Air Quality Overview

Learn how transportation planning helps improve air quality and public health by reducing emissions, supporting clean travel options, and meeting federal air standards.

Table of Contents

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Background

Since 1970, mobile source emissions have significantly decreased, even as population, employment, and vehicle travel have grown. This progress is largely due to cleaner vehicles and fuels. Transportation agencies have also helped by implementing transportation system management (TSM) strategies—like traffic signal improvements and high-occupancy vehicle lanes—and transportation demand management (TDM) strategies, such as transit, pedestrian, and bicycle programs. Despite these efforts, mobile sources still contribute heavily to emissions of VOCs, NOx, CO, and particulate matter, and remain a concern for regulators and the public. More recently, attention has expanded to include mobile source air toxics and greenhouse gases. Transportation planners address air quality at both regional and project levels: regional analyses assess how long-term plans affect overall emissions, while project-level analyses focus on localized “hot spots” and compliance with health-based standards.

National Ambient Air Quality Standards

Transportation agencies across the country have a vital role in implementing the laws and regulations established to protect the nation’s air quality. The primary law governing air quality is the Federal Clean Air Act (CAA). The Environmental Protection Agency (EPA) has set national ambient air quality standards (NAAQS) for six principal air pollutants, referred to as criteria pollutants: carbon monoxide (CO), lead, nitrogen dioxide, ozone, particulate matter (PM), and sulfur dioxide. In most urban areas, motor vehicles are important contributors to CO, nitrogen dioxide, ozone, and PM. If monitored levels of any of these pollutants violate the NAAQS, then EPA, in cooperation with the state, designates the affected area as “nonattainment.” By law, EPA must periodically review the health-based literature and modify the NAAQS as appropriate.

State Implementation Plans

State air, environmental or health agencies are responsible for the development of state implementation plans (SIPs) that explain how the nonattainment area will meet the requirements of the CAA. State and local transportation agencies are required to implement these measures, so it is important that they take an active role in development of the SIP. A SIP is required for each pollutant for which the nonattainment area violates the National Ambient Air Quality Standards. A SIP describes the measures a State will use to bring nonattainment areas into attainment, including public involvement. EPA must approve each SIP, and if a SIP is not acceptable or the State fails to submit one, EPA must impose sanctions and assume responsibility for enforcing the CAA in that State. Depending on an area’s nonattainment severity, the CAA requires various transportation-related activities, programs and strategies. States also have the option of choosing among a variety of additional voluntary transportation measures. States also are required to submit a “maintenance plan,” after they have attained the NAAQS, that demonstrates how the state will maintain the standards for 10 years. Eight years after the attainment date the area will have to submit a second maintenance plan to demonstrate how they will maintain the standards for a second 10-year period.

Emissions Inventories and Motor Vehicle Emissions Budgets

One of the first steps in the development of a SIP is the preparation of an emissions inventory, which is based on the actual or modeled emissions from all sources of air impacts within the nonattainment area. Mobile source emissions are categorized by on-road and non-road emissions. The emissions inventory helps define the extent of the problem, relative to the air quality standards in current and future years. State and local transportation agencies need to work closely with environmental agencies to develop the on-road emission inventory; this improves the SIP and helps avoid future conformity problems. Emission estimates for on-road mobile sources are a function of two measures: vehicle miles of travel (VMT) and emissions rates (the rate of pollutants emitted in the course of travel based on vehicle speed and other factors). EPA and U.S. DOT have developed tools and models to estimate on-road emissions.

The SIP allocates emissions reduction targets to each source category. For on-road mobile sources, this target usually becomes an area’s motor vehicle emissions budget. These budgets are, in effect, a cap on emissions and represent the “holding capacity” of the area. The SIP’s motor vehicle emissions budget will be used in the transportation conformity process to cap allowable on-road emissions resulting from TIP and RTP implementation.

Control Strategies

Once total emissions reductions targets are established, control strategies must be determined to demonstrate how emissions goals will be reached. Projected changes in population, industrial activity, travel, and other social and economic factors must be evaluated, and control strategies must reflect any projected growth to ensure that emissions are reduced to the actual level needed. The selected measures will be some combination of stationary, area, on-road, and off-road controls.

At the state and local level, on-road vehicle emission controls typically focus on fuel-based measures to reduce fuel volatility, improve fuel combustion and reduce toxic emissions; and programs to identify and address high-emitting vehicles through vehicle inspection and maintenance (I/M). I/M programs are of particular interest since research indicates that, for any given pollutant, a small fraction of vehicles contributes a disproportionate amount of emissions. In addition, most areas implement TSM measures to improve traffic flow and TDM measures to reduce trips and VMT. Together, TSM and TDM measures are often referred to as transportation control measures, or TCMs. Conformity determinations rely on timely TCM implementation.

