Category: Transportation and Climate Blog

Making Policy in the Absence of Certainty: Biofuels and Land Use Change

Biofuels are an important tool to help decarbonize our transportation system, and their role will likely grow in coming years. New tax credits authorized under the Inflation Reduction Act are being finalized; these would offer significant incentives for the biofuel alternatives to conventional jet fuel, so-called “Sustainable Aviation Fuels” or SAF. But it’s not always clear how sustainable these fuels truly are, or whether they offer a significant GHG advantage over petroleum. The proposed SAF tax credits limit eligibility to fuels that offer at least a 50% reduction in life cycle GHGs compared to petroleum and give additional incentives to those that exceed that threshold.

Biofuel technologies

Advanced fuel technologies – such as those that use inedible wastes, hydrogen, or fuels synthesized using renewable electricity – may eventually deliver very low carbon fuels, but they have yet to emerge at commercial scale. This means that current biofuel technologies that use food crops like corn or soybean oil, are likely to receive most of these tax credits for the next several years, at least. Assessing the GHG benefits of current biofuels requires a descent into the wonky world of lifecycle analysis—measuring the total emissions through farming, processing, and consumption. Much of this is fairly straightforward, by the standards of researchers and expert analysts, until one gets to the use and displacement of land.

While crop-based biofuels can reduce GHG emissions when used in place of petroleum fuels, they compete against food crops for arable land. When biofuel production increases demand for agricultural commodities like corn or vegetable oil, growers often meet this demand by expanding their planted area. Clearing land for cultivation releases much of the carbon stored in plants and soil into carbon dioxide, a greenhouse gas and the primary driver of climate change. This process of biofuel demand causing land conversion is known as Indirect Land Use Change (ILUC), and it has been a point of deep contention around biofuels over the past 20 years.

Experience has taught us that we ignore ILUC at our peril. European attempts to increase the use of biodiesel in the 2000’s overlooked the issue of emissions from land clearing. This led to widespread slashing and burning of tropical rainforest to expand palm oil plantations, much of it on sensitive high-carbon peat soil. Over 6 million hectares of tropical rainforest was lost in Indonesia and Malaysia between 2000 and 2012, demand for biofuels accounted for as much as half of this. Emissions from this conversion dramatically outweighed the benefits from additional biofuels.

Over the next few years, the key question for policy makers is: should we incentivize the consumption of more crop-based fuels, such as those made from corn or soybean oil and, if so, how much? Understanding the impacts of ILUC and getting the question right is essential to ensuring our climate policies actually reduce emissions.

All models are wrong, some models are useful

Estimates of ILUC arising from crop-based biofuel production are highly uncertain. The lowest estimates rate crop-based biofuels as up to 50% less carbon intensive than petroleum gasoline or diesel, while the highest estimates suggest that they’re several times more carbon intensive than petroleum.

Estimates of ILUC factors

Estimates of ILUC factors found in literature for a variety of fuels. Height of the gray bar represents the mean, black “x” represents the median, with uncertainty bars covering the full range of ILUC factors, and the numbers showing the number of studies contributing data points to each category. Red line shows the typical carbon intensity of petroleum. Source: Woltjer et al. (2017)

This uncertainty is due to three main factors:

  1. Complexity. Accurately modeling ILUC means accurately modeling the entire global agricultural commodity system, and the decisions of millions of individuals that make it up. Reducing this complex, dynamic system down to a single, fixed number is an impossible task, but nonetheless necessary to create a stable regulatory structure.
  2. Subjectivity. There’s no perfectly objective method for analyzing emissions from complex systems, like biofuel production or ILUC emissions. Modelers must set system boundaries and allocate impacts between products. Soybeans, for example, yield oil and high-protein meal, used as food for livestock or people. Assigning energy and pollution burdens to each of these is a subjective process. Two analyses of the same product can use equally valid assumptions and result in widely differing estimates of GHG impact.
  3. Calibration and validation. Developing ILUC models requires calibrating against real-world data about how growers make decisions about what to grow and where. These data are often incomplete, non-public, hard to interpret, or unavailable. More importantly, historical data don’t include future climate change impacts. We know that rising temperatures and changing weather patterns will make some highly productive growing areas too hot or dry to maintain yields, and other areas will become fertile. These factors will change how and where growers choose to expand cultivation – which means today’s ILUC model predictions will necessarily be calibrated with unrepresentative data.

Due to these challenges, estimates produced by any ILUC model will be rough representations of a dynamic system, based on subjective assumptions, and calibrated against data that does not reflect the world we’re trying to make policy for. In plain language: they’re going to be wrong. The thing is, there are a lot of different ways to be wrong, some of them worse than others. The question for policy makers in this case is: What’s the right way to be wrong

How to Make Decisions When Models Are Wrong

A perfect fuel policy would provide enough support to crop-based biofuels to maximize near-term decarbonization, but not so much that demand for agricultural products soars and exacerbates ILUC-driven emissions.

In a perfect world, we would use a perfect ILUC assessment to perfectly align incentives with real-world impacts, but that’s not possible in this case, so it helps to think through what happens when we’re wrong. Overestimating ILUC’s impact means biofuels will have higher carbon intensity scores on regulatory assessments, and so fewer of them will be eligible for credits and we would expect to see less of them enter the market; overestimating ILUC would therefore lead to consuming less than the theoretically optimal amount of crop-based biofuels. Underestimating ILUC’s impact means biofuels will have lower carbon intensity scores, more would be eligible for policy support and we would expect to over-consume them compared to a theoretical ideal.

Policy support for biofuel

If we get biofuel policy wrong in a way that causes us to under-consume biofuels, then we miss opportunities to reduce GHG emissions. Every billion gallons of renewable diesel made from soybean oil could offer the opportunity to reduce GHG emissions by about three million tonnes of CO2 equivalent.[1] This is not a trivial or meaningless loss. We know we must reduce emissions very quickly in coming years in order to achieve carbon neutrality by mid-century, and soybean oil based biofuels have demonstrated commercial success already.

There are significant GHG impacts of over-consuming biofuels, as well. Each billion gallons of soybean oil based renewable diesel requires about 15 million acres of land to grow – roughly the size of West Virginia. If this land were grassland in the U.S. Midwest before being converted to cultivation, the GHG emissions from land use change alone would be over two million tonnes of carbon equivalent, and much more if it were forested or land with high-carbon soil that was converted.

These direct GHG impacts are only part of the story, however. As we dig deeper into the risks associated with over- and under-estimation, critical differences arise.

  1. Timing. Carbon in land accumulates slowly but is lost quickly. Plants remove CO2 from the air over time, storing it as solid carbon and accumulating as organic matter in the soil. Converting natural land to cultivation releases that carbon in a matter of weeks to a few years. Once it’s lost, it often takes decades to recover, if it recovers at all.
  2. Diminishing returns. Higher-yielding land is likely to be converted from carbon-storing natural cover to crops first. Each additional million acres of cultivated land is likely to yield a bit less than those that were converted before it. As total demand increases, the additional land needed for every extra unit of production is expected to increase.
  3. Political momentum.  It is easier for politicians and regulators to give support to businesses than to withdraw it. Removing support can also reduce the effectiveness of future climate policy; if governments signal, via policy incentives, that investors should back biofuel projects and then quickly withdraw the incentives, those same investors would be justifiably skeptical about trusting other climate policy incentives. Starting off with lower levels of support and adding mores in the future, on the other hand, avoids sending mixed signals to the market.

