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Reference Sourcebook for Reducing Greenhouse Gas Emissions from Transportation Sources

1. Introduction

There is general scientific agreement that GHG emissions are contributing to a long-term warming trend of the earth. In the United States, rising seas already place coastal cities at risk, changing temperature and precipitation may alter the nation's food production capabilities, and increasingly extreme weather will take a toll on lives, the economy, and infrastructure (U.S. Global Change Research Program, 2009).

Yet the most severe changes in climate can still be avoided if greenhouse gas (GHG) emissions are curbed significantly. Successfully mitigating GHG emissions will require large and often difficult changes in all sectors of society, including transportation. From 1990 to 2009, GHG emissions from transportation increased 16%, and, today, transportation accounts for nearly one third of U.S. GHG emissions (U.S. Environmental Protection Agency, 2010). Thus, it is urgent that the transportation community takes steps today to reduce GHG emissions from this sector.

Some states have already implemented policies recommending or requiring that transportation agencies address climate change. California, for example, requires the California Air Resources Board to set targets for GHG emissions reductions. It further requires that Metropolitan Planning Organizations (MPOs) develop roadmaps for achieving those targets as part of their long-range transportation plans.[2] New York State's 2002 Energy Plan recommended that the state's Department of Transportation (DOT) and MPOs estimate the GHG emissions that would result from their long-range transportation plans (New York State Energy Plan, 2002). New York's 2009 Energy Plan calls for statewide GHG emission reductions to 80% below 1990 levels by 2050 (New York State Energy Plan, 2009).

As concerns over climate change grow, transportation agencies will increasingly seek ways to address it by assessing GHG emissions, setting targets for reductions, and developing ways to mitigate emissions and meet those targets (Grant et al., 2010). This will be especially challenging because federal, state, and local agencies simultaneously face reduced revenue, increased congestion, and growing demands for transportation. Therefore, agencies will need guidance and information in order to meet climate change mitigation goals amid these other challenges.

This sourcebook helps address that need. It presents the results of a literature review of GHG mitigation strategies, summarizing what has been published about the GHG effects of different strategies, their costs, and the social feasibility of implementing them. These results can be used by transportation agencies-principally DOTs and MPOs-to inform decision-making about strategies in their own jurisdictions.

This sourcebook is organized into seven chapters. Chapter 1 reviews the goals of this work, provides background on GHG emissions in transportation, and lists the strategies reviewed. Chapter 2 summarizes key findings about the strategies and offers recommendations to the FHWA about providing guidance to DOTs and MPOs. The research methodology is presented in Chapter 3 and the role of land use is presented in Chapter 4. Finally, Chapters 5, 6, and 7 present reviews of transportation demand management strategies, transportation system management strategies, and vehicle improvement strategies, respectively.

Goals of this Sourcebook

GHG mitigation strategies are policies or actions that can be used to curb greenhouse gas emissions. This sourcebook has two goals. The first is to provide DOTs and MPOs with a rich context in which to consider and evaluate GHG mitigation strategies for possible implementation in their jurisdictions. To this end, this review helps them answer the following key questions about the set of strategies as a whole:

These questions are answered in Chapter 2. The review also addresses questions about individual strategies:

The review of individual strategies in Sections 5-7 answers these questions. As described in Chapter 3, our approach to documenting each strategy is designed to help agencies quickly obtain the answers to each of these questions.

The second goal is to highlight for FHWA, DOTs, MPOs, and other stakeholders the most pressing needs and promising opportunities for future research. As climate change gains recognition nationally and internationally as a critical concern of our time, the demand for information will grow, and the research and practice of GHG mitigation strategies will accelerate. This sourcebook is designed to help shape those efforts by identifying gaps in the current research related to each strategy that could be filled by near-term research (described in individual reviews of strategies in Chapters 5-7). Where possible, it identifies longer-term research opportunities as well, though a comprehensive assessment of research gaps for each strategy is beyond the scope of this review. This sourcebook further discusses crosscutting research needs in Chapter 2 on summary findings.

GHG Emissions in Transportation

In transportation, vehicles' consumption of fuels-most often petroleum products-is the key source of GHG emissions, and most GHG mitigation strategies target emissions from these mobile sources. The quantity of emissions from mobile sources is a product of three factors: the carbon content of the fuel, the vehicle's fuel consumption per mile of travel, and the miles the vehicle traveled.

The carbon content of the fuel refers to the amount of carbon that is released into the atmosphere when a quantity of that fuel is consumed. Some fuels have higher carbon content and thus produce more emissions than others. Gasoline, for example, emits 19.6 lbs of CO2 per gallon, while diesel emits 22.4 lbs of CO2 per gallon.[3]

The vehicle's fuel economy is the number of miles the vehicle can travel on a particular quantity of a particular type of fuel.[4] In 2010, the average fuel economy of all light-duty vehicles (cars, minivans, sport utility vehicles, and pickup trucks) in the U.S. was 22.5 mpg (U.S. EPA (2010), p. iii), while the average fuel economy for heavy trucks and buses was in the single digits (US Bureau of Transportation Statistics, 2009), tables 4-11 and 4-12.

