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Reference Sourcebook for Reducing Greenhouse Gas Emissions from Transportation Sources

3. Research Methodology

This literature review summarizes what is known about a wide range of individual GHG mitigation strategies and describes trends and themes across strategies. To this end, a diverse body of literature, including academic publications and journal articles; government reports at the federal, state, and local levels; and reports issued by other credible institutions were reviewed. Relevant literature was identified through searches of online databases and libraries, as well as from the authors' professional knowledge of specific topic areas. Given the modest scope of this effort, and given that it sought to report on the major findings and trends (rather than to inventory the entire literature), development of this sourcebook focused our attention on publications that aggregate and summarize findings from several individual studies. The authors also sought literature that quantified the impacts of a strategy based either on theoretical studies or on real-world implementation and assessment. The bulk of this effort was conducted at the end of 2009 and therefore the review largely focuses on the literature published prior to January 2010. That this report does not include the most recently published work is a limitation inherent to literature reviews in rapidly advancing areas of research, but one that FHWA will continue to address through supplementary updates in the future.

In addition to using the literature, the authors also drew upon their own judgment and knowledge where appropriate (e.g., in identifying likely interactions between strategies and co-benefits). In many cases, the authors also calculated the GHG or cost effects of different strategies based on data in the surveyed literature. For instance, some studies report on the reductions in fuel consumption or VMT that a strategy achieved in a particular circumstance, rather than on the GHG emissions themselves. Where feasible, the authors used data from BTS, EPA, and other sources about the average fleet size, fuel economy, carbon content of fuels, etc. to estimate effects on emissions. The authors typically used the quantities described in Table 3.1. However, sources and data varied depending upon the purposes of the calculations and the data used in each case is clearly described. The sourcebook reports small quantities of GHGs in lbs of CO2 and larger quantities in metric tons of CO2 (MTCO2) or millions of metric tons of CO2 (MMTCO2).

To compare expenditures across projects, most dollars were converted to 2009 U.S. dollars (USD) using the Bureau of Labor Statistics (2010) inflation calculator. Where expenditures are reported in other currencies, these have been converted to 2009 USD using the OANDA conversion calculator.[18]

Table 3.1. Typical data used to estimate GHG emissions from data in the literature.

Data Value
Carbon content of gasoline (lb CO2/gal) 19.6*
Carbon content of diesel (lb CO2/gal) 22.4*
Average fuel economy of all registered cars (mpg) 22.6 (2008)+
Average fuel economy of all registered light trucks (mpg) 18.1 (2008)#
Average fuel economy of all registered cars and light trucks (weighted) (mpg) 20.7 (2008)O
Average annual VMT for all registered cars (mi) 11,800 (2008) +
Average annual VMT for all registered light trucks (mi) 11,000 (2008)#
Average annual VMT for all registered cars and light trucks (weighted) (mi) 11,460 (2008)O


*Federal Register / Vol. 75, No. 88 / Friday, May 7, 2010: Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy Standards; Final Rule.

+Bureau of Transportation Statistics, Table 4-11 (2009).

#Bureau of Transportation Statistics, Table 4-12 (2009). Calculated from data in Bureau of Transportation Statistics, Tables 4-11 and 4-12 (2009).

In order to consistently and clearly present findings for each strategy, this sourcebook employs a standardized framework for the reviews. This framework and a brief description of each section in the framework are provided in Box 3.1. Each review essentially consists of five major sections: (1) background information and a definition of the strategy; (2) GHG, cost, and other effects; (3) concerns associated with implementing the strategy; (4) the extent to which the strategy has been widely or scarcely adopted; and (5) opportunities for new research. At the end of each review, a list of references is provided.

Finally, there are a few strategies to address GHG emissions from motorized vehicles that the authors considered initially but that were subsequently determined to not yet be ready for review according to this framework, primarily because there is not enough information available in the literature at this time. These strategies include:

The authors recommend that as implementation and research on these strategies continue, they should be included in future reviews.

Box 3.1. Framework for individual strategy reviews.

Strategy Name

Each review begins with a brief overview of the strategy and summaries of GHG benefits and costs and implementation concerns.


Historical context or other information necessary to understand the strategy and its effects.

Policy and Implementing Organizations

Description of the specific policy action that is necessary to implement the strategy, and the roles of various federal, state, and local agencies in implementing the strategy.


Target Group

Note of which sectors or portions of the transportation socio-technical system the strategy affects.

GHG Effects

Discussion of the sequence of effects that lead to GHG emissions reductions, what is known in the literature about each kind of effect, and findings from individual studies.

Estimated Cost per Metric Ton of CO2 Reduction

Discussion of estimated cost per metric ton of reduction, and a demonstration and explanation of any calculations performed. Cost may reflect total public costs, agency cost, or other types of cost as appropriate.

Key Assumptions and Uncertainties

Qualitative discussion of key assumptions and uncertainties, both in findings from the literature and in findings that may be present in agencies' calculations.

Data and Tools

Annotated list of data sources, tools, and other guides that may be helpful to agencies.

Implementation Concerns

Agency Cost

Description of the kinds of costs transportation agencies may bear, and quantitative estimates of that cost where possible.

Agency Implementation Concerns

Discussion of institutional barriers or other concerns that may arise in the implementation of the strategy.

Social Concerns

Discussion of social concerns, including costs and their distribution, effects on driver behavior, etc.

Other Costs/Barriers

Discussion of costs and barriers that may not have already been articulated (e.g., costs to industry).

Interactions with Other Strategies

Annotated list of strategies that may interact with this strategy, beyond those common to all strategies in the class (TDM, vehicles, or systems).

Unique Co-benefits

Annotated list of other benefits of this strategy, beyond those common to all strategies in the class (TDM, vehicles, or systems) and not limited to transportation benefits.

Unique Negative Effects

Annotated list of negative effects of this strategy, not limited to transportation effects, beyond those common to all strategies in the class (TDM, vehicles, or systems).

Where in Use

Note of the extent to which the strategy has been implemented, and an annotated list of implementation examples.

Recommendations for Further Research

Discussion of key knowledge gaps and near-term research opportunities associated with this strategy.


Each strategy has its own list of references for easier use, including hyperlinks to sources where available.

Chapter 3 References

Beijing Strives To Improve Air Quality As Games Draw Near (June 23, 2008). As of May 17, 2011:

US Bureau of Labor Statistics (2010). Inflation calculator. As of August 13, 2010:

US Bureau of Transportation Statistics (2009). National Transportation Statistics 2009. US Department of Transportation.

US Environmental Protection Agency (2010). Inventory of U.S. Greenhouse Gas Emissions and US Environmental Protection Agency (February 2005). Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel, EPA420-F-05-001.

Updated: 03/27/2014
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