These issues show that the risks entailed from under- or overshooting optimal biofuel production volume are not equal. Insufficient support for crop-based biofuels could cost us the opportunity to reduce emissions, but excessive support for them could be significantly worse. Land conversion causes a loss of stored carbon that cannot be reversed on a time scale that allows us to meet mid-century GHG targets. Once over-production of biofuels occurs, any attempt to correct it could lead to stranded assets and break financial markets’ trust that policy signals are a reliable guide when making investment decisions.

We know that ILUC analysis is difficult to get right, and we cannot count on developing a model to predict the optimum level of crop-based biofuel consumption any time soon. If we can’t be sure we’re right, we should make a decision that recognizes the profoundly asymmetric risks around biofuel consumption. The risks entailed with underestimating ILUC impacts are far worse than the risks of overestimating them, so we should err on the side of overestimation.

Developing a Risk-Aware Approach to Biofuels

In practical terms, this means the arguments over which ILUC model is best are not terribly helpful since every model is going to be inaccurate. Instead, we should look at the full range of ILUC estimates we get from the many approaches of ILUC estimation that have been put forward by researchers. We’re relatively confident that the correct answer lies somewhere within this very wide range. The risk-aware approach to biofuel policy is to select a number high enough that it’s very unlikely we underestimate the real value.

“High enough to not be an underestimate” is still not precise enough to be helpful. Other landmarks can help us get closer to a risk-aware ILUC impact estimate. We know, based on extensive scientific research, that fuels from wastes and residues generally have lower ILUC impact, and that fuels from palm oil are very likely to be worse than the petroleum they try to displace. An effective ILUC assessment should align with the ample scientific evidence on these topics.

Support for additional research and modeling can help us both narrow the range of uncertainty around this issue, as well as identify what ILUC impact value within that range should be chosen to yield the best possible result. Expert-driven consultative processes, like the National Academies committee on biofuel life cycle assessment or CORSIA workgroups developing sustainability and GHG assessment methods can serve a vital role here, not only as clearinghouses for the latest research in this space but also to allow stakeholders to engage with the process. These groups can find mutually acceptable approaches to some, though certainly not all, of the subjective decisions that are an unavoidable part of LCA.

Just as there is no perfect ILUC model, there is no simple solution to alternative fuels. We know, however, that it’s going to be virtually impossible to meet critical decarbonization targets without lower-carbon liquid fuels. Avoiding the worst impacts of climate change will require risk-aware policies that succeed without perfect modeling. In the case of crop-based biofuels, it’s quite clear that the worst risks arise when we underestimate ILUC impacts. We know that our attempts to quantify ILUC risk are likely to be wrong, but we also know that there’s a less-damaging way to be wrong in this case. Policy should reflect this reality.

Colin Murphy is the Deputy Director of the UC Davis Policy Institute for Energy, Environment, and the Economy, and the co-Director of the Low Carbon Fuel Policy Research Initiative there. Any statements or inaccuracies herein are solely the responsibility of the author and should not be taken as representing UC Davis or the Policy Institute.

The material in this blog was originally presented as part of the EPA National Center on Environmental Economics seminar series, a recorded version is available via the UC Davis Low Carbon Fuel Policy Research Initiative website. The author would like to gratefully acknowledge the assistance of Amber Manfree, Dan Sperling, and John Schmitz in helping develop and refine this material.


[1] Assuming 65 gCO2e/MJ carbon intensity (derived from approximate average CI of such fuels under California’s LCFS), and that each gallon of renewable diesel displaces 1 gallon of petroleum diesel with 91 gCO2e/MJ carbon intensity (approximate average of U.S. diesel supply).

On the Governor’s Desk: What 2023 Transportation and Sustainability Bills Did California’s Legislature Pass?

At the UC Davis Policy Institute for Energy, Environment, and the Economy and the Institute of Transportation Studies, we hope to shine a light on which transportation bills might yield benefits to our communities and the environment. Of the 2,600 bills introduced to the California State Legislature in 2023, here are some of the most important for transportation and energy.

UC Day

Top Bills about Public Transit

Senate bill (SB) 434 is sitting on the governor’s desk. It was introduced by a savvy transportation lawmaker, Senator Min (a former law professor at UC Irvine). This bill would allocate funds to district public transit operators for rider safety surveys and outreach efforts. The survey would collect data regarding street harassment experienced by riders from underrepresented populations. The bill expands on San Jose State University research that found that “sexual harassment experienced by riders on buses and trains leads to reduced use of public transportation.” This bill would require transit agencies to collect and publish information on harassment reported by populations of interest including females, particularly those of color, those with low-income, and those in LGBTQ+ communities.

Another notable transit bill on the governor’s desk is Assembly Bill (AB) 971, introduced by Assemblymember Lee, which would amend the Vehicle Code to reflect the term “transit-only” instead of “for the exclusive use of public transit buses” when granting exclusive use of designated traffic lanes. ITS research supports the fact that transit-only lanes can encourage more ridership by improving bus speed and reliability. However, this bill is more procedural than substantive. It falls short of giving transit agencies what they really need, (but which local government’s hold the key) which is more control over where transit-only lanes crosscut busy roadways.

Despite many previous attempts to advance fare-free transit, this is not the year for Assemblymember Holden, a long-time transit advocate, to win on this issue. Holden’s AB 610 would have established the Youth Transit Pass Pilot Program and provided free transit service to youths attending certain qualifying educational institutions. It failed in its very last committee. Youth fare-free transit passes can increase accessibility and transit use among students, according to UC Irvine researcher Jean-Daniel Saphores. So far, the state has not found long-term sustainable funding sources for such programs.

Top Bill about Bikes

Also on the governor’s desk is SB 381, introduced by Senator Min. If signed, it will fund research on electric bicycle safety and regulation. Electric bicycles are defined as bicycles with fully operable pedals and a motor of less than 750 watts. UC Davis researcher Dillon Fitch has shown that the availability of shared e-bikes can reduce vehicle miles traveled. More research is necessary to understand safety and regulatory challenges that might inhibit scaling of e-bikes.

Bills about Emerging Technologies

The Legislature is attempting to prevent safety issues with medium and heavy-duty automated vehicles (AVs) weighing more than 10,000 lbs. These vehicles are currently not allowed to operate in the state due to a previous ban from the California Department of Motor Vehicles (DMV). AB 316 would add more limitations to the existing ban, requiring a safety driver in all automated trucks. It would also require more collision and deactivation data reporting and require the DMV to submit a performance evaluation regarding AV technology to the Legislature in five years. Researchers at UC Davis are currently studying this topic to assess whether AV safety driver requirements are the best way to increase road safety in the state. AB 316 passed the Legislature and is on its way to the governor’s desk for a signature or veto.

SB 800, authored by Senator Caballero, would require Caltrans to establish the Advanced Air Mobility and Aviation Electrification Advisory Panel. The panel would be tasked with assessing current infrastructure, developing a three year plan to advance infrastructure, and promoting pathways towards equitable access to advanced air mobility services. Members would include government and industry representatives. UC Berkeley, in partnership with NASA, is pursuing research that looks at how urban air mobility could fundamentally revolutionize regional travel.

