Greenhouse Gas Emissions and Mitigation

GHG emissions from transportation accounts for about 27% of GHG emissions in the U.S., second only to electricity generation (31%) (2013 USEPA). Within the transportation sector, light-duty vehicles were the largest category (60%) and medium- and heavy-duty trucks made up the second largest category (23%). From 1990-2013, GHG emissions in the transportation sector increased more in absolute terms than any other sector (i.e. electricity generation, industry, agriculture, residential, or commercial). (U.S. Transportation Sector Greenhouse Gas Emissions 1990-2013, U.S. EPA)

Transportation energy and climate change are closely intertwined since GHG emissions are a directly related to energy consumption. Global transportation energy demand is expected to increase substantially without implementation of aggressive policies, and could potentially be the fastest-growing end-use sector.

For the most part, efforts to meet energy use reduction goals also will reduce transportation’s GHG emissions.

GHG Reduction Strategies

Strategies to reduce transportation GHG emissions and energy include the following:

    • Vehicle technology strategies that reduce fuel consumption, including fuel efficiency improvements and engine technologies;
    • Fuel technology strategies using alternative transportation fuels such as biodiesel, compressed natural gas, electric vehicles and hydrogen fuel cell vehicles;
    • Land use planning and multimodal transportation strategies that reduce emissions through more compact land uses or changing travel behavior to reduce vehicle trips using transit, rail, ride-sharing, and telework;
    • Regulatory instruments and pricing strategies, including congestion pricing, pay-as-you-drive insurance, mileage-based user fees, tolls and fuel taxes;
    • Vehicle operations and highway system management strategies such as speed management, congestion relief, incident management, traffic smoothing, traveler information services, and other logistics improvements as well as more fuel-efficient “eco-driving” practices; and
    • Construction, maintenance and operations strategies that reduce GHG emissions, such as upgrading construction equipment; different pavement practices and mixes; work-zone management; and energy efficient practices for traffic lighting, transportation buildings, mowing and roadside vegetation management.

New York State DOT’s Climate Change and Energy Initiative is an example of the wide range of efforts transportation agencies are taking to reduce greenhouse gas emissions and reliance on petroleum products.

Conducting GHG Analyses

Transportation agencies use a variety of methods to estimate GHG emissions and analyze GHG reduction strategies, including the following examples:

FHWA Handbook

FHWA has developed a handbook detailing a variety of step-by-step procedures to help transportation agencies of all levels of technical capability evaluate the GHG implications of their plans.


For states with modeling expertise and advanced analytic needs, FHWA has produced the Energy and Emissions Reduction Policy Analysis Tool (EERPAT). The tool is designed to “assist state transportation agencies with analyzing greenhouse gas reduction scenarios and alternatives for use in the transportation planning process, the development of state climate action plans, scenario planning exercises, and to measure the reduction potential of various transportation strategies to meet state greenhouse gas reduction goals and targets.” EERPAT is based on the GreenSTEP model developed by the Oregon DOT.

Infrastructure Carbon Estimator, GreenDOT

Tools are also available to support analysis of GHG emissions associated with the construction and maintenance of transportation facilities. FHWA’s Infrastructure Carbon Estimator is a spreadsheet tool that estimates the lifecycle energy and GHG emissions based on simple data inputs related to projects or transportation plans. For projects with detailed engineering analysis, the National Cooperative Highway Research Program (Project 25-25/Task 58) developed a spreadsheet-based calculator tool, the Greenhouse Gas Calculator for State Departments of Transportation (GreenDOT). The tool estimates CO2 emissions from state DOT construction, maintenance, and operations, including emissions from electricity used in roadways, on-road vehicle fleets, emissions from off-road equipment, and embodied energy in materials used during construction.

Considering GHG Emissions at the Project Level

In 2016, the White House Council on Environmental Quality (CEQ) issued final guidance for how federal agencies should consider GHG emissions and climate change in project-level National Environmental Policy Act (NEPA) documentation.

