Why the 2002 Composite Estimates in this Report Differ from Preliminary Estimates
The 2002 composite estimates in this report were jointly developed by the Research and Innovative Technology Administrations Bureau of Transportation Statistics (BTS) and the Federal Highway Administrations (FHWAs) Office of Freight Management and Operations.1 An earlier BTS report, Freight Shipments in America, issued in 2004, also presents composite estimates for 2002, but the numbers were preliminary and are not the same.
The table below compares the estimates in this report for value, tons, and ton-miles with those in Freight Shipments in America. BTS considers the jointly developed composite estimates to be more reliable and complete than earlier estimates because of improvements in estimation methods and expanded industry-commodity coverage. As the table shows, the new estimates for total value and total tonnage are larger than the preliminary estimates in the earlier report. This is largely because the new estimates include previously uncovered sectors, such as construction, retail, services, and municipal solid waste. However, the new estimate for total ton-miles is about 2.1 percent (or 97 billion ton-miles) less than the BTS preliminary estimate. This difference is primarily due to revisions between the preliminary and final 2002 Commodity Flow Survey data. The final CFS ton-miles were 67 billion tons less than the preliminary CFS data. The remaining difference of 30 billion ton-miles was due to improvements in methodology, i.e., the use of a more disaggregated method of estimating the distance traveled per shipment. While the new composite estimates were derived at the commodity, industry, and mode of transportation levels, the former estimates were derived mostly at the modal level. The new estimates for water transportation are lower than the preliminary estimates because in the new method some shipments of crude oil and petroleum products were correctly reassigned to pipelines to avoid double counting.
In a separate effort to improve on the estimate of overall ton-miles (see figure 5 and its source), BTS reported 4,366 billion as the total national ton-miles for 2002, a number that is fairly comparable to the new composite estimates. The difference of 43 billion ton-miles between that estimate and the jointly developed composite estimate in the above table is less than one percentage point. This difference is due mainly to different methodologies and definitions.