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What Will the Advanced Clean Fleets Rule Do for California? 


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Trucks and buses on California’s roads and highways are responsible for the majority of lung-damaging fine particulate and ozone-forming nitrogen oxide emissions, and over 20% of the climate-warming greenhouse gas (GHG) emissions from vehicles. Diesel exhaust in particular is a well known carcinogen and has also been linked to adverse health conditions ranging from chronic heart disease to decreased lung-function in children. Given that a disproportionate amount of public health impacts from heavy-and medium-duty vehicle emissions are concentrated in lower income communities, mitigating, and eventually eliminating this significant source of pollution has been a priority in the state of California.

Passed last year, the Advanced Clean Fleets (ACF) rule is a first-of-its-kind emission standards regulation designed to ease the introduction of zero-emission vehicles (ZEVs) into California’s large truck and bus fleets and address the air pollution and carbon emissions from trucks and buses. The regulation came to be after a multi-year workshop process that had significant participation from environmental justice groups in an effort to achieve the goals set out by Governor Newsom’s 2020 Executive Order (N-79-20) related to transportation sector emissions. UCS also attended ACF workshops and contributed relevant research on estimated emission reductions based on proposed iterations of the rule, which helped to inform the true extent of climate benefits that could be reaped, prior to its final adoption.

UCS Senior Vehicle Analyst Sam Wilson has previously written how the ACF rule is desperately needed to ensure access to clean air and combat climate change. The California Air Resources Board (CARB) estimates ACF will reduce climate-warming emissions from the state’s truck fleet by more than 40% by 2050. For more specifics on estimated reductions the ACF will bring to emission levels, take a look at Sam Wilson’s breakdown of air quality benefits here.

Since the ACF rule was adopted by CARB in April of 2023, both policy makers and fleet operators have asked questions about the scope and structure of the regulation.

The rule can be broken down into three main parts: 

The “electrification” portion of ACF requires California’s largest and most polluting fleets, as well as state and municipal fleets, to gradually transition to zero-emission medium- and heavy-duty vehicles (MHDV) by 2050. 
The “drayage” portion requires vehicles operated for drayage (transporting containers to and from seaports and railyards) to transition to zero-emission vehicles (ZEV) by 2035.  
Lastly, the rule builds on the Advanced Clean Trucks (ACT) rule passed by CARB in 2020 and implements the requirement that all medium- and heavy-duty vehicles sold in California must be zero-emission after 2036, with some exceptions for emergency vehicles.   

Simply put, the rule applies to three types of fleets: fleets with 50 or more trucks, city and state-owned fleets of any size, and all drayage trucks. 

The electrification portion of the rule applies to fleets with 50 or more of any type of MHDV over 8,500 pounds and those whose annual revenue exceeds $50 million. Notably, ACF only applies to companies that own a lot of trucks, so many smaller fleet operators with as many as 49 vehicles have no requirements at all under the regulation.  

Also included under the rule are all state and local agency fleets—this includes any city, county, special district, department or agency operating at least one medium- or heavy-duty vehicle. Electrification requirements additionally apply to federal fleets and light-duty mail and package delivery vehicles.

The drayage electrification requirements apply to all vehicles operating drayage in California.

There are multiple compliance pathways for both commercial fleets and publicly owned fleets. Commercial fleets covered under the rule can choose from two different compliance pathways: 

Model Year Schedule: Fleets can begin to electrify now by only purchasing ZEVs for any new additions to a fleet, while continuing to use their existing internal combustion engine (ICE) fleet through the end of the vehicle’s useful life of 18 years or 800,000 miles, per Senate Bill 1 (SB 1).
ZEV Milestone Option: Fleets can gradually turn over their internal combustion fleet to zero-emission vehicles based on the California Air Resource Board’s gradual “ZEV Milestone” timetable. The timetable spells out when certain vehicles must be turned over. For example, the easiest-to-electrify vehicles like delivery vans, must be turned over to ZEVs in 2025, work trucks, day cabs, and three axel buses in 2027, and for sleeper cab tractors and specialty vehicles in 2030.  

Fleet managers of large public agencies are required to purchase at least 50% ZEVs when replacing vehicles beginning in 2024, and 100% ZEV purchases after 2027. Smaller public agencies are not subject to the 2024 requirements but are subject to the 2027 requirement. The rule includes exceptions for certain public safety and emergency vehicles. Public agencies of any size may choose to follow the gradual “ZEV Milestones” pathway instead.    

The ZEV requirements for drayage operators applies to all vehicles included under California’s drayage registry for port entry. Currently operating combustion drayage vehicles may operate through their SB 1 useful lives timeline or 2035, whichever comes first. All new vehicles added to the registry must be ZEVs.

The 2036 100% ZEV sales requirement applies to all manufacturers but does not apply to certain emergency vehicles.    

ACF is anticipated to accelerate the annual adoption of zero-emission trucks in California by an estimated 80%, which will reduce exposure-related health complications from air pollution. Additionally, the rule will reduce greenhouse gas emissions from the state’s trucking fleet by nearly 330 million metric tons through 2050, roughly equivalent to taking 90 coal-fired power plants offline for an entire year. This reduction is anticipated to significantly reduce illness and mortalities from exposure to air pollution, with an estimated 2,500 fewer deaths and over 2,000 avoided hospitalizations through 2050. The total health benefits are quantified at over $26 billion through 2050.  

ACF is also unique among many other air quality and climate change regulations in that it is estimated to have greater net financial benefits to regulated fleets than quantified health benefits. CARB estimates that regulated fleets will save nearly $50 billion through 2050, even when considering capital upgrade costs like charging infrastructure.  

While portions of the ACF rule are now in effect, enforcement has not yet begun, as CARB requires a specific waiver from the U.S. Environmental Protection Agency (EPA) to employ certain air quality regulations. The benefits that will come from ACF are both substantial and highly needed, as indicated by our previous coverage of the issue, which is why UCS urges EPA to approve the waiver sooner rather than later, as ensuring compliance is a vital part of the rule’s success.

You can learn much more about ACF by visiting CARB’s website. To get into the weeds on ACF funding, flexibility, and other frequently asked questions, check out Sam Wilson’s recent blog post.


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