The guidance urges agencies to quantify an action’s projected direct and indirect GHG emissions, taking into account available data and suitable GHG quantification tools. It also recommends that agencies use projected GHG emissions as a proxy for assessing potential climate change effects when preparing a NEPA analysis for a proposed agency action. For more information, link to the final guidance.

In 2008, Washington State DOT (WSDOT) first issued guidance for addressing project-level GHG emissions in environmental documentation, including a standard process and template. WSDOT has separate guidance on climate change evaluations on impacts of climate change to transportation infrastructure. The WSDOT Environmental Procedures Manual provides direction to project teams on addressing climate change and project-related GHG emissions in EAs and EISs.

Extreme Weather and Climate Impacts in Transportation

Projected changes in climate conditions and increasing instances of extreme weather events have become an ongoing challenge for transportation agencies charged with ensuring the long-term safety and functionality of surface transportation systems. According to the 2014 National Climate Assessment, in the coming decades “all regions and modes of transportation will be affected by increasing temperatures, more extreme weather events, and changes in precipitation.”

Current and future climate impacts that may affect transportation systems include rising sea levels, increased precipitation in some areas and decreased precipitation in other areas, higher risk of drought and wildfires, stress on ecosystems, and greater risk of flooding.

While infrastructure in coastal areas is particularly vulnerable to sea level rise, storm surge and flooding, inland states also are facing increasing impacts from extreme events, including riverine floods, winter storms, wildfires, and landslides.

Impacts to infrastructure include buckling of pavement from heat or freeze-thaw conditions; damage to culverts and roads, tunnels, bridges and transit systems from flooding, landslides, and erosion; inundation of infrastructure from sea level rise; bridge damage from high winds and scour from runoff and extreme river discharge. Transportation systems also face increasing road closures, transit system shut downs, increasing traffic disruption and emergency evacuations.

The National Climate Assessment notes that transportation agencies can build resilience to these potential impacts through a range of adaptation options. These include use of alternate routes around damaged elements and shifting the public to undamaged modes, new and improved infrastructure designs for future climate conditions, asset management programs, protection of at-risk assets, operational changes, and abandoning/relocating infrastructure assets that would be too expensive to protect.

“As new and rehabilitated transportation systems are developed, climate change impacts should be routinely incorporated into the planning for these systems,” the report noted.

Details on the expected changes in climate conditions and extreme weather, as well as the likely impacts on the highway system, are outlined in a comprehensive research report and practitioner’s guide produced under the National Cooperative Highway Research Program. (see NCHRP Report 750: Strategic Issues Facing Transportation, Volume 2: Climate Change, Extreme Weather Events, and the Highway System: Practitioner’s Guide and Research Report)

The guide outlines “the steps that should be taken if transportation officials want to know what climate stresses the transportation system might face in the future; how vulnerable the system will likely be to these stresses; and what strategies can be considered to avoid, minimize, or mitigate potential consequence.”

It recommends an eight-step diagnostic framework transportation agencies can use to undertake an adaptation assessment, including the following steps:

    1. Identify key goals and performance measures for the adaptation planning effort.
    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).
    8. Conduct site analysis or modify design standards (using engineering judgment), operating strategies, maintenance strategies, construction practices, etc.

Vulnerability and Risk Assessment

The Federal Highway Administration (FHWA) has been very active in research and developing tools to address the impacts of extreme weather and changing climate conditions.

FHWA has suggested a range of actions transportation agencies can take to build resilience into transportation systems. Key among these is assessing the vulnerability of transportation systems and assets. FHWA’s Vulnerability Assessment Framework provides a guidebook and online resource that outlines the key steps in determining vulnerability, including tools, case studies, videos, and other resources for transportation agencies. The framework has been used by numerous state DOTs and metropolitan planning organizations through FHWA’s Climate Change Resilience Pilots, and the results of many of those studies are documented on FHWA’s Climate Adaptation case studies web page. As of late 2015, FHWA has documented 24 state DOTs and 30 MPOs that have conducted vulnerability assessments of their highway systems to address climate change and extreme weather events.

