Heavy Vehicle Trends

Moving goods by truck, rail, and ship accounts for nearly a quarter of U.S. energy consumption and a growing share of criteria pollutant emissions. While considerable attention has been directed to modeling fuel use by light-duty vehicles traveling on public rights-of-way, much less effort has been devoted to understanding the usage and energy consumption patterns of heavy-duty vehicles, particularly those that do not travel on highways. Part of this discrepancy is due to wide variations in heavy vehicle attributes and use. Equally important, however, is the lack of consistent, time-series data on shipment characteristics, freight mode shares, and vehicle use.

The Commodity Transportation Survey (CTS), conducted in 1973 and 1977, documented both the volume of goods movement and its modal distribution. Along with carrier surveys, the CTS provided the data needed to model shipper behavior and mode choice. After the demise of the CTS, comparable data were not collected until the 1993 Commodity Flow Survey (CFS) began to track the origins and destinations of shipments by truck, rail, and ship. With the release of the 1997 CFS, the capability for modeling vehicle movements, as well as identifying long-term trends in modal choice and shipment characteristics, was greatly improved.

Ton-Miles by Mode, Metal Durables 1977 vs. 1997

For the U.S. Department of Energy's (DOE's) Energy Information Administration (EIA), we summarized commodity movements for input to the freight transportation model of the National Energy Modeling System (NEMS). In addition to the 1993 and 1997 CFS, data were summarized from the 1977 CTS, the 1997 Vehicle Inventory and Use Survey, and rail and ship carrier surveys. Methodological issues in summarizing these data center primarily on the consistency of commodity classifications over time and across surveys, the relationship of commodity groupings to NEMS' industrial sectors, and the handling of mode choice in the NEMS freight transportation model.

Results

Comparing raw data for 1977 and 1997 revealed dramatic changes in mode shares within industrial sectors. For virtually all sectors examined, the share of ton-miles moved by trucks rose at the expense of rail and waterborne modes. However, after controlling for differences in commodity classifications between the 1977 and 1997 surveys, mode shifts were shown to be confined largely to such manufacturing sectors as metal durables, paper, and chemicals. Further analyses of mode shifts by commodity and shipment type continues.

Future Plans

Like most transportation models, the NEMS does not model mode choice at the commodity and shipper levels. Until recently, it has been difficult to analyze how commodity, shipment, and modal attributes relate to shippers' mode-choice decisions either for single modes (e.g., rail vs. truck) or combinations of modes (e.g., solo truck vs. a combination of truck and rail) and to incorporate those relationships into newly calibrated freight demand models. In addition to incorporating new data, freight demand modeling must also adapt to a new group of "shippers," as wholesalers have moved into the business of shipping directly to consumers. With recent evolution in the supply chain, it has also been difficult to evaluate the energy savings potential of freight system improvements like inducing truck-to-rail mode shifts, encouraging intermodalism in place of solo truck movements, and evaluating the energy consequences of such changes in the supply chain as centralized package delivery. Future work in this area could address these possibilities.