How emissions regulations work in trucking, and today's major emission rules

The transportation sector 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 of GHG emissions. Within transportation, heavy-duty vehicles contribute 23.1% of the sector’s 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.

What is the Clean Air Act?

The 1970 Clean Air Act is the foundation of federal air quality law. It regulates air emissions from stationary or mobile sources and allows EPA to establish emissions standards.

Under the authority of the Clean Air Act, EPA can prescribe standards for any air pollutant from any motor vehicles that could reasonably endanger public health. EPA regularly makes new rules to better regulate motor vehicle emissions, including carbon dioxide, ozone, particulate matter, nitrogen oxides (NOx), carbon monoxide, diesel exhaust, and air toxins.

For trucking, EPA’s emissions regulate equipment manufacturers. These manufacturers must undergo rigorous testing and certification to ensure the equipment they sell meets emission requirements. However, manufacturers have occasionally tried to bypass the rules.

Among many other requirements, EPA's 2022 air pollution rule for heavy-duty vehicles tightens NOx standards for heavy-duty engines: from 200 milligrams per horsepower-hour down to only 35 mg/hp-hr.

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 hydrocarbons emissions standards in heavy-duty engines for model year 2027 and beyond.

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 now the Clean Air Act requires the EPA to regulate emissions of greenhouse gases.

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 GHG3 final rule brings tightening CO2 emission standards from heavy-duty tractors for model years 2027 and beyond.

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.

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.
Advanced Clean Fleets, which requires fleets to transition their heavy-duty vehicle mix to include a growing number of ZEVs over several years.

A CARB state won’t adopt every CARB regulation: currently, CARB expects 10 states to adopt ACT, nine states to adopt Omnibus, and no states to adopt Advanced Clean Fleets.