Bills about Carbon Offsets

AB 1305, a carbon markets bill, is on the governor’s desk. This bill would add clarity to public reporting of carbon offset purchasing, which is an area that could use additional public accountability mechanisms. A sister bill, SB 390, is also on the governor’s desk. It is similarly intended to provide clearer standards for voluntary carbon offsets by defining unlawful conduct and requiring sellers and buyers to provide explicit information. Improving the transparency of carbon offset is critical to the effective functioning of the State’s carbon market. These proposed bills aim to enhance the credibility and confidence of private investors within the green energy sector.

Some Wins and Losses for Electric Vehicles

Finally, it’s worth noting a few losses and one big win for California legislative actions on EVs. The winner first, AB 126 will extend certain vehicle fees till 2035 that fund state programmatic investment in a clean transportation future. There’s a few new EV priorities added to the updated program, including a requirement that half of funding goes to disadvantaged and low-income communities, and new requirements for data reporting for EV charging reliability. The bill requires new research led by the California Energy Commission to consider alternative funding strategies for ZEV infrastructure, and consider the equity impacts of the alternatives.

Other EV bills are worth noting, even though they didn’t make it out of the Legislature. These three bills failed to make it to the governor’s desk: 

AB 591, introduced by Assemblymember Gabriel, would have forced Tesla to open charging stations to the public and require universal connectors and public accessibility at almost all new and retrofitted EV charging stations, except those located at single- or multi-family residences. This bill would also have required CHAdeMO EV service equipment be maintained in good working conditions by owners for at least five years. UC Davis researcher Gil Tal suggests that charger reliability, including the prevention of highly disruptive charging failures, is a high priority for EV drivers. The challenge is that charger owners would prefer not to bear the costs of supporting charging standards for which there are very few vehicles on the road, such as CHAdeMO. This bill didn’t make it out of its second round of committees in time, but if legislators and stakeholders can find the right solution, it could still pass in next year’s legislative session.

Another EV bill worth noting is SB 529, which would have facilitated EV sharing services at affordable housing facilities. This type of program would have been transformational and expanded access to EVs in lower-income communities. UC Davis researchers Caroline Rodier and Brian Harold have done extensive work demonstrating that low-income EV carsharing in the San Joaquin Valley has successfully expanded access to EVs with short-term vehicle rentals to people living in low-income communities. These rental programs are a game-changer for rural community members, and will be an essential part of building an equitable and clean transportation system in California. This bill didn’t get the support it needed this year, but it was authored by Senator Gonzalez, Chair of the Senate Transportation Committee and champion for equitable transportation, so we will likely see this issue raised in future legislative sessions.

Another hot topic that didn’t advance through the Legislature was SB 425, introduced by Senator Newman. This bill would have expanded rebates for new electric pickup trucks. Rebates for electric pickup trucks would be $2,500 greater than rebates provided for other electric vehicles. This bill may not have advanced because additional research is needed to understand the role of EV pickup trucks in the overall clean vehicle fleet.

Conclusions and Looking Ahead

Governor Newsom has until October 14 to act on these bills. We’ll be watching out for which bills advance this year, and which topics may need more research and deliberation.


Mollie Cohen D’Agostino is Executive Director of the Mobility Science, Automation and Inclusion Center at the Institute of Transportation Studies at UC Davis

Colin Murphy is Deputy Director of the UC Davis Policy Institute for Energy, Environment and the Economy

Emily Chiu is a law student at the UC Davis School of Law

Laedon Kang is a Graduate Student at the UC Davis Transportation, Technology, and Policy Program

Dan Sperling is Founding Director of the UC Davis Institute of Transportation Studies, and distinguished Blue Planet Prize Professor of Engineering and Environmental Policy

Supporting California’s Move to Zero-Emission Vehicles: Creating a Viable, Large-Scale Fuel-Cell Vehicle and Hydrogen System

Hydrogen station

Photo: Adapted from Scharfsinn86 / Adobe Stock.

California is marching ahead with firm rules now in place for both light-duty and medium/heavy-duty vehicles to transition to zero emission stock by 2045. The State is requiring that all new vehicles sold from 2035 onward be “zero-emission vehicles” (ZEVs)—battery electric, plug-in hybrid, or hydrogen-powered fuel-cell vehicles. While battery electric vehicles currently dominate ZEV sales and discussions of the zero-emissions future, fuel-cell vehicles are expected to play a key role, especially in truck and bus fleets and some households. They offer a different set of strengths, such as extended driving ranges, fast refueling, and potentially greater payloads for trucks.

But creating an economically viable hydrogen system and scaling it up to meet 2035 targets will require massive investments over the next decade. While many initial investments have been made, there is no clear overarching strategy for what a full hydrogen system and supply chain infrastructure might look like in 5, 10, or 20 years. Some kind of system will be needed, given the projected needs of various sectors (transportation, industry, and buildings) and the need for low-cost renewable hydrogen to contribute to the goal of carbon neutrality by 2045 in California. Acknowledging the urgency of the moment, the state recently formed the ARCHES partnership to develop this system further.

For the past two years, a team at UC Davis has been working on the California Hydrogen Analysis Project to investigate potential future hydrogen systems and to assist in planning them through modeling. The current results of the project are described in detail in our full report. We modeled potential demands for hydrogen across sectors (with a focus on transportation), potential types and locations of hydrogen supply, and how hydrogen could be moved and stored between supply and demand locationss. We also analyzed the transportation sector, the electricity sector, and supply chains from production to end-use.

Our study has many findings across the various hydrogen sectors. Here are a few of the key findings and related policy recommendations.

Key findings

  • Transportation can lead hydrogen developments. California’s hydrogen system will need to be driven by growth in hydrogen demand from various end uses, and this growth can be led by transportation (especially by medium/heavy-duty road vehicles). By 2030 we estimate that road transportation, if properly incentivized, could create a hydrogen demand on the order of 500 metric tons per day. This should be sufficient to support development of a hydrogen production and distribution system that would be large enough to benefit from economies of scale.
  • Transportation is scalable. Rapid and incremental sales and adoption of light-duty and medium/heavy-duty fuel-cell vehicles, fostered by supply and demand-based incentives, can be supported by parallel growth of infrastructure to produce and distribute hydrogen. The decentralized nature of a transportation-focused approach can help to develop a regional hydrogen production/distribution network that can then be scaled with more stations and eventually other “offtakers”—i.e., end-users who contract to purchase hydrogen fuel when produced.
  • Strong early investment is needed. In the early years of developing hydrogen systems for transportation, many refueling stations will be needed to ensure adequate coverage so drivers can reliably find fuel as they travel. This can mean generally low utilization of stations and challenging station economics that may require policies to ensure profitability. The Low Carbon Fuel Standard (LCFS) credit systems, the Inflation Reduction Act (IRA) renewable hydrogen production cost credit, and other incentives can help. But the most important solution is to support investments in areas such as refueling stations and fleet vehicle purchases, which will quickly increase transportation demand.
  • On-going rapid scale-up should occur after 2030. Then, with lower hydrogen costs and prices available, the market should be able to further scale in a profitable manner to reach much higher fuel-cell vehicle shares and hydrogen demand. If fuel-cell vehicles succeed in growing to about 10% of light-duty vehicle shares and 25% of truck shares by 2045, hydrogen demand could be 10 times higher than in 2030, and refueling station numbers could eventually reach many hundreds or even thousands in California, depending on average station sizes.
  • Liquid hydrogen may play an important role. Currently all hydrogen is produced, stored, and moved as a compressed gas; but cryogenic liquid hydrogen may play an important role, especially for refueling large, long-haul trucks. Liquid hydrogen production/storage/station systems have significant advantages given their fuel density and potential for faster dispensing (even into gaseous storage on vehicles), particularly for vehicles, such as heavy-duty trucks, with a lot of hydrogen storage.

