Many recent studies have pointed out opportunities to improve the efficiency of petroleum-fueled vehicles. In new vehicles, fuel consumption (and GHG emissions) per passenger-mile can be reduced by improving today's gasoline-fueled and diesel oil-fueled vehicles, by shifting to hybrid or electric vehicles, and by improving today's hybrid vehicles. The extent to which these changes result in reduced emissions will depend on consumer preferences regarding vehicle weight and power. Reductions in emissions intensity will depend crucially on consumers' willingness to opt for constant or reduced vehicle weight and power, so as not to offset efficiency improvements. America's Energy Future (NRC, 2009d) judged that considerable reductions in vehicle weight will be required to meet the newest U.S. fuel economy standards for light-duty vehicles. Energy efficiency improvements are also under way in commercial passenger aircraft, but they are not expected to be large enough to counter the expected growth in demand for air travel over the next several decades.
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