AASHTO - American Association of State Highway Transportation Officials
AB 32 - California Assembly Bill 32
AR5 - Fifth Assessment Report
ARC - Atlanta Regional Commission
Caltrans - California Department of Transportation
CARB/ARB - California Air Resources Board
CH4 - Methane
CO2 - Carbon Dioxide
CO2e - Carbon Dioxide Equivalent
DOT - Department of Transportation
DRCOG - Denver Regional Council of Governments
DVRPC - Delaware Valley Regional Planning Commission
EERPAT - Energy and Emissions Reduction Policy Analysis Tool
EMFAC - California's Emissions Factors Model
EPA - Environmental Protection Agency
GBNRTC - Greater Buffalo Niagara Regional Transportation Commission
GHG - Greenhouse Gas
GTC - Genesee Transportation Council
GWP - Global Warming Potential
HFCs - Hydrofluorocarbons
HPMS - Highway Performance Monitoring System
IPCC - Intergovernmental Panel on Climate Change
LCDC - Land Conservation Development Commission
LRTP - Long Range Transportation Plan
MAP -21 - Moving Ahead for Progress in the 21st Century
MassDOT - Massachusetts Department of Transportation
MDE - Maryland Department of Environment
MOVES - Motor Vehicle Emission Simulator Model
MPO - Metropolitan Planning Organization
MTC - Metropolitan Transportation Commission
N2O - Nitrous Oxide
NAAQS - National Ambient Air Quality Standards
NCTCOG - North Central Texas Council of Governments
NJTPA - North Jersey Transportation Planning Authority
NYSDOT - New York State Department of Transportation
PBPP - Performance-based Planning and Programming
PSRC - Puget Sound Regional Council
RTAC - Regional Targets Advisory Committee
SACOG - Sacramento Area Council of Governments
SB 375 - California Senate Bill 375
SCS - Sustainable Communities Strategy
SHSP - Strategic Highway Safety Plan
SIT - State Inventory Tool
STIP - Statewide Transportation Improvement Program
TDM - Transportation Demand Management
TIP - Transportation Improvement Program
TPB - Transportation Planning Board
TRB - Transportation Research Board
VMT - Vehicle Miles Traveled
Climate models predict that the global climate will shift in a number of ways over the next century in response to continued emissions of greenhouse gases (GHGs). According to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), global mean surface temperature is likely to rise by 2.0°F to 8.6°F (1.1°C to 4.8°C) by 2100, based on different scenarios in which human-produced GHGs are either constrained by mid-century or continue to rise. As the ocean warms and the melting of glaciers and ice sheets accelerates, we are likely to see global average sea levels higher by 12 to 32 inches. Rainfall patterns are likely to change, with some parts of the world becoming wetter and experiencing more intense and frequent extreme precipitation events, and other parts becoming hotter and drier. The frequency and duration of heat waves is also very likely to increase. Climate changes are already clearly observable over the last century based on direct measurements and remote sensing from satellites, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. During the 20th century, GHG concentrations in the atmosphere have increased, the oceans have warmed, global sea levels have risen about 7 to 8 inches, and global average temperatures have increased by about 1.4°F. The IPCC has concluded that "[w]arming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia."
Most climate scientists now agree that increases in global concentrations of GHGs, largely attributable to humans, are the predominant cause of climate change. Human activities, such as driving cars, producing and consuming energy, and clearing forests, are significant contributors to GHG emissions, which are emitted into the atmosphere at a faster rate than they are absorbed back into the earth's land and water masses. The principal source of GHG emissions from human activities is the combustion of fossil based fuels, including oil, coal, and natural gas.
Climate change may have potentially catastrophic effects on both the natural and human environments as it disrupts ecosystems and threatens buildings, infrastructure, and human health. Expected shifts in climate may reduce crop yields, increase the risk of invasive species, exacerbate drought conditions, and threaten endangered species.
The built environment is also at risk. Human settlements in coastal and low-lying areas are particularly vulnerable to changes in sea level and to storm and precipitation events. These areas will almost certainly be at higher risk from flooding as the climate changes. Transportation infrastructure in particular will be threatened by shifts in the global climate. Changes in temperatures, precipitation, and water levels threaten to strain asphalt roadways, railroads, airports, and shipping lanes beyond the design conditions they were built to withstand.