Policy Recommendations

The analysis has led to a wide range of findings and conclusions. Some of the most important are policy recommendations for the California Energy Commission and other agencies and stakeholders to consider. These include:

  • Set a new vision for 2030/2035. Work with other agencies and ARCHES to create a clearer vision for the fuel-cell vehicle and hydrogen market in California for the 2030-2035 timeframe, with specific targets for vehicles, fuel, and infrastructure. Align investments in all areas to grow all elements of the system in parallel.
  • Create new fuel-cell vehicle support systems. For example, the state should link incentives and rebates for fuel-cell vehicle purchases to their incremental costs over diesel vehicles, at least for the next 5 years, until market scale can be achieved. This could also be adopted for battery electric vehicles, to keep the system technology-neutral.
  • Build more and larger stations oriented to heavy-duty vehicles. The state should fund a minimum hydrogen station infrastructure to 2030 with increased emphasis on heavy-duty trucks and some stations (such as highway rest stops) that can provide for both light-duty vehicles and all types of trucks. For heavy-duty trucks, at least 50 high-volume stations (each with a capacity of around 10 tons/day) will be needed by 2030 to support a system of several thousand trucks. Larger and potentially more profitable stations are also needed. Defining these levels is key. The ARCHES partnership is developing targets and specific roll-out plans that state agencies should coordinate with and build upon.
  • Find Champion Fleets. Within the Advanced Clean Fleets policy system, find champion fleets to help support major uptake of specific numbers and types of trucks to ensure demand that aligns with a roll-out of hydrogen stations and supply growth to serve these vehicles.
  • Create a data/tracking system for fuel-cell vehicles and hydrogen systems as they develop and grow, to ensure that investments are aligned and the system is functioning as planned for all stakeholders. This system must be kept up to date with annual statistics on numbers and types of vehicles, their usage and performance, refueling infrastructure characteristics and performance, and a range of other information considered important to fleets and policy makers. This database should be publicly available and well supported by the state.

In summary, a hydrogen production and distribution system that serves the growth of fuel-cell vehicles and other end-uses in California will be key to slowing climate change. It should be both feasible and eventually cost-effective, but navigating growth over the next few years will be key. We will continue our research to support planning and informed policymaking.


For more the full report that this blog is based on and information on the ongoing hydrogen research at ITS-Davis, click here and here.

Lew Fulton is the Director of Sustainable Transportation Energy Pathways Plus.

Cutting US road sector GHG emissions by 90% or more by 2050 takes both ZEVs and low-carbon fuels

Big reductions in greenhouse gas (GHG) emissions from the transportation sector are needed to limit the magnitude of climate change impacts. Understanding what kinds of policy and market dynamics are at play can help us meet national goals. Our recent study shows that there is an interplay between policy, vehicle types, and fuel sources, and that early investment in zero-emission vehicles (ZEVs) could yield big savings and big reductions in GHG emissions, by 2050. Low-carbon fuels for non-electric vehicles will also need to play an important role.

While the United States has not formally adopted long term targets for the sales of ZEVs, including battery electric, plug-in hybrid, and fuel cell vehicles, the Biden administration is a 50% sales share of light-duty ZEVs by 2030 and the US EPA has issued a proposed rule intended to slightly exceed this target.

California is leading the transition with nearly 20% ZEV market share in 2023, and with the most ambitious rules requiring a full transition of LDV sales to ZEV by 2035 and trucks to ZEV by 2040. Many states are following. So far, 16 states have committed to adopting the California LDV ZEV program, and at least 16 have signed the Multi-State Medium- and Heavy-Duty Zero Emission Vehicle MOU. If the Biden administration adopts the CO2 rules as currently drafted all 50 state vehicle markets will be required to move in the same direction. It then seems likely that most states will achieve 100% ZEV sales by 2045, 10 years after California’s target.

We recently published a major report on transitioning the US to ZEVs, along with other steps to achieve a very low carbon road-transport sector in the US by 2050. Our report considers a range of scenarios based on vehicle market and policy trends, extending trajectories to 2050. In each case, overall GHG emissions reductions are achieved sooner with the adoption of low-carbon fuels such as advanced biofuels. Our results show that it is possible to reach a 90% or greater reduction in road GHG emissions by 2050 compared to 2015, even in our slowest ZEV transition scenario.

Major findings include:

  • Fast ZEV uptake works but is challenging. Our Low Carbon California (LC CA) scenario is the most ambitious, reaching 100% ZEV sales nationwide by 2035, and 90% ZEV stock by 2050. It involves achieving 68% and 51% of ZEV sales by 2030 for LDVs and trucks, respectively, which will be challenging over the coming seven years.
  • Very high uptake of low-carbon fuels is another, complementary option. Our Low Carbon 10-to-15-year (LC 10-15) scenario is the least ambitious for ZEV uptake and therefore requires the most liquid fuels to reach a 90% GHG reduction. It does not reach 100% ZEV sales nationwide until 2050, resulting in about 54% ZEV stock in that year. These, along with a high uptake of low-carbon fuels in remaining ICE vehicles, achieves an overall GHG reduction of 90% in 2050.
  • Low GHG electricity and hydrogen are critical for both types of scenarios. All ZEVs must eventually be powered from these energy sources, with the electricity and hydrogen providing net zero carbon energy hopefully well before 2050.
  • The slower the ZEV uptake, the more challenging the biofuels component. The result of slower ZEV uptake is a build-up to very high—possibly infeasible or unsustainable—levels of advanced, very low-carbon biofuel use to ensure ongoing GHG reductions in the transportation energy sector. A transition will be needed from today’s dominant grain and oil-based biofuels to predominantly cellulosic biomass-based fuels to maximize their GHG benefits.
  • All scenarios save money, but ZEVs are likely to be cheaper than low-carbon fuels. Cumulative costs of the alternative scenarios from 2020 to 2050, aggregated across LDVs and trucks, are much lower than the business-as-usual (BAU) scenario. The faster the ZEV transition, the greater the net savings between now and 2050. This is mainly due to the lower need for maintenance and higher fuel efficiency of ZEVs. As ZEV prices fall over time, savings on vehicle costs of the alternative scenarios also contribute to overall savings. However, for some specific vehicle types, such as long-haul (LH) trucks that are dominated by fuel cell vehicles (FCV) with only a modest increase in fuel economy over diesel trucks, there are no fuel cost savings, so overall costs are higher than the BAU scenario.

Our analysis also evaluates battery electric energy vs. hydrogen fuel cells for 10 different vehicle types, including LDVs, trucks and buses of different sizes and types. The general results are shown here, with sales shares varying by vehicle type and year for our BAU and two fastest transition scenarios. Our background technology analysis shows that electric vehicles dominate LDV and most truck sales by 2035. However, for long haul trucks, we find hydrogen fuel cell trucks eventually could dominate. In any case, the ZEV sales share is 100% by 2050 in all our scenarios except the BAU.

Bar chart showing vehicle sales shares across vehicle types, scenarios, technologies, and years.

Comparing the fastest transition (LC CA) to BAU for costs, including purchase, fuels, and maintenance costs of all vehicles, we find that this scenario is more expensive than BAU until around 2030, then has lower net costs, becoming much lower very quickly. These higher “investment” costs pay off with around 54 times the savings after 2028 in a non-cost discounted scenario. Slower ZEV transition scenarios save less money, since it’s the ZEVs—particularly battery electric vehicles—that save money, while biofuels costs are generally higher than fossil fuels.

