Sustainability and Its Importance In Pavement Engineering
An ever-growing number of agencies, companies, organizations, institutes, and governing bodies are embracing principles of sustainability in managing their activities and conducting business. This approach focuses on the overarching goal of emphasizing key environmental, social, and economic factors in the decision-making process. In many ways, sustainability considerations are not new, since they were often considered indirectly or informally in the past, but recent years have seen increased efforts to quantify their effects and to incorporate them in a more systematic and organized fashion.
There are many reasons for this emphasis on applying sustainability, among which are a growing recognition of how human activity affects the environment (e.g., climate change, ecosystem changes, non-renewable resource depletion) and a better appreciation for considering key societal factors (e.g., land use, access, aesthetics) and economic considerations (net benefits, life-cycle costs) in decision making. Thus, a focus on sustainability reflects a commitment to address the entirety of impacts associated with human existence, not only in monetary terms but also in terms of environmental and social impacts. For example, greenhouse gas (GHG) emissions, a commonly used surrogate for assessing environmental sustainability, are known to trap heat in the atmosphere and contribute to climate change. The burning of fossil fuels (in manufacturing, electricity production, and transportation) is the largest contributor of GHG emissions, the most prevalent of which is carbon dioxide (CO2). According to the Environmental Protection Administration (EPA 2013), and using 2011 data as the basis, the transportation industry (including cars, trucks, aircraft, rail, ships, and pipelines) accounts for over 27 percent of all human-caused GHG emissions in the U.S. (see figure 1); this is second only to the amount of GHG emissions attributed to the electric power industry. In addition, the construction of transportation facilities also contributes to GHG emissions, which are represented as part of the industry section. As a result, any significant reductions in GHG emissions made in the transportation sector will have an effect on the total amount of GHG emissions in the U.S.
Figure 1. GHG emissions by economic sector in the U.S. (EPA 2013).
What is Sustainability?
Most definitions of sustainability begin with that issued by the World Commission on Environment and Development (WCED), often referred to as the Brundtland Commission Report (WCED 1987):
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 is focused on the concept of "needs" and the idea of limitations imposed by the state of technology and social organization on the environment's ability to meet present and future needs. In a shorter version of this, sustainability is often described as being made up of the three components of environmental, social, and economic needs, collectively referred to as the "triple-bottom line."
For many years, the economic component has been the dominant decision factor, but more recent years have seen the growing emergence of both the environmental and social components (even though there are some current limitations associated with their measurement and assessment). A focus on sustainability can then be interpreted in such a way that all triple-bottom line components are considered important, but the relative importance of these factors (and how each are considered) are case sensitive, very much driven by the goals, demands, characteristics, and constraints of a given project. Thus, "sustainable" in the context of pavements refers to system characteristics that encompasses a pavement's ability to:
- achieve the engineering goals for which it was constructed,
- preserve and (ideally) restore surrounding ecosystems,
- use financial, human, and environmental resources economically, and
- meet basic human needs such as health, safety, equity, employment, comfort, and happiness.
A "sustainable pavement" is, at present, an aspirational goal. That is, it is unlikely any pavement system based on current knowledge and technology could satisfy all or even most of the characteristics in the previous sustainability definition. However, continual improvement with an emphasis on each of these characteristics leads to more sustainable pavements, and, ultimately, to pavements that actually meet the rather demanding standards of sustainability. Progress towards sustainability may at first mean reducing bad outcomes (e.g., less pollution, reduced extraction of non-renewable resources, less waste). Further progress would transition to achieving a pavement system that is essentially a neutral player in the larger and surrounding systems (i.e., it does no harm). Importantly, however, progress should continue so that the pavements could ultimately produce positive outcomes (e.g., pavements that produce more energy than they consume, construction that restores more land than it uses). Current efforts at reducing the impact or amount of bad outcomes and improving efficiency should be viewed as good transitional strategies on the long path towards the ultimate goal of producing only positive outcomes. This interpretation of sustainability is substantially different, and ultimately more positive, than one limited to reducing negative outcomes.