According to the IPCC, limiting climate change will require substantial and sustained reductions of GHG emissions. Global GHG emissions must be reduced to 50 to 85 percent below year 2000 levels by 2050 to limit warming to 2.0°C to 2.4°C (3.6°F to 4.3°F). Limiting global warming to approximately 2.0° C is often considered a level that will minimize many of the worst effects of climate change. An increasing number of nongovernmental organizations and U.S. States are now calling for this scale of reduction in emissions. A short term target was identified by President Obama in 2009 when he made a pledge to reduce U.S. GHG emissions by roughly 17 percent below 2005 levels by 2020 if all major economies agreed to limit their emissions as well. This target was reaffirmed in the President's Climate Action Plan in 2013.
Transportation Greenhouse Gases
GHGs are heat trapping gases that are released into the atmosphere from a number of sources. The four main gases that comprise transportation GHGs are carbon dioxide (CO2), various hydrofluorocarbons (HFCs), nitrous oxide (N2O), and methane (CH4).
CO2, CH4, and N2O are all emitted via the combustion of fuels, while HFCs are the result of leaks and end-of-life disposal from air conditioners used to cool people and/or freight. Carbon dioxidemakes up the vast majority of transportation GHGs in the United States, at about 96 percent, weighted by 100-year global warming potential (GWP). CO2 is released into the atmosphere through burning fuels such as gasoline and diesel. HFCs are the second most important GHG from transportation comprising approximately three percent of GHGs measured in CO2 equivalent (CO2E). N2O comprises about one percent of GHGs and CH4 about a 0.1 percent.
The transportation sector directly accounts for roughly 28 percent of total GHG emissions in the U.S. It is also a significant source of indirect emissions through the extraction and refining of fuel, the manufacture of vehicles, and the maintenance of supporting infrastructure. In 2011, over 83 percent of direct transportation emissions came from on-road vehicles. Passenger cars and light-duty trucks (which include pick-up trucks and minivans) together make up the largest share of transportation GHG emissions at 61 percent (passenger cars at 43 percent and light-duty trucks at 18 percent). Medium and heavy-duty trucks, which are largely used in freight movement, made up 22 percent of transportation GHG emissions. Consequently, reducing transportation GHG emissions significantly will require reductions in emissions from on-road vehicles.
Transportation GHG Emsissions by Source
Most analyses have shown that a suite of strategies is necessary to make significant reductions in GHG emissions from transportation sources. It is important that States and MPOs identify the key drivers of GHG emissions in their areas and then analyze potential GHG reduction strategies to assess potential effectiveness in their specific State or regional circumstances.
Transportation GHG reductions can be achieved by implementing strategies in five broad categories:
Vehicle and fuel standards are regulated at the national level, but States also play a role. For instance, State DOTs and MPOs can influence State policies and take other actions such as education, supporting low-carbon fuel planning scenarios, and reducing fuel use by public fleets. Agencies can also support truck stop electrification, and related strategies to reduce vehicle fuel use and promote use of alternative fuels.
State Departments of Transportation (DOTs) and Metropolitan Planning Organizations (MPOs) can contribute to reductions in motor vehicle emissions through transportation system investments, such as multi-modal transportation options that reduce dependence on vehicle travel, and operational strategies and targeted capacity improvements to reduce recurring and non-recurring congestion. State and local governments can also implement a range of strategies, including those related to land use planning, incentives to purchase more energy efficient vehicles, or other pricing mechanisms. Moreover, agencies can reduce construction/maintenance-related emissions through policies and contracting requirements that include improving equipment fuel economy, using alternative equipment technologies and fuels, and using recycled and alternative materials, among others.
The purpose of this handbook is to assist transportation agencies in their efforts to reduce GHG emissions by integrating GHG emissions into a performance-based planning and programming process.
 IPCC, Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Quin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Climate effects are described for scenarios RCP4.5, RCP 6.0, and RCP 8.5
 IPCC, Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working
Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Also, supported by the IPCC Fifth Assessment Report.
 U.S. EPA. 2013. Climate Change Impacts and Adapting to Climate Change. http://www.epa.gov/climatechange/impacts-adaptation/.
 IPCC, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
 The President's Climate Action Plan. June 2013. http://www.whitehouse.gov/sites/default/files/image/president27sclimateactionplan.pdf.
 U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011, 2013.
 U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011, 2013.