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A Sampling of Emissions Analysis Techniques for Transportation Control Measures


Emissions Analysis Techniques for TCMs

* Purpose and Motivation
* Structure of Report

Forecasting Approaches

Selecting a Method

Descriptions of Available Methods

Key Inputs and Outputs for Each Method


List of Acronyms

Purpose and Motivation

The Congestion Mitigation and Air Quality improvement (CMAQ) program, established under the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA), is designed to assist regions in attaining ambient air quality standards by funding transportation projects and programs to improve air quality.1 CMAQ funding is apportioned annually to states, which then allocate funds to eligible areas within the state. The Federal Highway Administration (FHWA) and Federal Transit Administration (FTA) publish guidance on the CMAQ program.2

CMAQ funds are intended to support projects that result in measurable reductions in emissions of carbon monoxide (CO), ozone precursors including volatile organic compounds (VOCs) and oxides of nitrogen (NOx), or particulate matter (PM). To show that air quality objectives are being met, state and local governments must demonstrate the benefits of individual CMAQ projects. An annual report is required from each state which specifies how CMAQ funds have been spent and the expected air quality benefits. While quantitative analysis is required whenever possible, a qualitative analysis is also considered acceptable when project benefits cannot be quantified. Groups of projects may be also analyzed in conjunction with each other when appropriate.3

This report describes modeling tools and other methods that can be used to assess the emissions benefits of projects applying for CMAQ funds. The methods described in the report may be used by state and local planners to compare alternative projects to assist in selecting the most effective or cost-effective projects. They may also be used to quantify the benefits of implemented projects for the purposes of CMAQ reporting.

The report is intended primarily for state or local air quality/transportation program analysts, as well as for others interested in estimating the emissions benefits of CMAQ projects. Most of the methods in the report do not require an extensive background in travel or emissions modeling. A familiarity with basic transportation data sources, however, can be helpful.

The report includes a brief overview of 19 methods. The methods include pre-packaged and customizable software tools as well as worksheets or other procedures for calculating benefits. They collectively address a wide range of potential CMAQ projects, including travel demand management, traffic flow improvements, and vehicle and fuel technology strategies. The report also includes references to other sources of information on CMAQ program effectiveness.

Although project sponsors have been able to quantify benefits for a significant number of CMAQ projects, in many cases appropriate data and modeling tools to quantify benefits are not readily available. Nearly all of the public comments received on interim guidance for the CMAQ program under TEA-21 emphasized the need for project evaluation and selection criteria that could quantify air quality benefits more accurately and encourage the selection of the most cost-effective projects.4 This report is a response to the expressed need for more widespread knowledge of methods to estimate the benefits of CMAQ projects.

A number of disclaimers are in order. First, inclusion of a particular method in the report does not constitute its endorsement by FHWA or FTA. Conversely, failure to include a method does not imply that the method is not valid or should not be used. The available methods vary in their technical approaches, assumptions, and underlying data. Limitations in existing data and the uncertainties inherent in both travel and emissions forecasting mean that any analysis method should be applied carefully and judiciously. Care should be taken to utilize a method whose accuracy and required resources are commensurate with the scale of the particular measure or measures being analyzed and the magnitude of impacts expected.


Structure of Report

"Forecasting Approaches" contains a review of forecasting approaches and issues in evaluating CMAQ emissions benefits.

"Selecting a Method"  is designed to assist the user in selecting a method. Methods are identified by the types of CMAQ strategies addressed and by key characteristics such as level of effort.

"Descriptions of Available Methods" contains a brief overview of 19 specific forecasting methods. The overview covers data requirements, outputs, advantages, disadvantages, typical applications, related models, and the availability of each method.

"Key Inputs and Outputs for Each Method" contains a listing of methods organized by strategy type, along with the primary data inputs required and outputs produced for each strategy type.

"References" contains references that provide additional information on forecasting techniques or on evidence related to CMAQ project impacts.

"List of Acronyms" provides a list of acronyms referenced in the document.


Updated: 8/24/2017
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