Importantly, the fuel efficiency of a vehicle is not constant but varies based on driving and maintenance habits, traffic conditions, and other factors.[5] A well-maintained vehicle is likely to have higher fuel efficiency than one of an identical model and year that is poorly maintained. Additionally, most vehicles have different fuel efficiencies at different speeds and over different terrain.[6]

The third factor is the number of miles traveled by the vehicle, or vehicle miles traveled (VMT).[7] The more miles the vehicle travels, the more fuel it must consume, and thus the more GHGs it emits.

Equation 1.1 is used to determine emissions based on these factors:

(Eq. 1.1) equation: Emissions = Carbon Content x 1/fuel economy x vehicle miles traveled

Therefore, for example, driving a gasoline-powered car with a fuel economy of 21 mpg for 30 miles emits 27.7 lbs of CO2 as shown in Equation 1.2:

(Eq. 1.2) equation: 22.7 lbs CO2 = 19.4lbs CO 2/1 gallon x 1 gallon/21 miles x 30 miles

There are other sources of emissions in addition to fuel combustion in mobile vehicles. These include the production of construction materials like concrete; transportation system construction, operation, and maintenance; and vehicle manufacturing. This sourcebook considers these sources principally in terms of how they affect or interact with strategies to reduce emissions from mobile sources.

GHG Mitigation Strategies Reviewed in this Report

The strategies reviewed in this document were selected because they focused on actions that were or could be within the purview of DOTs and MPOs, as opposed to actions that only the federal government could undertake.[8][9] As shown below, these strategies are divided into three categories: transportation demand management, transportation system management, and improvements to vehicles.[10]

Transportation Demand Management Strategies Transportation System Management Strategies Vehicle Improvement Strategies
  • Road Pricing (including distance-based fees and cordoning)
  • Parking Management and Parking Pricing
  • Car Sharing
  • Pay-as-You-Drive Insurance
  • Ridesharing and HOV Lanes
  • Transit Incentives
  • Transit Improvements
  • Telework
  • Traffic Signal Optimization
  • Ramp Metering
  • Incident Management
  • Speed Limit Reduction and Enforcement
  • Roundabouts
  • Capacity Expansion
  • Resurfacing Roads
  • Alternative Construction Materials
  • Feebates
  • Scrappage Programs
  • Tax Incentives for Cleaner Vehicles
  • Heavy-Duty Vehicle Retrofits
  • Eco-Driving Education and Training and Dynamic Eco-Driving
  • Truck Stop Electrification and Auxiliary Power Units
  • Anti-Idling Regulations and Campaigns

This list covers many major surface GHG mitigation strategies available to transportation agencies, but it is not exhaustive for two reasons. First, the scope of this effort was modest and, given that new strategies are continually being introduced and evaluated, a comprehensive review is not possible. Instead, this review focuses on strategies that directly affect motorized transport. While strategies that enable and promote non-motorized transport-walking and cycling-are very important corollaries to changing motorized transport, this sourcebook is able to discuss them only briefly in Chapter 2 as they pertain to land use, and recommend that future literature reviews include those strategies as well. Second, for a few strategies considered, the body of literature is not yet large enough to allow for a review at this time. This sourcebook discusses such strategies briefly in Chapter 3 and recommends that, as research continues, these strategies be included in future reviews.

It is important to note that in some cases strategies do not appear on our list because they are better considered as components of other strategies. For example, intelligent transportation systems (ITS) can play an important role in improving the effectiveness of transportation system improvement strategies, and so this sourcebook discusses the role of ITS as part of those other strategies rather than as a stand-alone approach. Finally, although land use patterns are intimately tied to transportation, this sourcebook considers land use as a backdrop for transportation, rather than as a strategy per se. The sourcebook discusses this choice and the role of land use in Chapter 4.

Chapter 1 References

Grant, Michael, D'Ignazio, Janet, Ang-Olson, Jeff, et al. (2010). Assessing Mechanisms for Integrating Transportation-Related Greenhouse Gas Reduction Objectives into Transportation Decision Making, ICF International, Final Report for NCHRP Project 20-24(64).

New York State Energy Plan (2002). As of May 17:

New York State Energy Plan (2009). As of May 17:

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 Sinks: 1990-2008.

US Environmental Protection Agency (2010), Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2010, Executive Summary.

US Environmental Protection Agency (February 2005). Emission Facts: Average Carbon Dioxide Emissions Resulting from Gasoline and Diesel Fuel, EPA420-F-05-001. As of May 17: .

US Global Change Research Program (2009). Global Climate Change Impacts in the United States.

Updated: 10/20/2015
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