Sanctions

Sanctions can interrupt transportation project funding. Sanctions may occur due to deficiencies involving attainment SIPs. Sanctions are not imposed for maintenance plan failures. Under the CAA, two types of sanctions, mandatory and discretionary, are available to the EPA Administrator. EPA must impose sanctions through a rule making process. Once an area is officially notified by EPA of certain SIP deficiencies, a sanctions clock is triggered. Eighteen months after the sanctions clock has started, a 2-to-1 offset sanction on new or modified major stationary sources will be imposed. Under offset sanctions, each ton of emissions created by a new or modified stationary source must be offset by a two-ton reduction from existing stationary sources. Six months after offset sanctions, highway sanctions will be imposed. When highway sanctions are imposed, only those transportation actions identified as “exempt” under the CAA and those specific actions shown within FHWA’s exemption criteria policy may proceed toward final construction and implementation. Highway sanctions apply to projects where funds have not yet been obligated by FHWA by the date the highway sanctions are imposed. Projects that have already received approval to proceed and had funds obligated may proceed, if no other FHWA action is required. If the SIP failure is corrected by the state, the sanctions clock is stopped.

Transportation Control Measures (TCMs)

What They Are and Why They Matter

States can use Transportation Control Measures (TCMs) to reduce vehicle use or improve traffic flow, helping to cut emissions from on-road vehicles. While TCMs usually offer smaller emission reductions compared to programs like vehicle inspections or cleaner fuels, they are still important. To get approval for transportation plans (RTPs and TIPs), states must show that TCMs listed in their air quality plans (SIPs) are being implemented on time. If they can’t, project approvals are delayed. TCMs also get funding priority in TIPs.

Transportation and air quality agencies should work together to evaluate and include TCMs in the SIP when needed. Some projects—like transit improvements or HOV lanes—may act like TCMs but aren’t legally binding unless they’re officially listed in the SIP.

Legal Requirements

The Clean Air Act (CAA) requires areas with air quality problems to adopt “reasonably available control measures” (RACM), which often include TCMs. Section 108(f) of the CAA lists 16 example TCMs. These and other options are reviewed during RACM assessments.

Under Section 176(c)(2)(B), transportation projects can only move forward if SIP TCMs are being implemented on time. This rule is enforced through federal regulations (40 CFR 93.113), which also prohibit approving any project that would interfere with TCM implementation.

Conformity

Importance of Conformity

Transportation conformity, as required by the CAA, ensures that Federally-funded or approved transportation plans, programs, and projects conform to the air quality objectives established in the SIP. Failure to establish conformity will delay the advancement of these transportation plans, programs, and projects. Transportation conformity regulations are developed by EPA, with the U.S. Department of Transportation’s (DOT’s) input and concurrence. The U.S. DOT (through the FHWA and FTA) is responsible for implementing the conformity regulation in nonattainment and maintenance areas. EPA has a consultative role in the analysis and findings that are required.

Regional Analysis

In terms of transportation plans and transportation improvement programs (TIPs), FHWA/Federal Transit Administration’s joint conformity determination is based on a quantitative demonstration that projected motor vehicle emissions from the planned transportation system do not exceed the motor vehicle emissions budget established in the SIP. If the transportation plan or TIP cannot meet the motor vehicle emissions budget, then changes may be needed to the transportation plan or TIP, or the SIP. This regional emissions analysis must be based on the most current version of the motor vehicle emissions model specified by EPA, except in California. Conformity analyses started after August 1, 2007, in California must use new vehicle fleet data with the most recently approved emissions model, which currently is EMFAC2021. The conformity determination must also include an assurance that TCMs included in the SIP are being implemented in a timely fashion. If conformity is not determined according to the timeframes established in the regulations, a conformity “lapse” will occur. When conformity lapses, Federal projects may proceed only if they are exempt from transportation conformity (e.g., safety projects), TCMs in an approved SIP, or project phases that have already received funding commitments by FHWA or FTA.

Project Scale Analysis

In nonattainment and maintenance areas for CO, PM-10, and PM-2.5, localized (hot-spot) analysis must also be completed. To ensure conformity, such analysis must demonstrate that the project will not cause new violations or increase the severity of any existing CO, PM-10, or PM-2.5 violations within the time frame of the transportation plan. Such analyses are typically done as part of the NEPA process.

SAFETEA-LU Conformity Provisions

Section 6011 of the Safe, Accountable, Flexible, Efficient Transportation Equity Act, A Legacy for Users (SAFETEA-LU) revised transportation conformity requirements, such as establishing longer update cycles for conformity determinations, allowing conformity determinations to be based on a 10-year planning horizon under certain circumstances, and providing a one-year grace period for conformity lapses. An FHWA fact sheet describes key modifications to conformity requirements made by SAFETEA-LU.

