Climate Change, Extreme Weather Events, and the Highway System: Impacts and Adaption Approaches

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Parsons Brinckerhoff, now part of WSP | Parsons Brinckerhoff, led the development of NCHRP Report 750, Vol. 2, one of the first systematic efforts in the U.S. to identify a process for transportation agencies to: prepare for extreme weather events, manage agency operations during the event, and conduct post-recovery operations.

Over the past 10 years, the U.S. transportation community has become increasingly concerned about the impact of climate change and extreme weather events on transportation infrastructure and system resiliency. Partly in response to extreme weather events and major natural disasters such as hurricanes Sandy, Katrina, and Irene, massive flooding in the Midwest and large forest fires in the west, and in part due to a growing awareness of the potential threats of climate change described in research and policy studies, a growing number of transportation agencies are interested in understanding the risks associated with a changing climate.

A number of states, such as California, Massachusetts, and Washington, have legislative and executive directives for formally considering extreme weather and climate change factors in policy-making and agency decisions. At the federal level, the new Planning Order 5520 from the Federal Highway Administration (FHWA), the pilot studies on adaptation and vulnerability assessments supported by both FHWA and the Federal Transit Administration (FTA), and the new Executive Order on Establishing a Federal Flood Risk Management Standard and a Process for Further Soliciting and Considering Stakeholder Input (January 30, 2015) provide further motivation to better understand how adaptation measures need to be included in decision-making.

Although some question the projections of future climate conditions, most agree that the U.S. has experienced record extreme weather events over the past several years. The frequency and severity of such events have seemed to increase, infrastructure damage and community costs have risen, the impact of recovery costs on maintenance budgets and on regular operations activities continues to become more significant, and perhaps most importantly, public expectations of a transportation agency’s ability to recover the transportation system quickly and efficiently have increased greatly. In several instances, the recurring pressures on state transportation officials to prepare for, manage, and recover from extreme weather events have caused organizational change, the development of new management responsibilities (e.g., emergency management officials), the modification of standard operating procedures, and staff training in managing and administering recovery efforts.

WSP | Parsons Brinckerhoff led the development of National Cooperative Highway Research Program1 (NCHRP) Report 750, Vol. 2, which recommends steps that can be taken by transportation agencies to: prepare for extreme weather events, manage agency operations during the event, and conduct post-recovery operations.

NCHRP Report 750, Vol. 2 was one of the first systematic efforts in the U.S. to identify a process for investigating the vulnerability of transportation infrastructure to extreme weather events and, over the long-term, climate change. A successful vulnerability assessment as outlined in NCHRP Report 750, Vol. 2 incorporates an appreciation of the following:

Climate Change and Extreme Weather Events Present a Wide Range of Stresses on the Transportation System.

The report identified a large number of climate related stresses and resulting impacts that could affect the nation’s road network in the future. Thus, one of the first tasks of any climate change vulnerability assessment is an understanding of the types of stresses and impacts that are of most concern to the highway system managers. Depending on the type of environmental stress caused by climate change and extreme weather, a range of impacts on the highway system can be anticipated. These impacts include both impacts to the infrastructure itself (and thus how facilities are designed and constructed) and to operations and maintenance.

In addition to the direct effects of climate changes on highways, climate change will affect ecological dynamics in ways that will have implications for transportation systems. The report identifies the different types of impacts likely to be faced by transportation infrastructure, and the types of strategies that can be used to avoid or minimize the impacts. The strategies for dealing with climate change and extreme weather events will differ by functional activity within a transportation agency. For example, climate change adaptation can be considered in planning, environmental analysis, design, infrastructure retrofit, construction, operations, maintenance, emergency response and public outreach and communications. The report identifies the likely effects that a concern for climate change and extreme weather events will have on different units in a transportation agency.

Vulnerability Assessments of highways assets require several areas of expertise, including a level of engineering expertise that has at times been lacking in more policy and planning-oriented efforts.

Conducting substantive climate vulnerability assessments that can successfully lead to tangible and actionable measures is a challenge that requires experience and knowledge across a wide range of engineering disciplines. Vulnerability assessments and adaptation require an understanding of the design and operational performance characteristics of different types of assets and how these assets will respond to different types of climate and extreme weather stresses. For example, vulnerability assessments of the impacts of flooding benefit tremendously from expertise in hydrology and hydraulics and geotechnical knowledge is indispensable for slope vulnerability assessments.

Once vulnerabilities have been identified, decision-makers often want to know exactly what it means to apply an adaptation strategy to a particular asset or asset type. Cost, for example, is usually one of the most important factors for decision-makers interest and is a key factor in determining willingness to implement asset-related adaptation strategies. Likely effectiveness of different engineering strategies, under differing site and climatic circumstances, is another key factor.

Risk is a Key Factor in Vulnerability Assessments.

There is a growing understanding among researchers and highway officials that climate change and extreme weather events are a threat to many aspects of the highway system, which warrants the investigation of the specific risks it poses. Most agencies that are concerned about adaptation begin by conducting a risk assessment of existing assets.2 Most of these risk assessments remain largely qualitative and based on professional judgment, although the report presents different quantitative and qualitative approaches for considering climate change-related risks. Climate-related risk is more broadly defined in that risk can relate to impacts beyond simply the failure of the asset. It relates to the failure of that asset in addition to the consequences or magnitudes of costs associated with that failure. In this case, a consequence might be the direct replacement costs of the asset, direct and indirect costs to asset users, and, even more broadly, the economic costs to society given the disruption to transportation caused by failure of the asset or even temporary loss of its services (e.g., a road is unusable when it is under water).

