Center for Environmental Excellence by AASHTO
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This section provides a brief overview of the issue of sustainability as it relates to transportation. Topics include the following:



Global Concerns and Triple Bottom Line

Current and future transportation growth patterns and the way that we develop transportation systems are important factors in sustaining the world’s limited economic, environmental, and social resources and capacity.

Transportation represents 10 percent of the world’s gross domestic product, is responsible for 22 percent of global energy consumption and 25 percent of fossil fuel burning across the world, and produces 30 percent of global air pollution and greenhouse gases. As such, the transportation sector will play a key role in addressing global sustainability concerns, including depletion of resources, global climate change, disruption of ecosystems, and toxic pollution.

The American Association of State Highway and Transportation Officials (AASHTO) has acknowledged these challenges and as a result developed the visionary report “Transportation: Invest in Our Future.”

According to the AASHTO report, “America’s transportation system has served us well, but now faces the challenges of congestion, energy supply, environmental impacts, climate change, and sprawl that threaten to undermine the economic, social, and environmental future of the nation. With 140 million more people expected over the next 50 years, past practices and current trends are not sustainable.”

“To meet the transportation needs of the present and pass on a better world to our children and grandchildren, it is necessary to expand the transportation network’s capacity while simultaneously reducing the environmental footprint of the system,” the report said.

The report urged transportation decision makers to adopt the so-called “triple bottom line” approach to sustainability by evaluating performance on the basis of economic, social, and environmental impacts and allocating equal consideration to these driving forces.

The specific elements of the triple bottom line approach and the steps required to achieve them can be summarized as follows:

  • Robust economic growth: Deliver a sustainable, high-performance transportation system in support of a robust economy by first optimizing existing infrastructure, then reshaping demand, and lastly expanding judiciously.
  • Improved quality of life for all citizens: Enhance quality of life by integrating transportation with the built environment by using the full tool kit, including context sensitive solutions, land use policy, and diversified mode choice.
  • Better-than-before health of the environment: Embrace environmental stewardship as a preeminent approach to delivering transportation services that result in a zero carbon footprint and a “better-than-before” environment.

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Sustainable Development

In the early days of highway construction (1950s) enthusiasm for the benefits of transportation facilities overshadowed attention to potential negative impacts associated with those facilities. The National Environmental Policy Act ( NEPA) of 1969 and the Clean Air Act (CAA) of 1970 focused attention on environmental concerns. These environmental laws required agencies to include citizen ideas and consider environmental and social impacts of projects as well as conventional economic impacts. These factors led to the creation of the concept of sustainable development.

The term sustainable development was first used in 1980, and in 1987 the report by the World Commission on Environment and Development (the so-called Brundtland Commission) reemphasized the importance of sustainable development and provided a classic and widely used definition for sustainable development: “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” This definition contains two key principles identified by the Commission – the concept of needs and the idea of limitations. Currently numerous initiatives across the world focus on sustainable development and sustainable transportation, and this trend is accelerating with renewed emphasis on global climate change.

The overall goals of sustainable development are to meet human needs and improve quality of life; to live within the earth’s ecological carrying capacity and maintain or enhance its natural capital; and to protect future generations from reduced quality of life. These goals are achieved by addressing three dimensions of sustainability:

  • Economic development: Ensure that the financial and economic needs of current and future generations are met.
  • Environmental stewardship: Ensure a clean environment for current and future generations and use resources sparingly.
  • Social equity: Improve the quality of life for all people and promote equity between societies, groups, and generations.

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Sustainable Transportation

Transportation plays a key role in the global economy and in the challenges and opportunities associated with sustainable development. Sustainable transportation can be viewed as an expression of sustainable development in the transportation sector. Sustainable transportation addresses local, regional, national, and global issues and therefore requires considerable coordination. It is important to apply sustainable transportation in a holistic and integrated manner across the various sectors (external to transportation) to ensure that key concerns such as depletion of resources, global climate change, disruption of ecosystems, and toxic pollution are effectively addressed.

Numerous authors have developed definitions for sustainable transportation for different contexts. These definitions are based on the broader concept of sustainable development as outlined by the Brundlandt Commission, adapted to meet current and future mobility and accessibility needs without resulting in undue negative externalities. Context specific sustainable transportation definitions generally focus on the three dimensions of sustainable development – economic development, social equity, and environmental stewardship.

The figure below demonstrates the relationship between these dimensions: the resources and services provided by the earth’s natural systems (the environment) are critical for the functioning of our transportation systems and other infrastructure (social systems), and without well-functioning social systems our economic systems cannot be productive. Therefore, there is a need to address the dimensions in an integrated way while considering their relationships.

Dimensions of Sustainability

Dimensions of Sustainability 

A comprehensive definition of a sustainable transportation system developed by the Canadian Center for Sustainable Transportation states that sustainable transportation:

  • “allows the basic access needs of individuals and societies to be met safely and in a manner consistent with human and ecosystem health, and with equity within and between generations;
  • is affordable, operates efficiently, offers choice of transport mode, and supports a vibrant economy; and
  • limits emissions and waste within the planet’s ability to absorb them, minimizes consumption of non-renewable resources, limits consumption of renewable resources to the sustainable yield level, reuses and recycles its components, and minimizes the use of land and the production of noise.”