Plot graph showing total vehicle and operation and maintenance cost differences from 2015 to 2050 for light-duty vehicles and trucks combined for the LC CA scenario and BAU.

As our report describes, there are many details that are uncertain. Continuing research will be needed to better predict outcomes. For example, costs may change over time in unpredictable ways, and will depend to a large degree on scaling and learning. The level of policy support that may be needed to help manage the costs of transition are uncertain. The net societal costs of various types of policies and/or regulatory strategies are important, though often difficult to estimate. Our research over the coming one to two years will focus on better understanding fleet behavior, non-cost decision factors, electricity costs, and the potential role, sourcing, and costs of advanced biofuels as well as e-fuels.


Addressing the Impact of Lithium-ion Batteries on Low- and Middle-income Countries

The impacts of lithium-ion batteries on low- and middle-income countries are increasing as the global electric vehicle (EV) market continues to grow. The environmental and health burdens of production mainly affect countries that supply raw materials for EV batteries, while increasing exports of used EVs are going to poorer countries.

In the absence of strategic policies, the positive impacts of new EVs—such as decreased pollution and greenhouse gas emissions—could disproportionately benefit higher-income countries, while the negative impacts of second-hand EVs—such as battery disposal—could fall more on lower-income countries. 

To shed light on this problem and outline possible policy solutions, researchers at ITS-Davis collaborated with the United Nations Environment Programme (UNEP) and produced a March 2023 report entitled Electric Vehicle Lithium-ion Batteries in Lower- and Middle-income Countries: Life Cycle Impacts and Issues. Alissa Kendall, lead author and UC Davis professor of Civil and Environmental Engineering highlighted the aims of the study:

“The exponential growth of new EV sales in regions like Europe and the US is exciting to see given the key role that vehicle electrification will play in decarbonizing the transport sector. Second-hand or used vehicles from high-income regions are important sources of lower-cost vehicles in many lower- and middle-income countries, so the rapidly changing fleets have implications for the vehicles available in these regions. Unlike engines and other powertrain components in gasoline and diesel vehicles, which can be repaired, EV batteries aren’t as repairable, and as they age, their capacity and power inevitably fade. We undertook this research to make a first estimate of the magnitude of internationally traded second-hand EVs in the coming decades, and then explored the potential impacts, risks, and benefits to lower- and middle-income countries.”

The report and Figure 1 describe the life-cycle of lithium-ion batteries (LIBs)—from mineral extraction, to use in original vehicles, to secondary use in 2- and 3-wheel vehicles and microgrids, and finally disposal and recycling of components.

Life cycle of lithium-ion batteries in second-hand electric vehicle exports.

Figure1. Life cycle of lithium-ion batteries in second-hand electric vehicle exports.

The impact of second-hand EVs and batteries in lower- and middle-income countries, and whether they provide a net benefit or impact, is a function of the EV battery state-of-health at the time of import, the potential for repairing or replacing the battery, the availability of charging infrastructure, and the energy resources (fossil-fuel or renewable) used to charge batteries.

The authors of the report conducted an extensive literature review, consulted with an expert in a major used-EV importing country (Sri Lanka), and analyzed data from multiple sources on EV sales, imports, and exports. This last endeavor revealed major discrepancies in vehicle numbers reported by paired exporting and importing countries, such as the US and Mexico shown below.

Chart showing second-hand vehicle export and import estimates, with discrepancies in data from paired countries. The number of vehicles going from the US to Mexico surged when the North American Free Trade Agreement began, then fell when policies limiting imports went into effect.

Figure 2. Second-hand vehicle export and import estimates, showing discrepancies in data from paired countries. The number of vehicles going from the US to Mexico surged when the North American Free Trade Agreement began, then fell when policies limiting imports went into effect.

Policy suggestions stemming from this research, include:

  • Upon export, provide information on battery condition, technical information for safe repair and repurposing of batteries, and data on the movement of second-hand vehicles.
  • Ensure that secondary parties other than battery and vehicle manufacturers have the right to repair batteries and EVs and have access to real-time information on battery condition.
  • Create a harmonized reporting system for collecting data at the point of export and import.
  • Institute export and import controls, such as minimal requirements for the state-of-health of batteries.

Such measures can help prevent second-hand EV and battery exports from becoming a least-cost disposal option for exporting markets, burdening rather than benefiting importing markets.


Seth Karten is a science writer at ITS-Davis.

Design Strategies in Shared Vehicles to Prevent Disease Transmission

Ventilation figure

Ventilation figure

Through the evolving phases of the COVID-19 pandemic, most of us have had to consider, wonder, and worry how safe we or our personal contacts are when riding in vehicles with other people. The resulting decisions about whether to ride or work on a mode of transportation have far-reaching impacts: on individuals’ income, education, and social life, and, collectively, on transportation equity, road congestion, pollution, and greenhouse gas emissions. A new study from ITS-Davis will help researchers, transportation service providers, policymakers, and the public determine how well different design features in vehicles—such as ventilation and physical barriers—can prevent transmission of COVID-19 and other contagious diseases.

The study provides a comprehensive classification system for existing and proposed design strategies for vehicles that are used by multiple individuals at the same time (“pooled modes”) or one after another (“shared modes”). After conducting a literature search, the researchers classified design elements in modes including buses, trains, subways, ride-hailing cars and taxis, airplanes, ferries, and shared cars, bikes, and scooters.

The classification includes the following 12 categories:

  • Seating configuration—e.g., spreading out or changing the direction of seats
  • Pathways—e.g., changing how passengers move about and their proximity to drivers or other passengers
  • Barriers
  • Ventilation and air circulation
  • Air filtration and cleaning
  • Onboard surface sanitization—equipment installed in the vehicle that controls cleaning processes (e.g., ultraviolet light, heat, chemicals, or air)
  • Hygienic materials—easy to clean materials that are not porous
  • Hygienic construction—e.g., minimizing seams and joints; detachable or movable trays and seats to facilitate cleaning
  • Touchless technology
  • PPE (personal protective equipment) and supply provisioning
  • Communication and monitoring—e.g., public announcements to follow guidelines, displays that report on occupancy levels or air flow
  • Multimodal support —e.g., providing bike/scooter racks to shorten rides in pooled and shared modes

The published study includes an illustrated guide to these categories in a downloadable appendix, and it classifies 12 mechanisms through which each of the above strategies work, such as by physical distancing or increased air exchange.

A second part of the study is a survey of experts’ opinions on which design strategies may be most effective for reducing COVID transmission and where gaps in knowledge remain. The experts included physicians, engineers, epidemiologists, and social scientists with public health and communications backgrounds. Not surprisingly, they prioritized strategies that worked through increased air exchange, air flow, and air cleaning. The social scientists also emphasized the importance of affecting perceived safety, through education and communications, to make riders aware of strategies that are unfamiliar or, like air filtration, not visible.

Research and guidance on effective measures to reduce disease transmission on shared and pooled transportation has been published. However, much of the guidance focuses on behavioral rather than design interventions. And much of the design guidance has been industry specific, coming from disparate sources, such as the Centers for Disease Control, Department of Transportation, Occupational Health and Safety Administration, and American Society of Heating, Refrigerating, and Air-Conditioning Engineers. The new classification system builds on the earlier work and aims to make future research and guidance more widely applicable and robust.