An integrated risk assessment is performed on vulnerable assets with the assessment considering the likelihood of impacts and their consequences. These two factors are related to each other and their intersection determines the risk level facing an asset. Adaptation options can then be considered for high or medium risk assets while low risk assets are given lower priority.

Different Climate Change Stressors have Different Approaches and Analysis Methodologies.

From coastal storm surge along the Gulf Coast to high intensity stormwater runoff to melting of permafrost in Alaska, there is no one-size-fits-all solution to assessing vulnerability. How one analyzes the impact of extreme precipitation events and flooding, for example, is very different from how one would analyze higher temperatures/drought (resulting in more intense forest fires). For this reason, NCHRP 750, Vol. 2 was accompanied by a CD that allows transportation professionals to identify the type of environmental stresses of interest, the databases and approaches that could be used to analyze potential impacts, and the range of options available as part of the design process.

Data Availability and Quality is a Critical Foundation for Adaptation Analysis.

Every adaptation study depends on data that is not only reflective of actual conditions, but is readily available. This was noted in a recent meeting of all the FHWA adaptation pilot study grantees where data was universally recognized as the limiting factor in conducting the pilot studies as envisioned. Much of the data would come from existing databases, such as pavement type, previous flooding records, annual average daily traffic (AADT), etc., whereas other data would come from databases developed by others or would have to be generated.

The lack of engineering-relevant and spatially precise climate data and the uncertainty surrounding those data remain obstacles and will likely remain so for the foreseeable future despite the best efforts of climate modelers. This should not, however, be an excuse for inaction.

climate change highway systems

NCHRP 750, Vol. 2 developed a diagnostic framework that provides transportation professionals with a general step-by-step approach for assessing climate change impacts and deciding on a course of action. The framework, which can be applied at the systems planning level down to the scale of individual projects, consists of the following steps:

  1. Identify key goals and performance measures for adaptation planning efforts;
  2. Define policies on assets, asset types, or locations that will receive adaptation consideration;
  3. Identify climate changes and effects on local environmental conditions;
  4. Identify the vulnerabilities of asset(s) to changing environmental conditions;
  5. Conduct risk appraisal of asset(s) given vulnerabilities;
  6. Identify adaptation options for high-risk assets and assess feasibility, cost effectiveness, and defensibility of options;
  7. Coordinate agency functions for adaptation program implementation (and optionally identify agency/public risk tolerance and set trigger thresholds); and
  8. Conduct site analysis or modify designs, operating strategies, maintenance strategies, construction practices, etc.

This eight-step process is inherently a multi-disciplinary and collaborative one. It is not likely that a state transportation agency has internal staff capability on climate science. In most cases, these agencies have been working with the local university or the state climatologist in order to obtain such input. In many cases, the vulnerability and risk assessment process depends on local input on what is considered to be the most critical assets in an urban area. Importantly, the actions taken by local communities and governments, such as land use approval and street/drainage design, could have significant impact on the ability of state assets to handle larger loads, and thus the need for coordination.

Of particular interest, as agencies increasingly adopt transportation asset management (TAM) approaches, opportunities will exist to integrate consideration of weather risk and climate change into TAM objectives, data collection, performance measurement, monitoring, and resource allocation decisions. Over time, the integration of weather and climate information into TAM will help agencies make targeted investments or allocation decisions that will increase the resilience of the network and of individual assets to changing environmental conditions. A section in the report discusses the relationship between climate change and asset management.

The report concludes by discussing agency actions and initiatives. As noted, leadership is critical. Strong mandates (legislative or administrative) to consider adaptation and provide relevant data greatly encourage adaptation activities. That said, they need not be a prerequisite. Absent mandates, strong state or local leadership by individuals concerned about climate change can also spur action as is the case in most U.S. examples.

This article is adapted from “Strategic Issues Facing Transportation, Volume2: Climate Change, Extreme Weather Events, and the Highway System: Practitioner’s Guide and Research Report” by Michael Meyer, Michael Flood, Jake Keller, Justin Lennon, Gary McVoy, Chris Dorney. Readers can learn more about the series of reports at

1The National Cooperative Highway Research Program (NCHRP) is a forum for coordinated and collaborative research, which addresses issues integral to the state Departments of Transportation (DOTs) and transportation professionals at all levels of government and the private sector. The NCHRP provides practical, ready-to-implement solutions to pressing problems facing the industry. The NCHRP is administered by the Transportation Research Board (TRB) and sponsored by the member departments (i.e., individual state departments of transportation) of the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration (FHWA). Individual projects are conducted by contractors with oversight provided by volunteer panels of expert stakeholders.

2Vulnerability assessment looks at the likelihood that damage or some kind of disruption occurs to infrastructure given an extreme weather event. Risk assessment takes into account the actual likelihood of the event occurring along with the economic consequences of disrupted infrastructure.

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