In contrast, one state department of transportation (DOT) in the United States defines sustainable transportation concisely as “the provision of safe, effective, and efficient access and mobility into the future while considering the economic, social, and environmental needs of society.”

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Goals for Transportation

Goals for developing sustainable transportation systems are context sensitive – they differ depending on factors such as level of economic development (developed, developing, or undeveloped), institutional level (local, regional, state, national, or global), type of transportation system (highway, transit, marine, aviation, or intermodal), scale of facility (section, corridor, network, or system), and time frame (year, decade, or century). The following are sample goals that may help achieve sustainable transportation depending on a specific context:

  1. Improved accessibility
  2. Improved mobility
  3. Improved safety
  4. Improved security
  5. Improved equity
  6. Improved affordability
  7. Reduced air pollution
  8. Reduced greenhouse gasses
  9. Use of renewable resources at or below their rates of generation
  10. Use nonrenewable resources at or below the rates of development of renewable substitutes
  11. Appropriate land use
  12. Reduced noise pollution
  13. Maintain community cohesion
  14. Reduced ecosystem impacts
  15. Improved livability
  16. Improved public involvement
  17. Pricing that reflects true costs

Due to the broad range of goals that are applicable to sustainable transportation, several of the environmental issues included in AASHTO’s Center for Environmental Excellence website have relevance to sustainability: Air Quality, Context Sensitive Solutions, Environmental Considerations in Planning, Environmental Enhancement, Environmental Justice, NEPA Process, Noise, and SAFETEA-LU.   

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Relevant Laws, Regulations, and Policies

Since the emergence of sustainable development, many nations around the world have endorsed the concept as a national objective. However, although sustainable development has received attention in the United States, there is currently no integrated national strategy to pursue it. The closest the Federal government has come to creating a national policy on sustainable development was the formation of the President’s Council on Sustainable Development (PCSD) in 1993 under the Clinton administration.  During its six-year existence (1993-1999), the PCSD prepared three reports that are often referred to as a basis for a national strategy on sustainable development.

Even though there is no national law or policy for implementation of sustainable transportation, some Federal laws are relevant to sustainable transportation or provide flexibility that can help achieve sustainability goals.  The following are some of the most prominent U.S. laws that can help achieve sustainable transportation:

  • 1969 National Environmental Policy Act ( NEPA);
  • 1970/77/90 Clean Air Act (CAA);
  • 1972/77 Clean Water Act (CWA);
  • 1973 Endangered Species Act (ESA);
  • 1975 Energy Policy and Conservation Act ( EPCA);
  • 1991 Intermodal Surface Transportation Efficiency Act (ISTEA);
  • 1992 Energy Policy Act ( EPACT);
  • 1998 Transportation Equity Act of the 21st Century (TEA-21); and
  • 2005 Safe, Accountable, Flexible, Efficient Transportation Equity Act (SAFETEA-LU).

The turning point for transportation planning legislation in the United States was the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA). This legislation introduced several important policy innovations that support the objectives of sustainable transportation. First, ISTEA devolved decision making authority to metropolitan planning organizations (MPOs) by enhancing their role in transportation planning and programming. Second, ISTEA supported its objective to create an intermodal/multimodal transportation system by increasing the ability of MPOs and state DOTs to transfer Federal funds between transportation programs. Finally, a significant policy initiative in ISTEA was the integration of surface transportation legislation with the 1990 Clean Air Act Amendments (CAAA).

Since its creation, ISTEA has largely remained intact philosophically through its two reauthorizations – TEA-21, and SAFETEA-LU. The latter presents the following eight planning factors that provide for flexibility to address the goals of sustainable transportation:

  • Support the economic vitality of the metropolitan area, especially by enabling global competitiveness, productivity, and efficiency.
  • Increase the safety of the transportation system for all motorized and nonmotorized users.
  • Increase the ability of the transportation system to support homeland security and to safeguard the personal security of all motorized and nonmotorized users.
  • Increase accessibility and mobility of people and freight.
  • Protect and enhance the environment, promote energy conservation, improve the quality of life, and promote consistency between transportation improvements and state and local planned growth and economic development patterns.
  • Enhance the integration and connectivity of the transportation system, across and between modes, for people and freight.
  • Promote efficient system management and operation.
  • Emphasize the preservation of the existing transportation system.