A clear classification system should allow for a better understanding and comparative evaluation of a multitude of design possibilities. Evaluating which strategies are most effective and how they are perceived by the public can inform budgeting decisions by transportation providers and help raise ridership on pooled and shared modes. Many of these modes are key to transportation equity, employment of essential workers, and reduced greenhouse gas emissions.

Additional information: article in Transportation Research Recordproject web page.

Seth Karten is a science writer at ITS-Davis.

Angela Sanguinetti is a research environmental psychologist at ITS-Davis.

Beth Ferguson, Assistant Professor in the Department of Design at UC Davis, contributed significantly to the study described here as a co-principal investigator with Dr. Sanguinetti.

The study was made possible through funding received by the University of California Institute of Transportation Studies from the State of California through the Road Repair and Accountability Act of 2017 (Senate Bill 1).

Get in the Know About California Climate and Transportation Policy

Signing bill into law

It’s September–the month when Californians (especially policy wonks) wait with bated breath for the Governor’s end-of-the-month deadline for signing or vetoing bills. This year, Governor Newsom has championed a portfolio of ambitious climate bills and the Legislature has delivered them, with measures seeking to cut carbon from the electricity grid, transportation, buildings, natural lands, and industry. While nothing is final until the bills are signed, the time and effort the Governor’s Office has spent on these bills over the last month presages a strong chance that they will be signed into law.

Climate Planning Bills

First, let’s speak to two headline bills that escalate California’s ambitious climate policy. AB 1279 codifies the current target of carbon neutrality by 2045 into state law, where it was previously an executive order. It also specifies that greenhouse gas (GHG) emissions must be at least 85% below 1990 levels by that time, ensuring that no more than 15% of the goal could be met by carbon capture and sequestration, natural land uptake, or offsets.

SB 1020 enhances the state’s commitment to switching to zero-emission sources of electricity by specifying timelines and milestones. Existing law requires the state to supply 100% of its electricity from non-emitting sources—such as wind, solar, hydroelectric, geothermal, or nuclear—by 2045. SB 1020 requires that 90% of electricity come from such sources by 2035, and 95% by 2040, ensuring that utilities make significant progress immediately, while recognizing that switching the last 5–10% of supply over to clean sources may be more challenging than earlier parts of the transition.

Several other bills help define how the state would achieve its GHG targets. AB 2438 aligns transportation spending with California’s climate goals, AB 1322 requires the California Air Resources Board to map a plan to reduce emissions from commercial aviation to align with state goals and get 20% of aviation fuel from sustainable sources by 2030. SB 1137 prevents drilling to create new petroleum wells, or expand old ones, within 3200 feet of schools, hospitals, residential areas, or other sensitive sites. These, combined with many others, help align state policies with the strategy for achieving carbon neutrality in transportation laid out by a research team from UC Davis, UC Berkeley, and UC Irvine in a major report released last year.

Electric Vehicle Bills

Electric vehicle legislation advanced in the legislature, and the Governor will have a chance to consider AB 1738, which would empower the California Department of Housing and Community Development to come up with standards for EV charger installations for California homes. San Francisco has already set requirements for new buildings to include EV-ready parking spaces. But solidifying statewide standards is critical, given that most EV owners charge at home and home is the most influential charging location affecting the decision to purchase and continue owning an EV. Ensuring equitable EV access was a big focus of the legislature this year. If signed, SB 1382 will reform the state’s Clean Cars 4 All program to support more outreach to encourage people with low-incomes to buy EVs with state rebates, and it also adds additional tax benefits as an incentive.

Car Free California

Lawmakers are proposing to just pay people to give up their cars. SB 1230 would reform the Clean Cars 4 All program, including benefits for qualifying Californians who do not have a car by providing a “mobility option” voucher for transit or shared mobility services like bikeshare or scooter share. And if vouchers weren’t incentive enough to shed their vehicles, SB 457 takes it a step further, offering Californians cash for not driving. The bill will give a $1,000 tax credit to car-free households in the state. Together these two programs would put some real money in the pockets of car-free California households.

Bicycle and Pedestrian Bills

Perhaps some people who receive the car free bonuses will buy an e-bike and really max their state kickbacks. AB 117 would offer CARB $10 million to get a new and improved e-bike incentive program off the ground, separate from the state’s clean vehicle rebate program, where it is housed currently. Other e-bike bills include AB 1909, which would update bike law to authorize local governments to decide whether e-bikes can operate on bike trails (taking this authority from the state). On the pedestrian side, AB 2147 would decriminalize jaywalking, “unless a reasonably careful person would realize there is an immediate danger of collision with a moving vehicle or other device moving exclusively by human power.” According to research by Jesus Barajas, jaywalking policies have been historical tools for discriminatory enforcement against people who walk, and especially people of color.

Public transportation Bills

In the wake of many local transit agencies providing free rides in the early part of the COVID-19 pandemic, state lawmakers were actively debating how to make public transit free for Californians who need it most. The Governor will be considering AB 1919, which would provide funding to local agencies to offer a 5-year pilot for college and K-12 students to get free transit passes. A related transit bill, SB 942 would allow agencies to use certain existing funds for more discretionary purposes, including for free passes. Check out this Freakonomics episode featuring our UCLA colleague Brian Taylor explaining some of the pros and cons of fare-free transit.

Sustainable Land Use Planning Bills

There are also several sustainable planning bills for the Governor to consider. SB 922 would make it easier to build bike lanes, pedestrian projects, and transit, thanks to a streamlined environmental review process. Expediting these types of transportation investments can create a virtuous cycle that enables people to drive less. As Professor Susan Handy stated, “land use patterns shape travel behavior, transport investments shape land use patterns, transport is itself a sizable land use, and these relationships are self-reinforcing.” Another bill addressing the tension between land use and transportation is AB 2438, which seeks to align transportation projects with state plans. This bill will touch on some of the recommendations in the recent report by Betty Deakin that evaluated whether state planning efforts can succeed at reaching state climate goals. Other land use bills address parking, which is another hot (asphalt) topic. AB 2097 would restrict local governments from requiring parking minimums. This is another sign of research-backed policy, onsite parking has been shown to correlate with car ownership and use.

Bills that failed to get out of the Legislature

Some bills won’t get to the Governor’s desk. SB 917, the so-called “Seamless Transit Transformation Act” failed to make it out of its final committee. This bill aimed to streamline the San Francisco Bay Area’s 27 transit districts. According to the sponsor of the bill, Seamless Bay Area, it’s possible that the bill failed because many of the integration requirements are already being planned by regional regulators. Another bill that failed to get out of the legislature (dying in a nail-biter moment in its final committee) is AB 2133, which proposed to strengthen climate plans to strive for a 2030 goal of 55% of 1990 emissions levels (up from the 40% already required by law).

Looking Back to Inform Looking Forward

It’s important to note that all of these bills come on the heels of a record-setting budget package that was signed by the Governor in late June (with budget trailer bills tacked on in July). During the weeks of budget negotiations, lawmakers outlined plans to spend $308 billion (which includes a surplus of $49 billion more than initially projected), 6.4% of this to be spent on transportation. Big ticket items include $15 billion for the next four years of transportation infrastructure, $7.7 billion for transit, $4.2 billion for high-speed rail, $1.2 billion for goods movement and ports, and $1 billion for making active modes more safe and attractive. These historic investments are locked in, but it remains to be seen which transportation-related bills from this legislative session will receive the Governor’s signature and benefit from that windfall budget.