FHWA conducted a comprehensive study of the Gulf Coast region, including development of methods and tools for evaluating vulnerability and adaptation measures that are available for other transportation agencies. Tools developed under the study include the Sensitivity Matrix, Guide to Assessing Criticality in Transportation Adaptation Planning, CMIP Climate Data Processing Tool, and the Vulnerability Assessment Scoring Tool (see Gulf Coast Study Vulnerability Assessment Tools).

In addition, FHWA’s has provided technical guidance and methods for assessing the vulnerability of coastal transportation facilities to extreme events and climate change, focusing on sea level rise, storm surge, and waves (see Hydraulic Engineering Circular 25, Vol. 2: Highways in the Coastal Environment: Assessing Extreme Events (2014).

Incorporating Climate Risk in Planning, Design and Asset Management

Transportation agencies also are taking steps to incorporate climate risk into decisions related to design of transportation facilities as well as asset management processes and plans, and numerous tools have been developed or are underway to assist decision-makers.

FHWA urges agencies to use the 11-step process for engineering transportation assets to be more resilient to climate impacts developed under its Gulf Coast 2 Study. The process includes consideration of multiple alternatives and cost benefit analysis.

A tool to help transportation agencies incorporate climate risk into design practices was developed as part of NCHRP Report 750. The tool is a query-based system that provides specific guidance for assets including bridges, culverts, stormwater infrastructure, slopes/ walls, and pavement, and it is customizable based on climate threat.

Research conducted under the National Cooperative Highway Research Program has recommended a process for incorporating climate change and extreme weather into Transportation Asset Management Plans (see NCHRP 25-25, Task 94, Integrating Extreme Weather into Transportation Asset Management Plans.)

Incorporating Resilience in Project Planning and Development

In 2016, the White House Council on Environmental Quality issued final guidance regarding how federal agencies should consider GHG emissions and climate change at the project level in reviews conducted under the National Environmental Policy Act (NEPA). It calls on agencies to consider “both the potential effects of a proposed action on climate change, as indicated by its estimated greenhouse gas emissions, and the implications of climate change for the environmental effects of a proposed action.”

Where environmental analysis is required for projects, NCHRP Report 750 recommends that agencies consider factors including: the climate stressors that will affect the proposed action; the impacts of the stressors on the affected environment given expected changing climate conditions; the risk to the asset and affected environment; how the stressors will influence the proposed action; and potential strategies for protecting the function and purpose of the action.

Washington State DOT was one of the first state transportation agencies to issue project development guidance related to climate resilience, providing draft language to incorporate into project initiation documents and environmental reports. (Link here for Washington State DOT Guidance for NEPA and SEPA Project-Level Climate Change Evaluations and WSDOT’s separate guidance document on GHG evaluations focused on accounting for GHG emissions from transportation projects).

The transportation planning process also offers opportunities to address climate resilience early in the decision making process for transportation investments. These are discussed in FHWA’s report, Integrating Climate Change into the Transportation Planning Process.

Construction, Operations and Maintenance Activities

Construction, operations and maintenance activities face a range of impacts from extreme weather and climate events, and also play a key role in ensuring resilience of transportation systems.

Agencies may face changes in timing for construction activities, such as paving and changes in work hours for crews due to heat waves. More extreme precipitation events will necessitate better erosion and sedimentation control plans.

Operations and maintenance strategies that can help lessen extreme weather impacts include more frequent cleaning of storm-drains, improved plans for weather emergencies, closures and rerouting, traveler information systems, debris removal, early warning systems, prepositioning materials, damage repairs, and performance monitoring.

Numerous resources and research reports also are available from the Transportation Research Board’s Climate Change website.