Beyond 2020, continuing reductions in fuel consumption are possible. Plausible efficiency improvements and weight reductions in LDVs, alongside more extensive use of hybrid and plug-in hybrid (and possibly battery-electric) vehicles, could reduce transportation fuel consumption beyond 2020 to below the levels implied by EISA 2007 so long as a plausible rate of improvement can be sustained. An especially important R&D focus here will be developing marketable vehicles that use electricity, which will require improving the performance and reducing the cost of high-energy-storage batteries.

A parallel long-term prospect is fuel cells, with hydrogen as the energy carrier. But major improvements, especially in reducing costs, are needed if this option is to be attractive. Widespread implementation requires significant investment in low-emissions hydrogen supply and in ensuring efficient distribution systems. Onboard hydrogen storage is another key R&D issue. Because establishing a new propulsion-system technology and new fuel infrastructure on a large scale is a formidable task, significant deployment is unlikely before 2035.

Reduced energy use in freight transportation can occur both by improving vehicle efficiency and by streamlining freight-system logistics and infrastructure. Reductions of 10–20 percent in the fuel economy of heavy- and medium-duty vehicles appear feasible over a decade or so. Meanwhile, a broad examination is needed of the potential for further reductions in energy consumption stemming from improved freight-system effectiveness.

Most transportation-related energy efficiency studies and proposals have focused on the considerable gains that could be achieved with improved vehicles rather than on changing the transportation system as a whole. This emphasis is appropriate, given the potential impact of such gains. But major improvements will also come from a broader as well as deeper understanding of transportation-system issues for all transportation modes. The potential overall impact of systemic changes, such as densifying and reorganizing land uses and enhancing collective modes of travel, needs further exploration and quantification. Developing better tools for analyzing and forecasting the effects of different policies and investments on entire transportation systems is therefore an important task.

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