Trade-Off Considerations In Pavement Sustainability
Since sustainability is a broad systems characteristic encompassing virtually every impact a system has, most pavement features and qualities can be argued to support sustainability goals in one way or another. However, it is unlikely that all such features can be included in a pavement, either because some features support one sustainability objective but are in opposition to another, or because some features are mutually exclusive. For instance, an open-graded friction course (OGFC) may be desirable because it reduces tire-pavement noise and provides health benefits to the surrounding area (supports the social/equity component), but the same surface may also have a much shorter performance life (especially in the presence of studded tire wear), which would make its life-cycle cost substantially higher than a more traditional dense-graded AC surface (in opposition to the financial portion of the economy component). As another example, it may be desired to incorporate recycled materials in a rural paving project, but the nearest source of recycled material is 100 mi (161 km) away while an acceptable local extracted material is only 5 mi (8 km) away. In these instances, it is necessary to analyze the available options within the context of sustainability in order to make the best choice.
Essentially, this choice between multiple alternatives represents a consideration of "opportunity cost," the cost of an alternative that must be foregone in order to pursue a certain action (Investopedia 2012). In the previous example, if the local extracted material is selected in favor of the non-local recycled material, the difference in value between the two represents an opportunity cost. The difficulty is in determining the value of the alternatives in a sustainability context. In classic economics, value is usually expressed in monetary units (i.e., dollars). However, value in a sustainability context can have many different metrics expressed in many different units, some of which may be controversial or difficult to quantify. Some examples of sustainability value include life-cycle cost, greenhouse gas (GHG) emissions, energy use, water/air quality, waste generation, scenic views quality, art, community context, history, habitat continuity, and performance life. Historically, the value of alternative pavement features has been overwhelmingly based on economics, often being based on initial construction cost alone. While important, initial cost represents an incomplete view of the overall costs and benefits of a particular feature. Even standard life-cycle cost analysis (LCCA) [see Chapter 10 (.pdf) of the Reference Document for more details] procedures tend to ignore benefits and costs that are not easily monetized.
Ultimately, this consideration of trade-offs is essentially a benefit/cost analysis done in a more holistic sense (i.e., considering more than just economics). This section describes considerations when contemplating trade-offs for pavement sustainability best practices. Or, put differently, this section describes a few key items to be considered when conducting a benefit/cost analysis of sustainable pavement features. Even if benefits and costs are difficult to quantify, it is important to use a consistent approach in analyzing trade-offs to avoid introducing unintended bias. In general, these considerations involve the following: priorities and values of the organization or project, performance, cost, impact magnitude and duration, and risk. None of these considerations is new, so this section amounts to a formal articulation of what they are. These basic trade-off considerations are referenced throughout this document.
Priorities and Values of the Organization or Project
Since sustainability is such a broad system concept, most pavement features support some component goals and may be in opposition to others. Thus, judgment on the sustainability value of a pavement feature depends on the relative value of sustainability components. Therefore, organization or project goals and priorities should be considered in evaluating trade-offs. Ideally these goals and priorities should indicate (1) which sustainability components an organization or project particularly values, (2) an order of precedence for these values, and (3) a plan to operationalize those values and precedence. If sustainability goals and priorities exist and are clearly articulated, the first order trade-off consideration is to favor the feature that best supports those values.
In some cases, life cycle assessment (LCA) [see Chapter 10 (.pdf) of the Reference Document for more details] can be used to quantify and compare environmental impacts, while in other cases quantification is difficult, if not impossible. In these cases, it may be enough to determine the general duration of impact (that is, does it occur just during construction or is it over the entire life of the pavement) in order to make a decision.
All pavement sustainability choices involve an amount of risk. Generally, "risk" means that there is some uncertainty regarding the impact and cost of a selected alternative and such uncertainty leaves open the possibility of less desirable outcomes than predicted on average. For instance, a composite pavement may be selected as the preferred alternative because it results in the lowest life-cycle cost among alternatives considered. However, if inadequate bonding is developed between the surface and underlying layers, it may be that performance life is substantially reduced, resulting in a much higher life-cycle cost. Metrics that provide a probabilistic-based analysis (e.g., RealCost [FHWA 2011], Construction Analysis for Pavement Rehabilitation Strategies-CA4PRS [Caltrans 2008]) can help quantify risk due to uncertainty. Some metrics, like LCA, are only now beginning to incorporate uncertainty into their analysis.
California Department of Transportation (Caltrans). 2008. Construction Analysis for Pavement Rehabilitation Strategies (CA4PRS) Caltrans 'Rapid Rehab' Software. California Department of Transportation, Sacramento, CA.
Federal Highway Administration (FHWA). 2011. Life-Cycle Cost Analysis Software. Federal Highway Administration, Washington, DC.
Investopedia. 2012. Opportunity Cost. Investopedia, New York, NY.