Key Drivers of Emission Intensity in Transport Systems

Transport systems power our daily lives-moving people to work, goods to market, and connecting communities around the globe. But this mobility comes at a significant environmental cost. According to the U.S. EPA, transportation accounted for 29% of total U.S. greenhouse gas emissions in 2022, making it one of the largest contributors to climate change. Globally, the picture is similar, with CO₂ emissions from transport increasing by nearly 80% since 1990. Understanding what drives these emissions-and how to reduce them-is essential for anyone working toward climate solutions.

Emission intensity in transport depends on three interconnected factors: how much we travel (Vehicle Miles Travelled), how efficiently our vehicles use fuel (Fuel Economy), and how carbon-intensive our fuels are (Fuel Carbon Intensity). Each of these drivers offers different pathways for reducing transport’s climate footprint. Let’s examine each one in detail.

Vehicle Miles Travelled (VMT): the foundation of transport emissions 🔗

At its most basic level, emissions increase when people and goods travel more distances. Vehicle Miles Travelled (VMT) measures total travel demand and serves as a fundamental indicator for transportation planning. The U.S. Department of Transportation defines VMT as the total annual miles of vehicle travel divided by the total population in a state or urbanized area.

The challenge is stark: from 1990 to 2022, the number of vehicle miles traveled by light-duty motor vehicles increased by 47% as a result of population growth, economic growth, urban sprawl, and periods of low fuel prices. This growth has outpaced efficiency improvements, keeping total emissions elevated even as individual vehicles became cleaner.

Strategies for reducing VMT 🔗

Tackling VMT requires rethinking how we design cities, plan transportation networks, and organize our daily lives. Research from the Urban Institute shows that households living in the densest parts of metropolitan areas consistently produce lower carbon emissions than those in suburban, exurban, and rural areas because they have more nearby destinations, reducing driving time and enabling alternative travel methods like transit and walking.

The IPCC’s 2022 report projected that concentrating growth near transit in dense, mixed-use developments could reduce emissions by up to 26% compared to business-as-usual scenarios. This approach, known as transit-oriented development, puts housing, employment, and services within walking distance of public transportation.

The Coalition for Clean Air identifies several practical interventions: building protected bike lanes, sidewalks, and bus shelters, increasing the reliability of transit, lowering fares, and slowing down traffic in residential areas can encourage households to reduce their driving in favor of using transit, walking, or bicycling.

The concept of “15-minute cities”-where all basic needs are located within a 15-minute walk or bike ride-has gained momentum as a planning framework. Rather than requiring residents to drive for every errand, these compact urban designs integrate housing with schools, shops, healthcare facilities, and workplaces.

Beyond urban planning: behavioral and technological shifts 🔗

Remote work, which surged during the COVID-19 pandemic, offers another pathway to reduce travel demand. Washington State’s VMT analysis noted that as of 2021, the only time the state met VMT reduction goals for light-duty vehicles was during the COVID-19 pandemic -highlighting both the potential and difficulty of achieving sustained reductions.

Various jurisdictions have set ambitious targets. According to transportation research, Washington State aims for 30% VMT reductions by 2035 and 50% by 2050, while Oregon targets a 20% reduction in light-duty vehicle travel by 2040. Minneapolis has committed to reducing VMT by 40% by 2040 through increased non-auto travel and compact development.

Fuel economy: getting more miles per gallon 🔗

While reducing travel is important, improving how efficiently vehicles use fuel delivers immediate emissions benefits without requiring changes in travel behavior. Fuel Economy (FE) measures how far a vehicle travels on a unit of fuel-typically expressed in miles per gallon (mpg).

The CAFE standards story 🔗

The United States’ experience with Corporate Average Fuel Economy (CAFE) standards demonstrates both the power and limitations of regulatory approaches. According to NHTSA, CAFE standards were enacted by Congress in 1975 to reduce energy consumption by increasing the fuel economy of cars and light trucks.

The original legislation, passed in response to the 1973-74 oil crisis, had a near-term goal to double new car fuel economy from 13.6 mpg in 1974 to 27.5 mpg by model year 1985. The results were dramatic: fuel economy roughly doubled from a fleetwide average of 13.4 mpg in 1973 to 27.5 mpg ten years later.

