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Transportation Sector

Bumper-to-bumper traffic jams the I-25 corridor in Denver, Colorado, U.S.A. Source: National Renewable Energy Laboratory.
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Transportation accounts for 27% of energy use in the U.S. (EIA, 2019) and is the largest contributor of greenhouse gases, accounting for 28% of U.S. emissions (EPA, 2019). Globally, transportation accounts for 19% of end-use energy consumption and 14% of global emissions (EPA, 2019). Transportation energy efficiency can be improved through two primary strategies – 1) improving individual vehicle efficiencies and 2) improving overall transportation system efficiencies.

Share of U.S. Transportation Sector GHG Emissions by source, 2018 (EPA, 2020).


National Renewable Energy Researcher working on co-optimization of fuels and engines (CO-OPTIMA) to boost fuel economy and vehicle performance, while reducing emissions. This work advances the underlying science needed to deliver better engines and fuels sooner. Source: National Renewable Energy Laboratory.

Individual vehicle efficiencies can be improved through fuel efficiency enhancements, reducing vehicle weight during the design phase, use of alternative fuels, and through cognizant driving. The Department of Energy Office of Energy Efficiency and Renewable Energy (EERE) has been carrying out research to improve fuel efficiencies with a specific focus on combustion engine drives, lubricant technology and vehicle light-weighting. Light-weighting has proven to be especially effective, with research suggesting that a 10% reduction in body weight can improve fuel efficiency by 6-8% (Scientific American, 2012). Alternative fuel vehicles and electric vehicles also have significant primary energy consumption reductions per mile through lightweighting.

Driving speeds can also have an effect on fuel efficiency; the following chart shows how driving speeds influence fuel economy by vehicle type. Policies and private programs (such as those offered through car insurance companies) could incentivize efficient driving speeds. 

Fuel economy at various driving speeds and by vehicle type. Source: U.S. DOE EERE, Alternative Vehicles Data Center.

The transportation system as a whole can be made more efficient through a slew of measures, including mass transit improvements, ride-sharing programs, effective city planning that promotes walking and biking, and advanced transportation such as autonomous and connected vehicles.

Well-utilized mass transit drives energy and emissions savings across the transportation system by substantially reducing per capita energy consumption. The following table represents the passenger-miles-per-gallon values for various transportation methods. In high usage-rate scenarios, mass transit is the clear winner from an energy efficiency perspective.

Passenger-miles per gallon by scenario and transit type. Source: U.S. DOE EERE, Alternative Vehicles Data Center

Ride sharing, including ride hailing, can increase the utilization rate of individual vehicles and has the potential to reduce the number of cars on the road. That said, ride-hailing can inadvertently detract from mass transit utilization as well as walking and biking, as detailed by a UC Davis report. Ride-hailing can also drive up emissions through “deadheading,” when a ride-hailing vehicle travels without a passenger between hired rides. This climate risk can be curtailed through electrifying ride-hailing vehicles, which can cut emissions between 50-80% (Union of Concerned Scientists, 2020).

Connected and autonomous vehicles (CAVs) are the future of transportation: vehicles that require no interaction from passengers but interact with one another to ensure smooth transportation. CAVs can be energy intensive to operate given the physical fixture requirements and large amounts of data transmitted. Continuous research is being carried out on CAVs across the globe. Research carried out at the University of Michigan has shown that CAVs could enable a base-case energy use reduction of 9% (Gawron et al., 2018). The energy efficiency benefit of CAVs can be offsetted, however, by the increase in their use (“rebound effect”), since lower fuel costs and time costs of travel can induce increases in travel and energy use. They can also lead to “home-shifting decisions,” encouraging people to sprawl to suburbs and exurbs (Taiebat et al., 2019). 

Levels of interactions between CAVs and the environment with positive (+) and negative (-) impacting mechanisms. Source: Taiebat et al., 2018.

An important factor to consider is that, through systemic efficiency enhancements to transportation, the overall access and reliability of transit can be increased. Communities of color have shown to be affected the most by unreliable transportation. Having systematic improvements such as enhanced mass transit access would have life-changing effects. Multiple states in the U.S. have policy recommendations to make transportation more equitable. This PolicyLink report sheds light on transportation inequities and offers policy solutions to curb them.


Wind turbine blades being transported by train through Denver, Colorado, U.S.A. Source: National Renewable Energy Laboratory.

Heavy duty and long-distance transport have long posed a significant challenge to those seeking to decarbonize the transportation sector: Aviation, heavy-duty road transport (i.e. trucking), and shipping (maritime and rail) collectively account for over 12% of all global anthropogenic emissions. As in many subsectors, no single solution will be sufficient to meet decarbonization goals for the highly heterogeneous long distance shipping and freight (which includes heavy duty trucking, maritime shipping, and freight trains), but efficiency improvements will be critical.

Heavy-duty vehicles (HDVs) such as long distance trucking represent more than 60% of energy consumption and fuel use in freight transportation globally, reaching as much as 80% in some regions. HDVs account for less than 5% of the global vehicle fleet on the roads but 40% of energy consumption. Many HDVs rely on diesel fuels which contribute a fifth of anthropogenic black carbon emissions, a carcinogen that worsen urban air quality with severe human health impacts (Miller & Jin, 2018). These disproportionate contributions make HDVs an effective target for fuel consumption and emissions control, contributing to the reduction of climate impacts and costs for truck operators. Vehicle simulation tools can be used by fleet or green freight managers to estimate the potential fuel savings of truck technologies without extensive real-world testing (ICCT, 2019).

Share of freight energy consumption by mode in 2015 (ICCT, 2019).

Maritime shipping also has an outsized impact on global energy consumption and emissions. It accounts for 3.5% of all global emissions, and is forecasted to increase by 150-250% by 2050 if unmitigated. To stay below 2°C warming by 2100, collective international maritime emissions need to decrease by 85% by 2050. This high target may seem daunting, but reduction in emissions at that scale is possible by 2050 with existing technologies. Opportunities for maritime emissions reductions include improvements in vessel size, hull shape, ballast water reduction, hull coating, hybrid power propulsion, propulsion efficiency devices, speed optimization, and weather routing. If implemented at scale, these measures alone could account for 55% reduction in maritime emissions (Bouman et al., 2017). Additional opportunities include measures such as reduced speed in fairway channels, reduced turnaround time for ships at berth (time at berth accounts for half of port emissions), and portside availability of alternative and renewable fuels (Styhre, et al., 2017ICCT, 2020).

The International Maritime Organization (IMO) regulates international shipping and has set GHG reduction standards of at least 50% reduction by 2015 (compared to 2008) for oceangoing vessels, and 70% reductions by 2050 when only considering CO2 emissions (ICCT, 2018). The following chart shows these scenarios:

CO2 emissions from international shipping under IMO’s initial strategy (green and blue lines) vs. Business As Usual (BAU), with cumulative emissions from 2015 to 2075 (ICCT, 2018).