U.S. DOT Congestion Mitigation and Air Quality Improvement (CMAQ) Program

Importance of CMAQ

The CMAQ program, initially developed under the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 and reauthorized under subsequent Acts, funds transportation projects which help reduce emissions in ozone, CO, and PM nonattainment and maintenance areas. CMAQ funds can be used for a wide range of programs and projects such as transit improvements, HOV lanes, traffic flow improvements, demand management strategies, pedestrian and bicycle programs, I/M programs, shared ride services, etc.

FHWA CMAQ website includes links to the CMAQ Program Guidance; CMAQ evaluation and assessment reports; and additional guidance on topics including eligibility of projects for CMAQ funding. Resources on CMAQ project reporting, brochures, and other CMAQ-related documents are also included.

Air Toxics

Importance of Addressing Air Toxics

The Federal Highway Administration (FHWA) continues to require consideration of air toxics—also known as hazardous air pollutants—in transportation project NEPA documentation. These substances are linked to cancer and other serious health effects. In urban areas, mobile sources remain the dominant contributors to air toxics emissions and associated health risks.

FHWA’s updated January 2023 interim guidance outlines a tiered approach for Mobile Source Air Toxics (MSAT) analysis in NEPA documents. Projects are categorized into three levels based on their potential MSAT impacts:

  • Exempt Projects (no meaningful potential MSAT effects),
  • Low Potential MSAT Effects, and
  • Higher Potential MSAT Effects.

This guidance includes prototype language, resources for addressing incomplete or unavailable information, and mitigation strategies. FHWA also recommends using the EPA’s MOVES model for MSAT analysis.

EPA’s Regulatory Updates

The Environmental Protection Agency (EPA) continues to regulate air toxics under the Mercury and Air Toxics Standards (MATS). In May 2024, EPA finalized amendments to strengthen emission standards for coal- and oil-fired electric utility steam generating units. However, in June 2025, EPA proposed repealing some of those amendments, citing regulatory reconsiderations.

While MATS primarily targets stationary sources, mobile source air toxics remain a priority. EPA’s 2007 rule identified eight priority MSATs:

  • Benzene
  • 1,3-Butadiene
  • Formaldehyde
  • Acrolein
  • Acetaldehyde
  • Diesel Particulate Matter (DPM)
  • Polycyclic Organic Matter (POM)
  • Naphthalene

EPA projects continued declines in MSAT emissions due to vehicle and fuel standards.

Relationship Among Road Proximity, Concentrations, and Health Effects

Research consistently shows elevated pollutant concentrations near major roads. EPA’s findings indicate that concentrations are highest within 200 meters of roadways and typically return to background levels around 300 meters downwind. FHWA has identified near-road air quality—especially particulate matter—as a top research priority.

Project-Level MSAT Analysis

FHWA’s guidance recommends tailoring MSAT analysis to project scope:

  • No analysis for exempt projects
  • Qualitative analysis for low-impact projects, considering traffic volumes, speeds, vehicle mix, and routing
  • Quantitative analysis for projects with higher potential impacts

Qualitative assessments should also reference the overall downward trend in MSAT emissions due to regulatory improvements.

Global Climate Change

Importance of Climate Change

There is growing consensus among scientists around the world that greenhouse gas (GHG) emissions are a primary driver of global climate change. Carbon dioxide (CO₂), the most prevalent GHG, is largely produced by the combustion of fossil fuels. In the United States, the transportation sector is responsible for approximately one-third of CO₂ emissions, making it a critical area for climate action.

As economic growth continues both domestically and globally, the demand for transportation—and consequently fossil fuel consumption—is expected to rise. This trend positions transportation vehicles as one of the fastest-growing sources of GHG emissions in the coming decades.

In response, many states and municipalities are implementing strategies to reduce transportation-related emissions. These include:

  • Transportation Control Measures (TCMs) and Congestion Mitigation and Air Quality (CMAQ) programs, which aim to reduce traffic congestion and improve air quality. These initiatives often have the co-benefit of lowering GHG emissions by improving fuel efficiency and reducing idling.
  • Investment in public transportation, which emits significantly fewer GHGs per passenger mile compared to private vehicles.
  • Promotion of electric vehicles (EVs) and the development of EV infrastructure, which can drastically reduce emissions when powered by clean energy.
  • Land use planning and transit-oriented development, which encourage compact, walkable communities that reduce the need for car travel.

Transportation officials are also beginning to integrate climate resilience into infrastructure planning. This includes accounting for sea-level rise, more intense storms, and temperature extremes in the design, operation, and maintenance of transportation systems.

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