However, progress then stalled. As American consumers developed a taste for larger vehicles, the standards were relaxed to 26 mpg for passenger vehicles and stagnated for a full two decades. This plateau illustrates a persistent tension: consumer preferences for larger, more powerful vehicles often work against efficiency gains.

Modern fuel economy standards 🔗

The 2007 Energy Independence and Security Act revitalized fuel economy regulation. The legislation shifted the one-size-fits-all standards to attribute-based standards for cars based on vehicle size and boosted federal agencies’ authorities. As a result, CAFE standards for light-duty vehicles rose by roughly 40% to 35 mpg by 2020.

In 2011, President Obama announced an agreement with thirteen large automakers to increase fuel economy to 54.5 miles per gallon for cars and light-duty trucks by model year 2025. These automakers, including Ford, GM, Chrysler, BMW, Honda, Toyota, and others, together accounted for over 90% of all vehicles sold in the United States.

By model year 2014, many of the program’s goals were being met, with the average new vehicle fuel economy reaching 30.7 mpg. Future improvements depend on continued technological innovation in engines, materials, and aerodynamics-but also on whether efficiency gains translate into fuel savings rather than increased vehicle size and power.

Technology pathways for better fuel economy 🔗

The EPA identifies several approaches for improving fuel efficiency: developing advanced vehicle technologies such as hybrid vehicles and electric vehicles that can store energy from braking; reducing the weight of materials used to build vehicles; and reducing aerodynamic resistance through better shape design.

Operating practices also matter. Driving sensibly by avoiding rapid acceleration and braking, observing speed limits, reducing engine idling, and improved voyage planning for ships through weather routing all contribute to fuel savings without requiring new vehicle purchases.

Fuel Carbon Intensity: the quest for cleaner fuels 🔗

Even with reduced travel and highly efficient vehicles, transport emissions cannot reach zero while burning fossil fuels. Fuel Carbon Intensity (FCI) measures the greenhouse gas emissions associated with producing and using a fuel-and changing it requires switching to fundamentally different energy sources.

According to the International Energy Agency, transport continues to rely on oil products for nearly 91% of its final energy, down only 3.5 percentage points from the early 1970s. Breaking this dependence is essential for deep decarbonization.

Low-carbon fuel options 🔗

Electric vehicles (EVs) represent the most mature low-carbon option for passenger transport. Statista research identifies EVs as the single most important technology for decarbonizing the sector, as they have lower lifecycle emissions compared to internal combustion engines. However, the emissions benefit depends heavily on how electricity is generated-EVs charged from coal-fired power plants offer smaller improvements than those using renewable electricity.

Biofuels offer another pathway, particularly for sectors difficult to electrify. UC Davis researchers note that biofuels are an important tool to help decarbonize our transportation system, and their role will likely grow in coming years, with new tax credits under the Inflation Reduction Act offering significant incentives for Sustainable Aviation Fuels.

Hydrogen holds promise for heavy-duty vehicles and long-distance transport where batteries remain impractical. When produced using renewable electricity, hydrogen can offer very low carbon intensity. However, scaling production and developing refueling infrastructure remain significant challenges.

The indirect land use change complication 🔗

Not all biofuels deliver the climate benefits they promise. The issue of indirect land use change (ILUC) has become one of the most contentious aspects of biofuel policy.

The European Environment Agency explains the problem: where pasture or agricultural land previously destined for food and feed markets is diverted to biofuel production, the non-fuel demand will still need to be satisfied either through intensification of current production or by bringing non-agricultural land into production elsewhere. When this process involves converting forests or wetlands with high carbon stocks, it can generate significant greenhouse gas emissions.

Studies published in academic journals found that GHG emissions released from ILUC could more than offset the direct GHG savings by producing biofuels and replacing liquid fossil fuels, creating a “carbon debt” with a long payback period. The estimates of this payback period vary widely across different feedstocks and modeling assumptions.

Policy responses have varied. In April 2009, the California Air Resources Board approved specific rules and carbon intensity reference values for the California Low-Carbon Fuel Standard that included ILUC impacts. The European Union has similarly established limits for biofuels with high ILUC risk, with these limits gradually decreasing to zero by 2030.

