The transportation sector is a major source of harmful emissions. The sector—including trucking, rail, shipping, and light-duty operations—is the largest source of U.S. greenhouse gas emissions, contributing 28.5% of total GHG emissions, according to the U.S. EPA’s 2023 inventory. Within transportation, heavy-duty vehicles contribute about 23% of the sector’s GHG emissions.
Heavy-duty vehicle emission regulations fall on two major authorities: the U.S. Environmental Protection Agency and the California Air Resources Board. These authorities have shaped the heavy-duty vehicle market for over 50 years and continue to do so with new rulemakings to this day.
Enforcing the Clean Air Act, these authorities require OEMs and fleets to meet specific (and sometimes expensive) emissions standards.
One of EPA’s latest major emission regulations is “Control of Air Pollution From New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards.” The final rule adopted by EPA in December 2022 introduces many requirements but notably tightens nitrogen oxides, particulate matter, carbon monoxide, and hydrocarbon emission standards in heavy-duty engines for model year 2027 and beyond. The rule is also known as Heavy-Duty NOx.
See also: Fleets Explained: DEF and NOx
EPA’s other recent major emission regulation focuses on greenhouse gases.
How does the Clean Air Act regulate greenhouse gases?
It wasn’t always the case, but currently the Clean Air Act requires the EPA to regulate emissions of greenhouse gases. (This could soon change—more information below).
This requirement became official following the 2007 Supreme Court decision in Massachusetts v. EPA. In its decision, the Court ruled that greenhouse gases—gases that affect Earth’s atmosphere and contribute to climate change—are air pollutants.
In 2009, the EPA formally recognized that six major greenhouse gases, including carbon dioxide, threaten public health. Since then, EPA has published several rules to regulate greenhouse gas emissions.
EPA’s latest major greenhouse gas rulemaking is “Final Rule: Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles – Phase 3.” Adopted as a final rule in March 2024, the standard introduces tightening carbon dioxide emission standards for heavy-duty vehicles from Model Year 2027 through 2032 and beyond.
The emission standard varies by equipment type and model year, ranging from 103.4 grams per ton-mile for MY2027 high roof Class 7 tractors to 45.4 g/t-m for MY2032 high roof Class 8 day cab tractors.
See also: Is GHG3 an EV mandate? Almost
What is CARB?
EPA is not the only major regulatory authority for vehicle emissions. The California Air Resources Board sets emissions standards for California. Other states have the option to adopt CARB’s standards.
The Clean Air Act prohibits states from setting their own emission standards—but with one key exception. The Clean Air Act allows states with pre-existing emissions standards to seek a waiver to set their own emissions standards. California was the only state with existing emissions standards before the Clean Air Act, due to the state’s severe air quality problems.
Los Angeles established the first air pollution control program in the nation in 1947. In 1959, the state legislature founded the California Motor Vehicle Pollution Control Board, which regulated vehicle emissions and would later become the California Air Resources Board. The Board established the first tailpipe emissions standards in the nation in 1966.
The federal government consistently granted California’s emissions waivers since passage of the Clean Air Act. In 2009, after EPA recognized the public health threat posed by greenhouse gases, the agency granted California a waiver to regulate vehicles’ greenhouse gas emissions.
Other states have the option to adopt California’s emissions standards without asking for authorization. These states are sometimes called CARB states.
CARB regulations
Currently, the biggest CARB regulations affecting heavy-duty vehicles are:
- Heavy-Duty Omnibus, which establishes engine emissions standards for equipment in model year 2024 through 2027 and limits sales on legacy engines.
- Advanced Clean Trucks, which requires OEMs’ sold heavy-duty vehicles to include a growing certain percentage of zero-emission vehicles over several years.
A CARB state won’t adopt every CARB regulation: currently, CARB expects 10 states to adopt ACT and nine states to adopt Omnibus.
Emissions standards will likely weaken
This explainer article will change over time. The Trump administration intends to roll back EPA's standards and revoke CARB's regulatory autonomy.
Since November 2024, the trucking industry has expected the Trump administration to combat emissions regulations in favor of a weaker regulatory landscape. Industry groups tend to prefer this outcome: Emissions standards benefit public health but also raise fleet equipment costs.
EPA plans to weaken its rules
In March, EPA Administrator Lee Zeldin announced the agency would "reconsider" its emissions standards. Zeldin made clear that he means to replace GHG3, heavy-duty NOx, and beyond with more business-friendly rules.
Most significantly, EPA will reconsider its 2009 endangerment finding and all regulations that depend on it. If the agency decides to remove the endangerment finding, it could nullify all of EPA's GHG emissions standards.
It is unclear how far or how quickly the rollback could occur. Jim Mullen, an industry leader familiar with the first Trump administration, said he expects EPA to revert to GHG Phase 2 standards for a year or more.
CARB faces strife with White House
Trump has vowed to revoke CARB's emissions autonomy.
The Trump-era EPA has already allowed Congress the opportunity to revoke CARB waivers. In the days preceding Trump's inauguration, CARB gave up pursuing a waiver for its electric truck mandate, Advanced Clean Fleets.
A waiver revocation will not be easy. California legislators are preparing for conflict, and Trump was unsuccessful in revoking CARB waivers during his first term.