[UPDATE: AB 1919 was vetoed by the Governor on September 13th. A veto statement can be found here.]

Mollie Cohen D’Agostino is Policy Director at the UC Davis Policy Institute for Energy, Environment, and the Economy (PIEEE)

Colin Murphy is Deputy Director at PIEEE

Josh Stark is Policy Analyst at PIEEE

New Mobility for Sustainable Suburban and Rural Travel

Sustainable Suburban and Rural Travel

There are many reasons to reduce our reliance on cars—they are polluting, inefficient, dangerous, and expensive to own. In dense urban areas, travel modes such as walking, bicycling, and public transit can be reasonable alternatives for many peoples’ transportation needs.

But what about suburban and rural areas with lower population densities and longer distances between destinations? Land use patterns in these regions make it challenging to get around without a private vehicle. Can emerging mobility options reduce car dependence in these environments?

Based on our research so far, the answer appears to be yes. Our research teams are at the forefront of evaluating how new mobility services such as microtransit (on-demand, small shuttles providing shared rides), carsharing, and ridesharing are being used and the extent to which they are substituting for private car travel in California’s suburban and rural contexts.

Microtransit in Suburban Sacramento

Suburban communities around Sacramento are not well served by fixed-route transit. In 2018, Sacramento Regional Transit launched a microtransit service called SmaRT Ride to fill this gap. SmaRT Ride is available in eight outlying areas and the downtown core that allows travelers to request a “door-to-door” or “corner-to-corner” ride via a smartphone app. Rides cost the same as fixed-route transit.

Dr. Xing’s team studied early adopters of SmaRT Ride, conducting surveys and focus groups of both users and non-users of the service. When asked about transportation choices, more than 40% of riders said that, without SmaRT Ride, they would have made their last microtransit trip by car instead. This suggests that microtransit has real potential to reduce dependence on driving.

The study also found that more than half of SmaRT Ride users had an annual household income of less than $50,000, and that users of the service are more likely than non-users to have physical limitations. Finally, people who do not like fixed-route transit or have a neutral attitude towards it are more likely to use SmaRT Ride than are those who like fixed-route transit. This suggests that microtransit is more of a complement to, rather than a replacement for, fixed-route transit.

Innovative Mobility in the San Joaquin Valley

Dr. Rodier has been engaged with regional partners since 2014 to plan, launch, and evaluate three mobility pilots in the rural San Joaquin Valley. The region is characterized by high levels of poverty and air pollution and long distances between destinations, presenting particular challenges for providing clean and affordable transportation.

In 2019, three services were launched in the region: Míocar, an electric carsharing program with vehicle hubs at affordable housing complexes in Tulare and Kern counties; VOGO, a volunteer ridesharing service; and Vamos, a Mobility-as-a-Service app that facilitates trip planning and ticket purchasing across the valley’s many transit services.

So far, the research shows that each of these services is helping people move around the region in new ways. More than 60% of Míocar trips would not have occurred without access to the service, and three-quarters of the miles traveled on these “new” trips were made by travelers from households below the median income level in their county. People who would have used another mode to make their trip in the absence of Míocar would have traveled almost exclusively by gasoline-powered cars. So the service reduces greenhouse gas emissions for those trips.

Similarly, most of those using the VOGO ridesharing service would not have otherwise been able to make their trips. Most VOGO riders do not have access to a personal vehicle and are uncomfortable driving vehicles due to medical issues or other concerns, leaving them with few options for trips that cannot be made via existing transit services. Finally, early study results suggest that Vamos is a valuable transit fare payment tool and contributes to an improved transportation experience for its active users. More study is needed to assess the effects of Vamos on transit use and mobility as this app expands its service area and user base.

Key Takeaways

Small mobility programs like these are proliferating as communities seek affordable, sustainable alternatives to private vehicle ownership. Our research is showing early indications that these programs can be successful in meeting rural and suburban transportation needs, particularly for low-income travelers. However, these services need continued support. Private ridehailing and carsharing companies typically offer their services in higher-income urban areas that already have plentiful transportation options. To increase clean transportation access in underserved communities, new business models may be needed. These may include direct service provided by a transit agency, such as SmaRT Ride, or a non-profit service operated with public subsidies, such as Míocar. Further coordination among transportation providers and community-based organizations will be essential in identifying and developing transportation solutions to meet the needs of individual communities. Rigorous evaluations of these programs as they develop can assess their contributions to policy goals related to transportation equity and climate change, and inform longer-term investments.

Further reading:

NCST SmaRT Ride report and policy brief:

UC ITS Míocar report and policy brief:

NCST San Joaquin mobility report and policy brief:


Yan Xing is a postdoctoral researcher at ITS-Davis.

Caroline Rodier is a researcher at ITS-Davis and the associate director of the Urban Land Use and Transportation Center at UC Davis.

Brian Harold is the policy evaluation specialist at the UC Davis Policy Institute for Energy, Environment, and the Economy.

Mike Sintetos is the policy director at the National Center for Sustainable Transportation (NCST) and UC ITS Statewide Transportation Research Program.

Ecology 101: Protecting Wildlife from Transportation

Wallis Annenburg Wildlife Crossing | Photo: Courtesy of the National Wildlife Federation

(Photo: Courtesy of the National Wildlife Federation)


The Wallis Annenberg Wildlife Crossing in Agoura Hills, California, which will soon be under construction, is unprecedented in its size, cost, and primary purpose. Estimated to cost $90 million, it is the first major wildlife over-crossing primarily aimed at bringing genetic diversity to isolated animal populations rather than preventing roadkills—though it will do that, too.

The crossing, also known as the Liberty Canyon Wildlife Crossing, will be an overpass covered with vegetation, spanning ten very busy lanes of US Highway 101. It will allow mountain lions and other species to cross between the Simi Hills to the north and the Santa Monica Mountains to the south. Without this crossing, the mountain lion populations in the area would likely disappear in the next few decades because of inbreeding, vehicle strikes, and limited space to escape from wildfires.

Traffic Light and Noise

The planned crossing features design elements that will encourage mountain lions and other species that are sensitive to light and noise to actually use it. Barriers and berms will be built to reduce the amount of traffic-generated noise and light that reaches the areas that animals will use to approach the crossing. Three of our research projects at the Road Ecology Center on roadway light and noise helped influence this design. These projects were supported through state (SB1) and federal (USDOT) funds to the UC Davis Institute of Transportation Studies and the National Center for Sustainable Transportation, respectively. 

In the first of our related projects, we found that wildlife crossings are used by larger proportions of nearby, light- and noise-sensitive animal species if the crossings are comparatively dark and quiet. Our second study showed that the different behaviors among animal species in response to traffic noise and light determined how much they used wildlife crossings with varying traffic disturbance conditions. In the third study, we investigated ways of mitigating light and noise near the Wallis Annenberg crossing and a proposed over-crossing for I-15, near Temecula, California. First we found that light and noise from traffic could be detected more than 100 meters away from the highway in the animal approach zones of both crossings. We then used 3D-design software and traffic-noise modelling software to show that changing the configuration of barriers and berms near the crossings could reduce the traffic noise and light in the approach zones (Figure). Based on the results, we made recommendations to the designers to increase the chance that wildlife will approach the crossings.

