization, expansion has allowed the reclamation of vast amounts of land from rural activities towards other uses. Economic globalization and the associated rise in the mobility of passengers and freight have required the expansion of terminal facilities such as ports and airports that have a large footprint. Also, the duplication of infrastructure, public and private alike, have resulted in additional land requirements. This is notably the case for large transport terminals such as ports and airports that were built because they belonged to different administrative jurisdictions. The general aim was to convey a high level of accessibility to answer mobility demands. While in several regions, road transportation infrastructures are overused, a situation of under-capacity exists in others. The formation of compact and accessible cities must be allowed to contend with the already existing built environment while considering several limits to development and urban renewal through temporal constraints and common limitations in capital availability.

The geographical growth of cities has not been proportional to the growth of their population, resulting in lower densities and higher space consumption. This also concerns manufacturing and freight distribution that have the propensity to expand horizontally with the expansion of the transportation and storage functions, particularly for distribution centers. An increase in the quantity of energy consumed and waste generated has been the outcome. Consequently, changes in urban land use and its transport system have expanded the environmental footprint of cities.

2. Spatial Form, Pattern and Interaction

The structure of urban land use has an important impact over transport demand and over the capacity of transportation systems to answer such mobility needs. This involves three dimensions influencing the environmental impacts of transportation and land use:

  • Spatial form. Relates to the spatial arrangement of a city, particularly in terms of the setting and orientation of its axis of circulation. This form thus conveys a general structure to urban transportation ranging from centralized to distributed. The dominant influence has been expansion and motorization. The resulting polycentric cities are economically and functionally flexible but consume more energy.
  • Spatial pattern. Relates to the organization of land use in terms of location of major socio-economic functions such as residential, commercial, and industrial uses. The prevailing trend has been a growing specialization, disconnection, and fragmentation between land uses. Also, different types of land use can be incompatible with their proximity to the source of additional externalities. For instance, residential land use is incompatible with the majority of industrial, manufacturing, warehousing, and transport terminal activities. They generate noise and congestion externalities to which residents are highly susceptible. In such a context, buffers, which apply different barriers effects to promote physical separation, can help mitigate incompatible land uses.
  • Spatial interaction. Relates to the nature and the structure of movements generated by urban land uses. The prevailing trend has been a growth in urban interactions in terms of their volume, complexity, and average distance.

The location of activities such as residence, work, retail, production, and distribution is indicative of the required travel demand and the average distance between activities. With specialized land use functions and spatial segregation between economic activities, interactions are proportionally increasing. It is over the matter of density that the relationships between transportation, land use, and the environment can be the most succinctly expressed. The higher the density level, the lower the level of energy consumption per capita, and the relative environmental impacts. A remarkable diversity of urban densities is found around the world, which is reflective of different geographical settings, planning frameworks, and levels of economic development. This complexity is compounded by how density changes in relation to the city center.

Paradoxically, the outward expansion of cities and suburbanization has favored a relatively uniform distribution of land use densities, notably in cities with prior low-density levels. In recent decades, the average density of several large metropolitan areas has declined by at least 25%, implying additional transport requirements to support mobility demands. Further, residence/work separation is becoming more acute as well as the average commuting time and distance. It is consequently increasingly challenging to provide urban transit services at an efficient cost. This underlines that the future of sustainable mobility will require accomodating personal mobility requirements, even if this mobility is considered less sustainable than collective mobility.

An important effect of land use pattern and density on the local environment concerns the heat island effect. It is an outcome of differences in albedo between an urban surface composed of buildings and paved surfaces (roads, parking lots) and the natural landscape. The urban landscape absorbs more heat during the day, which is released during the night and can result in ambient temperatures up to 5 degrees Celsius higher than normal. The land use pattern plays a role in the heat island effect with grid patterns (or other ordered patterns) retaining more heat than other disordered patterns, mostly because buildings and other structures reabsorb the heat emitted by others.

A higher level of integration between transportation and land use, particularly density, often results in increased accessibility levels without necessarily increasing the need for automobile travel. The slow transformation of urban land uses, with annual rates lower than 2%, makes it difficult to establish sound transportation/land use strategies that could have effective impacts over a short time period. As it is generally market forces that shape such changes, it is uncertain which drivers of change would significantly impact the transformation of urban land use.

3. Environmental Externalities of Land Use

As a spatial structure, land use is linked to a number of externalities that impose significant economic, social, and environmental costs that communities are less willing to assume. This has led to various land use regulations, mostly under the umbrella of “smart growth” initiatives, to increase density and promoting modes other than the automobile. Strategic indicators that are recurrent in evaluating the environmental externalities of land use involve vehicle-mile (km) traveled, transit ridership, and average commuting time to the workplace, which are all spatial interaction variables.

The last half a century has been associated with a declining role of public transit, a more disorganized spatial structure, and the prevalence of suburbanization. This trend could be reversed with two possible and interdependent paths of land use changes unfolding, depending upon the concerned urban setting:

  • Densification. It involves a more rational and intensive use of the existing land uses to minimize the environmental footprint and the level of energy consumption. Initiatives such as smart growth are trying to change the urban planning framework towards forms and densities that are more suitable for walking, non-motorized modes, and public transit. If this occurs in proximity to a transit station, the term transit-oriented development is used to characterize the densification process. Yet this implies higher levels of capital investment and the provision of an adequate public transit service since, in a car-dependent context, densification easily leads to congestion and other externalities.
  • Devolution. Due to economic and demographic trends, several cities could lose a share of their population, imposing a rationalization of urban land uses. In industrial regions of Europe and North America, several cities have lost a share of their economic base and, correspondingly, their population. This involves dismantling urban infrastructure and closing sections or whole neighborhoods, leading to the emergence of urban forests and even forms of urban agriculture. Detroit is a salient example since the population of the city dropped by more than a half from 1.8 million in 1950 to 713,000 in 2010. Yet, the population of Detroit’s metropolitan area has remained relatively stable since the 1970s, hovering around 4.2 million. This implies that the process of devolution is very location-specific.

What could shape land use towards a more environmentally beneficial structure in the future is uncertain since many policies appear to be not particularly useful. Since it took 30 to 50 years for North American, Australian, and to some extent, European cities to reach their current patterns of automobile dependency, it may take the same amount of time to reach a new equilibrium if specific conditions apply. This transition could even be more complex in developing economies where the forces of motorization are gaining momentum with economic development. Since the price of energy is an important component in the cost of personal mobility, energy costs are likely to be a significant force shaping urban development. If the energy component does not change significantly, congestion and infrastructure capacity limitations will likely play a more important aspect. Consequently, the environmental impacts of transportation and land use are likely to stay prevalent for several decades.

Related Topics


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