transportation-energy efficiency. Advances in railroad operation could contribute to improved efficiency as well.

Shifting some freight from trucks to rail could save fuel. Candidates for diversion include trailers and containers carrying commodities that are not time sensitive and are being transported more than 500 miles.

The main fuels used in waterborne shipping—by ocean, inland, coastal, and Great Lakes routes—are diesel fuel (about 70 percent) and heavy fuel oil (30 percent). Waterborne freight accounts for about 3.5 percent of all shipments by weight.

Measured in tonnage, the oceangoing segment accounts for about half the freight moved on water into or within the United States. In terms of energy consumed per ton-mile, ocean shipping is highly efficient, as the vessels carry large payloads over long distances. Gains in energy efficiency are still possible, however. For example, one study estimates that improvements of 20–30 percent could be achieved in ocean shipping by 2020 (Kromer and Heywood, 2008). Speed reduction offers the greatest potential, although there are strong incentives to move shipments rapidly.

Potential System-Level Improvements in Transportation

Transitions in transportation systems—such as expanded use of rail for freight or passenger service—provide opportunities to boost overall energy efficiency. Such changes are usually costly and complicated, however, and are often driven by factors other than energy efficiency (such as productivity). Nevertheless, energy costs can play a motivating role.

The freight sector offers examples. The use of shipping containers has stimulated intermodal transfers among trucks, rail, ships, and even cargo airplanes, leading to dramatic productivity improvements, although gains in overall energy efficiency are less clear. Rail is much more energy-efficient than trucking is; thus, enhancing the quality of rail services and facilitating intermodal transfers should lead to significant gains in freight-transport energy efficiency.

In passenger transport, the opportunities for systemic approaches to improve energy efficiency may be even greater. Some studies have suggested that greater use of advanced information and communication technologies—“intelligent” transportation systems that electronically link vehicles to one another and to the infrastructure—could enable better traffic management. These and other studies have also examined the potential for reducing vehicle use by enhancing collective



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