Noise glare mitigation

Figure: Noise and glare mitigation in the approach zone to the crossing. (A) Typical approach to crossing structure without noise and light abatement. (B) Quiet and dark paths created by excavating and redistributing landscape materials (tan areas) and adding barriers along the highway.

The Need for Fencing and Crossings

Every year, more than 7,000 vehicle collisions with large, wild animals (e.g., deer, black bear) are reported in California, according to our California Roadkill Observation System (, and this is likely a significant undercount. For example, State Farm Insurance Co. estimates that there are upwards of 20,000 claims/year for deer-vehicle collisions in California. Not only can these crashes lead to loss of human life, property, and animal life, they can also affect the balance of ecosystems. 

One way to slow the decline of wildlife species is by lowering direct and indirect mortality from traffic. We know that we can reduce the number of vehicle collisions and alleviate genetic isolation with simple tools: 1) traffic calming and reduction, 2) fencing alone, and 3) fencing combined with wildlife crossing structures. Research done in California by the Road Ecology Center has shown where wildlife crossings and fencing are most needed and could provide greater economic benefits than their cost ( The state is already home to more than 100 wildlife crossings, used by a wide range of reptile, amphibian, and mammal species. But there is a grave need for at least ten times as many. 

Policy Environment

In the past few years, public support for wildlife crossings has grown significantly. Knowledge and awareness of crossings has spread from research scientists, to transportation planners and engineers, to the wider world. But policies and related budgets remain inadequate to the needs. Two Legislative efforts reflect this picture. 

First, the 2021 Federal Bipartisan Infrastructure Law initially authorized $350 million for new wildlife crossings. Ultimately, however, Congress appropriated $0 of the $350 million for crossings. Second, California Assembly Bill (AB) 2344 initially required the Department of Fish and Wildlife and Caltrans (the California Department of Transportation) to investigate areas that are essential to wildlife movement and habitat connectivity, to develop a plan to address these areas, and for Caltrans to implement 10 crossing structures per year. This last and strongest requirement was supported by hunting and environmental groups, but was recently removed from the bill. Although compromise of environmental legislation is an all-too-common occurrence in California, the future will tell whether this critical requirement will be negotiated back into the bill. 

In California, Road Ecology Center research shows that wildlife-vehicle collisions cost the state upwards of $250 million per year. While the state’s transportation budget is $20-25 billion per year, Caltrans has claimed that only two to three wildlife crossings are built per year in California, which, at most, would account for about 0.1% of the annual transportation budget. Yet, surveyed taxpayers consistently report that they would be willing to pay more taxes in order to protect wildlife. And state policy may finally be starting to reflect this outlook. SB 790, signed by Governor Newsom in October 2021, included $61 million for building wildlife crossings, $7 million of which was allocated for the Wallis Annenberg crossing. 

In sum, we know where to build wildlife crossings and fences; research is improving the effectiveness of these tools; and we can approximate their cost and benefits. We have a lot of information for decision support on how to address habitat fragmentation, wildlife deaths, vehicle accidents, and the cost to humans and nonhuman animals. While the policy and transportation planning response has grown significantly, we still need more from implementing transportation agencies to protect wildlife and from legislative bodies to require action and allocate more funding to make it possible.


Fraser Shilling is the director of the Road Ecology Center at the UC Davis Institute of Transportation Studies (ITS-Davis); Seth Karten is a senior writer at ITS-Davis.

How Providing Emissions Information Can Begin Greening Aviation

Illustration of flight schedule board with blank rows and positive or negative CO2 indicators at the end of each row

Those carefree, pre-pandemic days of hopping on a flight for a meeting or vacation without a second thought may seem like a distant memory to some of us right now. Still, air travel this fall has far exceeded that of 2020, even if it has yet to recover to pre-pandemic levels. Fairly soon, flying will almost certainly return to the upward trend that showed global air travel more than doubled from 2004-2019.

That’s a problem for the climate.

Flights accounted for about 2.5% of worldwide greenhouse gas emissions in 2019. While the federal government has announced an emissions reduction target of 20% by 2030 and a key airline industry group has recently proposed a net-zero plan, there is no firm consensus on a path to decarbonization for the aviation sector.

Can individual travelers play a role in reducing emissions?

Many climate hawks are cutting back or choosing not to fly at all. And it turns out that even when we do fly, our choices matter–different flight itineraries on the same route can vary quite a bit in the amount of greenhouse gases they generate, primarily due to the number of layovers (take-offs and landings produce a lot of emissions), the locations of the layovers, and the fuel efficiency of the aircraft. For example, a traveler heading from Sacramento, CA to Washington, DC might see a selection of flights that vary in emissions by as much as 30%.

That’s why the new emissions information you see when searching on Google Flights is so important. Our research indicates that people will use this information to choose lower-emissions flights.

Illustration of flight schedule board with blank rows and positive or negative CO2 indicators at the end of each row


How Emissions Information Can Make a Difference

In 2015 we began work on, a demo flight search website that shows the emissions of each flight as prominently as the price, labels the lowest emissions options “Your GreenFLY,” and, by default, sorts flights from lowest to highest emissions. Experimenting with GreenFLY helped us demonstrate the wide variation in emissions among flights with the same origins and destinations, especially for long cross-country or international flights.

We conducted two experiments to test whether this method of providing emissions information on available flight options would nudge travelers toward choosing lower-emitting flights. After all, someone using a flight-search website is already balancing many factors, such as when to fly, price, length of layover and perhaps a favorite airline; why not put emissions into the mix? In our studies we asked people to choose between a few flights, and we used that data to build a predictive model of how much value people put on different factors such as price, layovers, and emissions.

As most other researchers into flight choice have found, price was the most important consideration. But we also found that people were willing to pay more for a lower-emissions flight. For instance, an international flight might emit around a metric ton of carbon (1000kg); and people were willing to pay $20 more for a ticket that avoided 100kg of CO2 emissions, or about 10%. This translates to a rate (“willingness to pay”) of about $200/ton of CO2 saved, much higher than the average price of carbon offsets, which is about $3-6/ton! This surprisingly strong effect was consistent between two studies looking at different populations (one of UC Davis employees and one of more broadly recruited American travelers).

Real-World Emissions Reductions

Our studies presented travelers with hypothetical trips, and our GreenFLY site was just a demo. But now that Google Flights is using an emissions-focused interface, we will have an opportunity to see how emissions information will influence consumer behavior on a huge scale. We’re optimistic that we’ll see an impact on the travel choices of Google Flights’ many users and, possibly, on overall aviation emissions.

The possibilities are exciting. If more travel booking and search platforms follow the lead of Google and others like Kayak, Skyscanner, and Lite Flights, this could have a cascading effect. Many consumers choosing lower-carbon flights could push airlines to invest in more fuel-efficient aircraft as well as sustainable aviation fuels. Repeatedly seeing emissions information could raise traveler’s awareness of the environmental cost of flying. This may ultimately build support for emissions reduction regulations and investments in more sustainable travel alternatives.


More information on the authors’ GreenFLY research is available in an NCST ReportNCST Policy Brief, a 2021 Transportation Research Record paper, and a 2017 Design, User Experience, and Usability Conference Paper.


Angela Sanguinetti is a Research Environmental Psychologist at the UC Davis Institute of Transportation Studies and Energy & Efficiency Institute.

Nina Amenta is professor of Computer Science at UC Davis.

Mike Sintetos is Policy Director for the National Center for Sustainable Transportation (NCST) and SB1 Research Program at the UC Davis Institute of Transportation Studies.