Increasing the efficiency of transportation—especially light-duty vehicles—has been a major strategy for reducing U.S. petroleum consumption. The companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c) includes a summary of changes in fuel economy standards over the past 30 years, the effectiveness of these standards, and their implications for climate policy. For example, the fuel economy potential of new passenger cars and light trucks (measured in terms of ton-miles per gallon) has improved at a rate of about between 1 and 2 percent per year since 1975 (EPA, 2009c), mainly through a series of technological advances in engines and aerodynamics. However, this potential has not been reflected in actual new vehicle fuel economy; since the mid-1980s, the fuel economy of new automobiles and light trucks as tested by the Environmental Protection Agency (EPA) has essentially been stable. Instead, vehicles have become heavier (by about 900 pounds on average [Davis et al., 2008]) and have improved their acceleration performance (average 0 to 60 mph times have declined from just over 14 seconds to about 9.5 seconds [Davis et al., 2008]). The EPA estimates that if the potential improvements in fuel economy had been realized, model year 2008 cars would have averaged 33 to 34 mpg instead of the 30 mpg they did average, and new light trucks would have averaged 27 to 28 mpg instead of 22 mpg.
Congress has called for a fleetwide combined fuel economy for cars and light trucks that reaches 35 mpg by model year 2020, representing a 30 percent increase over current levels (Energy Independence and Security Act of 2007, P.L. 110-140). In addition, new EPA GHG-performance standards for cars and light trucks will acclerate these fuel economy improvements by 3 or 4 years (EPA, 2009c). Tapping the reservoir of unrealized fuel economy potential with continued modest improvements in the efficiency of conventional gasoline and diesel engines would be the easiest way for motor vehicle manufacturers to meet these new efficiency standards. Doing so, however, would require consumers to sacrifice certain desired performance attributes such as acceleration capabilities. In order to meet the new standards under these constraints, manufacturers will need to increase the use of hybrid-electric propulsion systems, make cars and trucks lighter (typically through the use of materials such as fiberglass and carbon fibers), and develop next-generation propulsion systems—batteries and fuel cells being the two main candidates (see next subsection). It will be important with respect to some of these vehicle technologies to consider the life-cycle energy costs associated with producing more efficient vehicles; for example, some of the materials used for lightweight and hybrid vehicles are associated with significant energy production costs, which may offset some fuel savings.
To advance the technologies required to enable the production of more fuel-efficient light vehicles, the federal government has over the years funded cooperative research and development programs such as the Program for a New Generation of Vehicles. In addition to such federal actions, some states, led by California, have set their own fuel economy standards and taken other actions, such as requirements to sell a certain number or fraction of low-emissions vehicles.
In addition to improving the efficiency of the vehicle fleet, there are behavioral changes that may be able to increase the energy efficiency of the operations of existing vehicles in the light-duty fleet, such as maintaining properly inflated tires, reducing time spent idling, and removing excess weight from trunks. Each of these alone is a minor factor for the individual driver, but small changes multiplied across the U.S. passenger vehicle fleet could have an impact (Dietz et al., 2009b). More information is needed on the prevalence and effectiveness of these behaviors as well as on how they might be further encouraged.
It merits noting that Congress has called for fuel efficiency standards for medium- and heavy-duty trucks (P.L. 110-140). EPA may also develop GHG performance standards for trucks and other transportation vehicles (EPA, 2010b). Developing efficiency standards for trucks presents a particular challenge, because these vehicles are used in so many different ways that a single metric for efficiency is impractical (e.g., using miles per gallon as a metric would encourage smaller trucks with less payload and would reduce ton-miles per gallon). A recent NRC report examines the issues surrounding the development of such standards (NRC, 2010i). As this report and others have pointed out, trucking and the other long-distance freight and passenger modes of transportation already have powerful economic incentives to care about energy efficiency, since they are highly competitive and cost-conscious industries in which fuel is a main operating cost.
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