By Chris McCahill
There is an important growing consensus around the need for achieving net zero carbon emissions by 2050. The White House, Ford Motor Company, and many state and local governments seem to be on board. Yet the transportation sector—still an afterthought in some bold plans—may be our biggest challenge. Transportation emissions continue to rise and now make up a larger share than any other sector.
“Zero emissions” is a lofty goal, and one worth aiming for, but this line of thinking has caused some in the transportation profession to lean heavily on widespread electrification for hitting the mark. Instead of focusing on that ideal outcome, however, it is important to think about what missing the mark could look like, so that we can take all the necessary steps to limit further environmental damage. We should be asking, for instance, what if our clean energy transition takes longer than we hope? What if gas-powered cars keep getting larger and less fuel efficient? And what if we experience another driving boom, spurred by outward growth and self-driving vehicles?
We at SSTI modeled several different scenarios to answer these questions and to understand how far off from our goal we might land in each case. The results (shown below) suggest that without considering factors like total vehicle miles traveled (VMT), we could be producing the same transportation emissions decades from now as we are today, or worse. This should be no surprise, since it is the same trend we have seen over the last 30 years.
A handful of state agencies have recognized that challenge and taken steps to manage VMT growth. A state law in California, for instance, requires state and local agencies to limit and mitigate any new VMT caused by developments or transportation investments, and a proposed rule in Colorado could have a similar effect. Meanwhile, a longstanding state law in Washington and advisory council recommendations in Minnesota are compelling their state DOTs to formulate more proactive VMT management strategies. Our analysis helps paint a picture of why steps like these could be so important.
Our approach is based on a recent study from Carnegie Mellon that argues for “travel budgets” as a critical consideration in reducing emissions by 80% in 2050. Focusing just on light-duty vehicles, which account for around 60% of transportation emissions, it found that if VMT rises to 4 trillion (a 37% increase), at least 73% of travel would need to be electrified to reduce emissions by 80%. If VMT drops to 2 trillion (a 32% decrease), less than half of travel would need to be electrified.
Unfortunately, even those ambitious targets are built on rosy assumptions. For instance, the study assumes that average CO2 emissions from electrical production will drop by 75%, compared to a 25% reduction projected by the U.S. EIA, and which the authors call “incompatible with the climate targets under consideration.” And recent industry forecasts estimate electric vehicles could account for 30 to 50% of total VMT in 2050, which barely puts us on the right track.
Assuming the electrical grid becomes at least 20% cleaner, and the average fuel economy of gas-powered vehicles improves by 46% (Scenario 1), we could expect emissions to drop by anywhere from 30 to 60% in 2050, depending on total VMT. The U.S. EIA forecasts an 18% VMT increase, in which case emissions could drop by around 43%. Keeping VMT flat or reducing it by 20% could help get emissions down by 50 to 60%. If VMT increases at the same rate it has for the last 30 years, however, emissions would only drop by about 30%.
The outcomes in Scenario 1 all depend on electric vehicles accounting for more than 40% of travel. However, if weak incentives and slow production persist, and we see an electric vehicle market closer to what the U.S. EIA anticipates (around 15% of VMT), emissions could drop by only 13 to 50% (Scenario 2).
Assuming we do stay on track to electrifying at least 40% of travel, the remaining 60% will still be gas-powered and the fuel economy of those vehicles will be critically important. The scenarios above assume those numbers will improve from around 25 miles per gallon today to 36 MPG in 2050. At the current 30-year rate, however, we could expect something closer to 29 MPG. The outcomes in that case are like those under a weaker EV market, except that rising VMT could cause emissions to hover around the same level for another decade before beginning to drop off (Scenario 3).
And in one of the worst-case scenarios—one with modest EV uptake and fuel economy improvements—emissions are likely to stay roughly where they are through 2050, or even increase, unless we can slow VMT growth (Scenario 4). Lowering VMT or keeping it flat, however, could help us cut emissions by 30 to 40%.
This is not to suggest that we cannot meet ambitious clean energy goals. Instead, it is to remind those leading the charge that no strategy should be left off the table. To learn more about taking a comprehensive approach, interested state officials can read a recent brief from the Colorado DOT or reach out to SSTI staff for opportunities to engage with peers and experts working in the field.
Our assumptions and methods
| Scenario | Light-duty VMT (trillions) | EV share of VMT | Average fuel economy (gas) |
| 2019 | 2.9 | 0.02 | 24.9 mpg |
| Scenario 1 | 2.3 to 4.1 | 0.42 | 36.4 mpg |
| Scenario 2 | 2.3 to 4.1 | 0.15 | 36.4 mpg |
| Scenario 3 | 2.3 to 4.1 | 0.42 | 29.4 mpg |
| Scenario 4 | 2.3 to 4.1 | 0.15 | 29.4 mpg |
We used an equation developed by Alarfaj et al. to estimate CO2 emissions from light duty vehicles, including their electric loss factor (L = 0.12 + 0.045), electric vehicle fuel economy (6 miles per kWh), gasoline fuel economy (36.4 miles per gallon in 2050), and gasoline carbon intensity (821 grams per gallon). We assume gasoline fuel economy in 2019 is 24.9 miles per gallon, based on estimates from the U.S. EPA. For modest fuel efficiency improvements, we assume gasoline fuel economy in 2050 is 29.4, based on the 30-year trend. We also assume the carbon intensity of electric power drops from 428 grams per kWh in 2018 to 329 in 2050, based on the U.S. EIA projections reported by Alarfaj et al.
We estimate the electric vehicle share of VMT from two sources. Our industry forecast is the average of three forecasts provided by the Columbia Center on Global Policy and our other forecast is from the U.S. EIA’s Annual Energy Outlook. We base our high VMT scenario on the 30-year trend in FHWA’s Highway Statistics (+40%) and our medium-high VMT scenario on the Annual Energy Outlook (+18%). We also include a constant VMT scenario and a 20% reduction.
Our historical estimates of light duty vehicle emissions are from the latest inventory of greenhouse gas emissions from U.S. EPA.