skip navigation
print icon Print

Air Quality


This section provides a brief overview of air quality laws and regulations as they relate to transportation. Topics include the following:


Since 1970, significant progress has been made in reducing mobile source emissions, despite increases in population, employment, and vehicle miles traveled.  Most of the emissions reductions have resulted from cleaner vehicles and cleaner fuels.  Transportation agencies have contributed to emission reductions through the implementation of transportation system management (TSM) strategies such as traffic signalization projects, and high occupancy vehicle lanes, and through transportation demand management (TDM) strategies such as transit facilities, and pedestrian and bicycle programs.  Nevertheless, mobile sources are a major contributor to volatile organic compound (VOC), oxides of nitrogen (NOx), carbon monoxide (CO), and particulate matter (PM) emissions and remain an area of concern to regulatory agencies and to the general public.  More recently, mobile source air toxics and greenhouse gas emissions have also become concerns.  Transportation planners address air quality issues in two contexts:  the regional and project scales.  Regional-scale analyses estimate how regional transportation plan (RTP) and transportation improvement program (TIP) implementation affect region-wide emissions.  During the transportation conformity process, these regional-scale emissions are often compared to allowable levels, or "budgets."  Project-scale analyses involve "hot spot" assessments focused on whether pollutant concentrations near roads exceed health-based standards, or whether project implementation increases or decreases emissions.

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 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 pollution 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 pollution 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 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.

[back to top]

Transportation Control Measures (TCMs)


States may choose to implement on-road mobile source SIP measures to reduce vehicle use or improve traffic flow or congestion conditions. These measures are known as TCMs. Although TCMs generally produce only modest emission reduction benefits compared to fleet turnover, I/M, and fuel-based programs, their timely implementation is a prerequisite for RTP and TIP transportation conformity determinations.  A failure to adequately demonstrate timely implementation of SIP TCMs stops the conformity approval process and delays project implementation. In addition, TCMs receive funding priority in TIPs.  TCMs should be evaluated jointly by transportation and air agencies and incorporated into the SIP, if applicable.  In addition, many RTPs and TIPs include TCM-like projects such as transit investments or HOV lanes, which are not included in the SIP.  These measures can improve air quality but are not considered legally enforceable commitments unless they are identified as TCMs in the SIP.


CAA 172(c)(1) requires that all nonattainment areas implement "reasonably available control measures" or RACM.  Although there are a number of TCM provisions in the CAA, the RACM requirement is the most important since it compels consideration and, where necessary, adoption of TCMs. CAA section 108(f) includes 16 example TCMs, and these and other measures are typically evaluated during a RACM assessment.  CAA 176(c)(2)(B) requires that conformity approvals proceed only after a determination that SIP TCMs are being implemented on a timely basis; this requirement is included in the conformity regulations (40 CFR 93.113).  In addition, the conformity regulations (40 CFR 93.113) prohibit approving any transportation project that interferes with TCM implementation. 

[back to top]


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 EMFAC2014. 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.

[back to top]

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.

[back to top]

Air Toxics

Importance of Air Toxics

FHWA interim guidance requires states to consider air toxics analyses when completing transportation project NEPA documents. Air toxics, also known as hazardous air pollutants, are substances known to cause or are suspected of causing cancer or other serious health effects.  In urban areas, on- and off-road mobile sources are estimated to contribute the vast majority of emissions that result in estimated excess cancer risk from airborne toxic contaminants.  FHWA issued its guidance, in part, in response to litigation over the U.S. 95 highway widening project in the Las Vegas, Nevada area (see:  Sierra Club v. Mineta).

In 2001, EPA issued a final rule that identified 21 mobile source air toxics (MSATs).  The rule specified six of these 21 substances as priority MSATs:

  • benzene,
  • 1,3 butadiene,
  • formaldehyde,
  • acrolein,
  • acetaldehyde, and
  • diesel particulate matter (DPM) and diesel organic gases.

In a February 2007 final rule, EPA expanded the priority MSATs to include polycyclic organic matter (POM) and naphthalene.  EPA’s 2007 rule projects that total MSAT emissions will decline substantially by 2020 due to fuel controls and vehicle standards.

According to FHWA, there are limited tools and analysis techniques to assess project-specific MSAT impacts.  In addition, EPA has not established regulatory concentration targets for MSATs that can be used in the project development process.  Unlike criteria pollutants, there are no air toxics NAAQS.

Relationship Among Road Proximity, Concentrations, and Health Effects

The growing emphasis on MSATs is, in part, a function of research documenting higher pollutant concentrations and potential adverse health impacts with decreased distance from major roads.  EPA, in its February 2007 MSAT rule, noted that elevated pollutant concentrations are generally observed within the first 200 meters of a road, and that studies of several pollutants found that concentrations reach background approximately 300 meters from a road.  EPA also noted that “pollutants emitted along roadways will show highest concentrations nearest a road, and concentrations exponentially decrease with increasing distance downwind.” FHWA has identified the need to obtain more scientific data about near-road pollutant concentrations as its top research priority for particulate matter.

In February 2006, FHWA issued interim guidance to assist transportation agencies with project-level MSAT analyses. The interim guidance was updated in September 2009 and December 2012. The interim guidance suggests three options for NEPA documentation: no analysis, a qualitative analysis, or a quantitative analysis, depending upon the project’s scope and potential for meaningful MSAT effects.  Qualitative assessments should consider project impacts on traffic volumes, speeds, vehicle mix, or traffic routing, and expected changes in MSATs.  Qualitative analyses can also discuss the overall downward trend in forecasted MSAT emissions. 

[back to top]

Global Climate Change

Importance of Climate Change

There is growing consensus among scientists around the world that greenhouse gas (GHG) emissions are contributing to global climate change.  Carbon dioxide (CO2), the major greenhouse gas, and the transportation sector accounts for about a third of the CO2 emissions nationwide. Transportation vehicles are expected to be one of the fastest growing sources of GHG emissions in the future. Carbon dioxide emissions are produced, in part, from the burning of fossil fuels and are expected to continue to increase as the demand for fossil fuel continues to grow with the predicted growth in the U.S. and global economies.  As a result, a number of states have started to address the challenge of reducing GHG emissions.  Many of the TCMs and CMAQ funded projects that are intended to reduce criteria pollutants will have the additional benefit of reducing some GHG emissions by improving system efficiency and reducing fuel consumption.  Transportation officials are also beginning to consider the consequences of climate change such as sea-level rise, more intense storms, and changes in temperature on the design, operations, maintenance, and location of new transportation infrastructure.

More detailed information and resources on climate change issues related to transportation may be found on this website under the Energy and Greenhouse Gas Emissions Section and the Infrastructure Resilience sections or on AASHTO's Resilient and Sustainable Transportation Systems Technical Excellence Center.

[back to top]

Featured Case Study

  • Case Study Photo

    Georgia DOT supports a range of efforts such as the Georgia Commute Options Program to help reduce auto-related air emissions.

    Read Case Study >Photo: Georgia DOT

View All Case Studies for this Topic >

New - items posted in the last 7 days

AASHTO (American Association of State Highway and Transportation Officials)
skip navigation