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Measuring International Trade on U.S. Highways
The quantity of ton-miles is the product of the tonnage of a shipment times the miles the shipment traveled to its destination. The quantity of value-miles is the product of the declared value of a shipment times the miles the shipment traveled. Both ton-miles and value-miles are derived from separate estimates of either weight or value and miles traveled.
Ton-mile and value-mile figures are combined from separate estimates of export and import trade traffic for a variety of categories. Despite growing attention to measures of tonnage, miles, and value, both ton-mile and value-mile estimates have many substantial sources of error. Because the data sources are not designed to directly yield ton-mile and value-mile measures, the estimates require application of model-based estimation techniques that rely on unvalidated assumptions. The models that were used in the BTS study included the Oak Ridge National Laboratory’s (ORNL) highway network model to derive estimates of the distances traveled by imports and exports. The accuracy of the distance estimates from this model is unknown. Moreover, data on imports were available only at the state level. The accuracy of the assumptions used to distribute state-level imports to counties within a state is unknown because of a lack of data at the substate level, and substate level allocations are needed to determine the likely route taken, which provides a better assessment of the miles traveled within a state.
Moreover, the various data sources utilized to produce estimates of international ton-miles and value-miles transported by highway have known and unknown errors and biases that affect the accuracy and reliability of the estimates. The errors and biases are due not only to typical sources, including sampling variance, nonresponse variance, and measurement error of various types, but also to various data deficiencies, even though the models mentioned above are used to overcome these deficiencies.
The BTS study identified a number of sources of error for a variety of data sets used to estimate volumes and values for freight arriving here as imports or departing as exports, which we summarize below. In addition, there are shipments involving Canada and Mexico that are not straightforward exports or imports: truck shipments between Canada and Mexico and shipments to or from the United States that are transshipped via Canada or Mexico are not fully captured in current trade data for all modes of transportation, which is also a source of error. Also, some truck transportation was not included in the estimates of the amount of international trade because no reliable data were available, especially including drayage, or short-haul shipments.