Improving sustainability can be achieved through the adoption of "sustainable best practices"; these are practices that work to either (1) go above and beyond required regulatory minimums or standard practice, or (2) show innovation in meeting these minimums and standards. Because a pavement must exist and function within larger systems, practices that support sustainability must contribute to more sustainable systems and thus depend on context. As a result, a full accounting of surrounding systems and a pavement's influence on them is necessary in order to define the most appropriate sustainability practices associated with a particular pavement system.
Importance of Sustainability in Pavement Engineering
The nation's roadway system is one part of a transportation network that provides mobility and access to a range of users. The roadway network is not only important to the nation's overall economic vitality by providing for the movement of freight and commodities, but it also provides societal benefits as well (e.g., access to schools, services, and work; leisure travel; and general mobility). Pavements are an integral part of this roadway network. Pavements provide a smooth and durable all-weather traveling surface that benefits a range of vehicles (cars, trucks, buses, bicycles) and users (commuters, commercial motor carriers, delivery and service providers, local users, leisure travelers). Given their key role and widespread use, there is a unique opportunity to improve the sustainability of pavement structures with the potential to deliver tremendous environmental, social, and economic benefits. With regard to those components, listed below are just a few examples of how pavements can impact sustainability:
- Environmental component: energy consumption; GHG emissions; noise; air quality; stormwater treatment.
- Social component: safety (fatalities, injuries, property damage); smoothness; vehicle operating costs; GHG emissions; access, mobility; aesthetics.
- Economic: construction, maintenance, and rehabilitation costs; vehicle operating costs; crash costs.
Moreover, the current timing is such that transportation agencies and the general public alike are demanding increased consideration of sustainability principles and practices. This evolution in the role that transportation plays in society is well summarized as follows (AASHTO 2009):
Transportation's mission is no longer about just moving people and goods. It's much broader. Transportation fundamentally allows us to achieve economic, social, and environmental sustainability. Transportation supports and enhances our quality of life. As state transportation professionals, we need to model the way toward achieving a sustainable future...Sustainable transportation requires innovative approaches and partnerships like never before.
Transportation and highway agencies are already making advancements to improve and enhance overall sustainability. Recent years have seen significant strides being made to better align current practices and technologies with more long-term sustainable strategies. In fact, the pavement engineering community has adopted a number of technologies as a way of improving sustainability, such as the increased use of recycled materials in pavement structures, the incorporation of modified binders to increase pavement performance, and the development of rating systems to measure sustainability. At the same time, there is considerable research being conducted on energy use, GHG emissions, and other impacts associated with pavement materials and construction activities to support the development of life-cycle assessment tools.
Nevertheless, there are no universal characteristics or design features that describe a sustainable pavement. Although a general sustainability framework for pavement can be defined, it is context sensitive in that each situation is unique, with specific needs depending on the location, climate, available materials, facility type, required level of service, and so on, as well as on the overall goals of the organization. Furthermore, it is important to recognize that, in some cases, it may even be counterproductive to try to introduce certain features that are thought to be sustainable without a complete assessment; for example, trucking in recycled materials from a great distance when an acceptable local aggregate is readily available could actually have negative environmental consequences.See Chapter 1 (.pdf) and Chapter 2 (.pdf) of the Reference Document for more information.
American Association of State Highway and Transportation Officials (AASHTO). 2009. "Transportation and Sustainability Best Practices Background." Sustainability Peer Exchange - Center for Environmental Excellence (.pdf). American Association of State Highway and Transportation Officials, Washington, DC.
Environmental Protection Agency (EPA). 2013. Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2011. EPA 430-R-13-001 (.pdf). Environmental Protection Agency, Washington, DC.
World Commission on Environment and Development (WCED). 1987. Our Common Future: the Report of the World Commission on Environment and Development. Document A/42/427 (.pdf). Oxford University Press, University of Oxford, United Kingdom.