The National Environmental Policy Act of 1969 (NEPA) also provides clear direction to pursue a sustainable transportation system. Under Section 1 of Title 1, Congress recognizes the profound impact of human activity on the natural environment, particularly the influences of population growth, high-density urbanization, industrial expansion, resource exploitation, and new and expanding technological advances. The section also recognizes the critical importance of restoring and maintaining environmental quality and highlights the roles of Federal, State, and local governments and other concerned public and private organizations in this regard. The section identifies six goals that must be pursued, and these goals also provide flexibility for addressing sustainable transportation:

  1. Fulfill the responsibilities of each generation as trustee of the environment for succeeding generations.
  2. Assure for all Americans safe, healthful, productive, and aesthetically and culturally pleasing surroundings.
  3. Attain the widest range of beneficial uses of the environment without degradation, risk to health or safety, or other undesirable and unintended consequences.
  4. Preserve important historic, cultural, and natural aspects of our national heritage, and maintain, wherever possible, an environment that supports diversity, and variety of individual choice.
  5. Achieve a balance between population and resource use that will permit high standards of living and a wide sharing of life’s amenities.
  6. Enhance the quality of renewable resources and approach the maximum attainable recycling of depletable resources.

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Measuring Sustainable Transportation

An agency can move toward implementing a sustainable transportation system by following three steps – conceptualization, operationalization, and utilization:

  • Step 1: Conceptualize: Define what sustainability means to your agency, and what actions are within the agency’s power to approach sustainability.
  • Step 2: Operationalize: Identify appropriate indicators and performance measures that the agency can use to track progress toward sustainability.
  • Step 3: Utilize: Use performance measures individually or in combination as indexes to track progress, compare projects, and make decisions.

Agencies can use indicators or performance measures to quantify or operationalize sustainable transportation. An indicator is a variable that represents an operational attribute of a system. Indicators condense complex information from underlying data into a simplified form, and when compared with standards or objectives, become performance measures. For example, assume the sustainability goal to be addressed is to improve accessibility. Possible indicators associated with this goal include number of travel objectives that can be reached within an acceptable travel time, ability of nondrivers to reach employment centers and services, land use mix, and percentage of employees within x miles of major services.

Indicators or performance measures are commonly used across a broad range of agency tasks such as: system performance, alternative evaluation, trend tracking, project selection, impact assessment, program performance, internal communication, and external communication.

Vast numbers of possible performance measures can be selected within the context of sustainable transportation. Selection of appropriate performance measures is a very important task because poor performance measures can lead to poor decisions and poor outcomes. Performance measures should align with strategic goals and objectives. Typically, each objective needs only one or two performance measures. Performance measures need to be appropriate to the type of analysis (planning, operational, or strategic), level of analysis (project, local, or regional), and the specific purpose for which the measure should be used (system performance, project selection, or impact assessment). The four-R test – relevant, robust, repeatable, and responsive provides a broad indication about whether a measure will be effective.

Several studies have identified indicators or performance measures for sustainable transportation. In a current initiative, the Transportation Research Board (TRB) Subcommittee on Indicators (ADD40-1) has prepared a preliminary report on possible indicators for sustainable transportation.

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Applying Sustainable Transportation

Decision making in the context of sustainable transportation can take several forms: policy evaluation, project selection, alternatives evaluation, and impact assessment. It can also take the form of evaluating progress toward sustainability goals. Decision making is generally based on quantified performance measures that are used individually or in combination in the form of indexes. Performance measures are quantified using collected data and equations describing the various performance measures.

The more common of the two types of decision making is the evaluation of progress toward sustainability goals. Most state DOTs perform this type of evaluation to some extent to track the progress toward their goals. These evaluations are normally to track trends over time, compare with predetermined benchmarks, and compare between different areas. Washington and Hawaii have examples of such applications.

Decision making for sustainable transportation where alternatives are assessed is still in the early stages of development. Multi-criteria decision making is an example of a methodology that can be used in this context to evaluate the multiple and often conflicting objectives of sustainability. Recent examples of using multi-criteria decision making for sustainable transportation can be found at the regional level, corridor level, and street level. (For more information, see Applications of Portfolio Theory and Sustainability Metrics to Civil Infrastructure Management Project Website (regional level); Transportation Corridor Decision-Making with Multi-attribute Utility Theory, International Journal of Management and Decision Making, 2006 (corridor level); and Green Highways Partnership website (street level).)

Applying sustainable transportation goals depends heavily on the specific scale of application. The following are examples of applications of sustainable transportation at the regional level, local level, or project level:

  • Regional level: At the regional level the authority is concerned with broad issues of sustainable transportation such as greenhouse gas reduction, environmental protection, economic development, and quality of life. Decisions at this level generally apply to the demand side of transportation. For example, decisions relate to aspects such as modes of transportation, selection of corridors, travel demand management, and policies related to alternative fuels. In other words, how can the sustainability goals be met at the regional level? 
  • Local level: At the local level the authority is concerned with more detailed supply side decisions once demand aspects have been addressed at the regional level. For example, decisions relate to aspects such as the optimum location of a bridge, provision of sidewalks and bike lanes, and scheduling of bus services. In other words, how can the sustainability goals be met at the local level?
  • Project level: At the project level the authority is concerned with detailed project level decisions once the specific infrastructure for the project has been determined. For example, decisions relate to aspects such as final alignment of the road, materials that can be used for recycling and energy conservation, and fuels that can be used in construction equipment to reduce emissions.

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