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Multi-Pollutant Emissions Benefits of Transportation Strategies-FHWA

1. Introduction

Background and Purpose

The National Ambient Air Quality Standards (NAAQS) are Federal standards that set the allowable concentrations and exposure limits for certain pollutants. Air quality standards have been established for several pollutants associated with transportation, including carbon monoxide (CO), ozone, and particulate matter (PM-10 and PM-2.5). If monitored levels violate the NAAQS, then the Environmental Protection Agency (EPA), in cooperation with the State, will designate the contributing area as "nonattainment." In addition to direct pollutant emissions, motor vehicles emit precursors that contribute to pollutant concentrations, including nitrogen oxides (NOx), volatile organic compounds (VOCs), sulfur oxides (SOx), and ammonia (NH3).

Transportation is a major source of air pollutant emissions. Nationally, on-road transportation sources are responsible for 27 percent of VOCs emissions, 35 percent of NOx emissions, and 55 percent of CO emissions1. Although emissions from most transportation sources have been declining for the last two decades, and are projected to continue to decline due to the beneficial effects of improved emission control technologies and more stringent emission regulations, transportation will continue to contribute to regional air pollution for years to come.

Transportation agencies have a long history of implementing strategies to reduce air pollutant emissions. Since 1991, the Congestion Mitigation and Air Quality Improvement (CMAQ) program has devoted more than $14 billion in highway funds for projects that reduce emissions and relieve congestion, most of which have been implemented by transportation agencies2. Some State Implementation Plans (SIPs) include transportation control measures (TCMs), many of which are implemented by state, regional or local transportation agencies. Thus, there is a wealth of experience implementing emission reduction strategies by transportation agencies.

Despite the past efforts, there are few resources, especially comprehensive compilations, of the full range of strategies available to transportation agencies. Moreover, an important deficiency in the existing literature is the lack of documentation of the types of effects of strategies on all transportation-related pollutants. For instance, many studies report only effects on NOx and VOCs, and many do not include PM-2.5 impacts. Traditionally, transportation agencies have focused their emissions reduction strategies on CO; the ozone precursors, VOCs and NOx; and PM-10 from road dust. The recent designation of nonattainment areas under the fine particulate matter standard (PM-2.5), however, has also brought new attention to the role of transportation in direct emissions of PM-2.5, and emissions of PM-2.5 precursors, which may include NOx, SOx, VOCs and NH3.

In some cases a control strategy that is successful in reducing one pollutant may actually increase emissions of another pollutant. In some other cases, a control strategy may be beneficial to multiple pollutants. Because many regions are facing multiple air quality objectives (e.g., either designated nonattainment for multiple pollutants or addressing multiple precursor emissions), it is important for transportation agencies to understand the effects of emissions reduction strategies on different pollutants. There is also an increased need for transportation agencies to consider and understand the effects of non-traditional emissions reduction strategies, such as truck idle reduction projects, diesel retrofits, and alternative fuel vehicle programs, which may be effective in addressing some pollutants.

The purpose of this report is to help transportation practitioners consider appropriate transportation strategies for reducing transportation-related emissions of concern. Specifically, this report provides a compendium of traditional and innovative transportation-related control strategies, and for each type of strategy, identifies effects on the following seven pollutants: CO, PM-10, PM-2.5, NOx, VOCs, SOx, and NH3. Strategies included are those that can be implemented by policy makers at a state or local level (Note: strategies that would require a change in federal law or federal action, such as new vehicle emissions standards, are not included). Although many strategies can be funded or implemented directly by transportation agencies (e.g., programs eligible for CMAQ funding), others included in this document are more typically implemented by state air agencies (e.g., inspection and maintenance programs) or require state or local government implementation (e.g., land use policies, fuel tax increases).

For each strategy, the document reports on the direction of emissions impacts (increase, decrease, neutral or uncertain) that typically are expected for each pollutant. It also includes calculations of emissions impacts for sample projects, based on real project examples, and identifies EPA guidance documents that should be referenced and sample methodologies for calculating impacts.

How to Use this Document

Overall Organization

This report is divided into the following chapters.

Summary of Findings (Chapter 2) - This section provides an overview of the impacts of different types of transportation strategies on emissions of the seven pollutants, and provides context regarding targeting of emissions reductions to specific pollutants of concern.

The following five chapters are organized into categories based on the primary objective of each strategy, as follows:

Transportation demand management (TDM) strategies (Chapter 3) - These strategies focus on reducing vehicle travel.
Transportation system management (TSM) strategies (Chapter 4) - These strategies focus on improving the operating characteristics of vehicles, such as by affecting traffic flow, vehicle speeds, or idling.
Vehicle technology and fuels strategies (Chapter 5) - These strategies focus on reducing vehicle emission rates by changing vehicle characteristics or fuel composition.
Non-road transportation strategies (Chapter 6) - These strategies address railroads, marine vessels, and other non-road engines.
Road dust reduction strategies (Chapter 7) - These strategies focus specifically on reducing fugitive dust emissions from paved and unpaved roads.

Some individual strategies may fall into more than one of these categories (e.g., a high-occupancy vehicle lane can be considered both a TDM and TSM strategy, since it is designed to encourage ridesharing and also improve traffic flow). Strategies that fit into more than one category are included in one chapter only, but the discussion and impacts assessment accounts for all expected effects.

Conclusion (Chapter 8) notes gaps in the findings.

The appendices include a listing of potential (traditional and innovative) transportation emissions reduction strategies (Appendix A), a summary of the contribution of transportation and other mobile sources to national emissions of each pollutant (Appendix B), and an overview of emissions factors and assumptions used in the sample calculations (Appendix C).

Information on Each Strategy

For each of the strategies presented in this report, information is presented using the following structure:


Note that although emissions calculations are provided for sample projects, the methodologies used for the sample calculations are often simplifications of more complex methods.4 The user should consult EPA guidance to determine appropriate and accepted methodologies for use in quantifying emissions reductions as part of a State Implementation Plan (SIP) or for use in a conformity determination. In some cases, these methods require use of travel forecasting models or other tools, accounting for indirect impacts, or additional steps in the calculation that are not accounted for in the sample calculations. Moreover, the approach and data sources that should be used for calculating emissions impacts will depend on whether the analysis is being conducted to forecast impacts prior to implementation or as a post-project evaluation. In a post-project evaluation, additional survey data, field measures, or other data sources on actual transportation system performance should be used.
The assumptions used in the sample calculations are generally drawn from actual projects. However, given the variations across projects and range of factors that influence effects, this document is not intended to provide results that can be expected for all similar projects and the assumptions used in the sample calculations are not meant to be used as standard defaults for calculations of emissions impacts for other projects. Factors such as utilization rates, days of operation, and average trip lengths should be defined based on locally-available data and project-specific information.

1 National Emissions Inventory Air Pollutant Emissions Trends Data, 2002,

2 National Highway Institute, 2006,

3 If examining the effects of one project over time, one might expect the travel or speed implications to grow or shrink over time, depending on the strategy.

4 For instance, for most of the TDM strategies included in this report, the methodologies used for the sample calculations do not incorporate the secondary or indirect effects associated with travel speed changes and do not account for the potential need to increase transit services. These effects are not significant in most cases.

Updated: 9/25/2017
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