Second-generation biofuels and advanced alternatives 🔗

Not all biofuels carry ILUC risks. California’s analysis identifies feedstocks with minimal land use impacts: biofuels produced using waste products as feedstocks will have insignificant land use effects, including corn stover for cellulosic ethanol, native grasses grown on land unsuitable for agriculture, and waste stream feedstocks such as yellow grease, waste cooking oils, and municipal solid waste.

These second-generation biofuels, along with synthetic fuels produced using renewable electricity, represent the future of low-carbon liquid fuels-particularly important for aviation and shipping where electrification remains challenging.

Integrating the three drivers 🔗

Real-world emissions reductions require progress across all three dimensions simultaneously. Research from the Institute for Transportation and Development Policy found that ambitious zero-emission vehicle sales and avoid-and-shift measures together could avoid nearly 90 billion tonnes of cumulative road transport CO₂ emissions globally by 2050, representing about 60% of total mitigation potential.

The World Resources Institute emphasizes this integrated approach: transitioning to zero-emission transport requires a comprehensive suite of improvements addressing clean fuels, vehicle efficiency, how we build cities, and how we move people and goods.

The IEA’s Net Zero Scenario illustrates what success looks like: transport sector emissions must fall by around a quarter by 2030 even as transport demand continues to grow, requiring rapid electrification of road vehicles, operational and technical energy efficiency measures, and commercialization of low-emissions fuels.

The path forward 🔗

Reducing transport emissions is not about choosing between VMT reduction, fuel economy, and cleaner fuels-it requires pursuing all three simultaneously. Cities must be redesigned to reduce travel demand. Vehicles must continue becoming more efficient. And the fuels we use must shift decisively away from petroleum.

The tools exist. The EPA notes that average new vehicle fuel economy has improved almost every year since 2005, slowing the rate of increase of CO₂ emissions. Electric vehicle adoption is accelerating. Transit-oriented development is gaining support in planning departments worldwide.

What remains is political will, sustained investment, and individual choices that collectively reshape how we move through the world. The transportation sector’s emissions trajectory is not fixed-it responds to the policies we enact, the infrastructure we build, and the technologies we develop and deploy.

What do you think? Given that reducing VMT often requires significant changes to urban planning and lifestyle patterns, while improving fuel economy and fuel carbon intensity can happen through technological innovation, which approach do you believe offers the most realistic pathway to cutting transport emissions in your community? How might cultural attitudes toward car ownership and driving need to shift to support these changes?

References

1. https://www.epa.gov/ghgemissions/transportation-sector-emissions
2. https://www.statista.com/topics/7476/transportation-emissions-worldwide/
3. https://www.planetizen.com/definition/vehicle-miles-traveled
4. https://www.urban.org/urban-wire/reducing-transportation-emissions-through-land-use-policy-and-investments
5. https://www.ipcc.ch/report/ar6/wg3/
6. https://www.ccair.org/advocacy/vehicle-miles-traveled/
7. https://wsdot.wa.gov/sites/default/files/2023-06/VMT-Targets-Final-Report-June2023.pdf
8. https://www.vtpi.org/vmt_red.pdf
9. https://www.nhtsa.gov/laws-regulations/corporate-average-fuel-economy
10. https://dieselnet.com/standards/us/fe.php
11. https://www.ase.org/sites/ase.org/files/resources/Media%20browser/cafe-standards-factsheet.pdf
12. https://en.wikipedia.org/wiki/Corporate_average_fuel_economy
13. https://www.iea.org/energy-system/transport
14. https://its.ucdavis.edu/blog-post/making-policy-in-the-absence-of-certainty-biofuels-and-land-use-change/
15. https://www.eea.europa.eu/en/analysis/indicators/greenhouse-gas-emission-intensity-of
16. https://pmc.ncbi.nlm.nih.gov/articles/PMC3262259/
17. https://en.wikipedia.org/wiki/Indirect_land_use_change_impacts_of_biofuels
18. https://energy.ec.europa.eu/topics/renewable-energy/bioenergy/biofuels_en
19. https://ww2.arb.ca.gov/sites/default/files/classic/fuels/lcfs/iluc_assessment/iluc_analysis.pdf
20. https://itdp.org/2024/02/21/cutting-urban-emissions-with-policies-that-promote-alternatives-to-driving/
21. https://www.wri.org/insights/everything-you-need-know-about-fastest-growing-source-global-emissions-transport