society moves from vehicles propelled by internal combustion engines using petroleum-based fuels to vehicles using more varied types of propulsion systems and fuels, it will be increasingly important to understand the full life cycle of GHG emissions generated by various vehicle and fuel combinations, including the emissions and energy implications associated with vehicle production. The move from tank-to-wheels to well-to-wheels emissions analyses represents an important step in this understanding. For example, our understanding of the true life-cycle emissions from various biofuels is still incomplete, as is understanding of trade-offs and consequences for other resources and environmental issues. Also, the construction and maintenance of trans portation infrastructure is an additional source of GHG emissions, but little is known of the relative emissions associated with different transportation modes or infrastructure types even as large investments are being planned for constructing new systems such as high-speed rail.
Improve understanding of what controls the volume of transportation activity. While there is potential for tempering growth in vehicle miles traveled by increasing land development densities, a recent NRC report (NRC, 2009e) found a lack of sound research on the potential for increasing metropolitan densities to affect travel, energy use, and emissions. Further research is needed on the relationships among household location, workplace location, trip-making activity, and light-duty vehicle travel, and on the effectiveness of various policy mechanisms to influence these relationships. Technological improvements such as online shopping, telecommuting, and virtual conferencing also have the potential to significantly reduce total transportation activity, but further research is needed on how to facilitate and promote expanded use of these technologies (and this research will require data on current levels of usage of these technologies—an example of a climate-relevant observation that falls outside the rubric of traditional climate observations).
Conduct research on the most promising strategies for encouraging the use of less fuel-intensive modes of transportation. Any increase in fuel prices, whether a result of climate or energy policy or other factors, can be expected to promote a shift toward more fuel-efficient modes of transportation, both at the personal level and through major private-sector transportation providers. However, as noted earlier in this chapter, there are a variety of strategies that might be employed to encourage less energy-intensive modes. As with overall reductions in travel volume, additional research is needed on the factors that influence travel mode choice—understanding how, for example, intermodal service can be made more attractive to shippers or public transit more attractive to passengers. Research is also needed on potential large-scale changes in the built environment and infrastructure that would encourage less energy-intensive modes, and the policy mechanisms that might be used to facilitate these changes.
Continue efforts to improve energy efficiency. In addition to the continued improvement of more efficient vehicle designs and propulsion systems, there could potentially be major energy efficiency gains in other transportation modes. For example, there is room for improvement in medium- and heavy-duty truck aerodynamics and means of reducing idling (NRC, 2010i). Ultralight materials such as carbon fiber are already beginning to see widespread application in new commercial aircraft (e.g., the Boeing 787), and additional research by both public and private sectors may help ac celerate this and other efficiency improvements, such as "blended wings" and open fan propulsion systems.
In addition to technology development and deployment, there is a wide range of research needed on human behavior as it relates to transportation use and on the best policies for influencing both technology development and human behavior. For example, there are behavioral changes that increase the efficiency of existing vehicles, such as maintaining properly inflated tires, but we lack basic data on the prevalence of these behaviors as well as on how they might be effectively encouraged. Further research is also needed on factors that encourage the purchase of more efficient vehicles—fuel prices are certainly one factor, but, as with the adoption of any new technology, prices are only part of the explanation and a more nuanced understanding might lead to the design of effective policies. There may actually be substantial proprietary information on what influences consumer choice and technology adoption, but there is little open literature on this subject or on how policies, programs, and institutions might influence vehicle or mode choice. Finally, the history of U.S. fuel economy over the last 35 years, where efficiency improvements were offset by consumer demands for larger, more powerful vehicles (with little resulting fuel consumption penalty, because efficiency had increased), suggests a need for better understanding of how to design regulatory policies that have the intended results.
Accelerate the development and deployment of alternative propulsion systems, fuels, and supporting infrastructure. New, less carbon-intensive fuels and alternative propulsion systems will ultimately be needed to make major reductions in GHG emissions from the transportation sector. The two primary candidates for replacing internal combusion engines are batteries and hydrogen fuel cells, and major technological advances are still needed to make these methods competitive with current propulsion systems. Moreover, while these alternative propulsion systems would reduce petroleum consumption, they will only reduce GHG emissions significantly if the needed electricity or hydrogen is produced using low-emissions fuels and processes. As discussed in the companion report Limiting the Magnitude of Climate Change (NRC, 2010c) and elsewhere, widespread adoption of these technologies also implies a major restructuring of the nation's transportation infrastructure, and reasearch will play an important role in optimizing that design.
Advance understanding of how climate change will affect transportation systems and how to reduce the magnitude of these impacts. One of the most difficult tasks for transportation planners in addressing climate change is obtaining relevant information in the form they need for planning and design (NRC, 2008g). Improved regional-scale climate information is needed, but so is a better understanding of how projected climate changes, such as changes in temperature and precipitation, will affect different kinds of infrastructure in different regions, and improved methods of providing information to transportation decision makers. Practical research on adaptation measures, both for current transportation systems and for the design of new systems and infrastructure, is needed to better inform all kinds of transportation-related decisions as climatic conditions continue to exit the range of past experience.
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