The freight transportation services index combines monthly truck tonnage, air revenue from freight and mail, weekly rail carloads, rail ton-miles, tons moved by water and pipeline transportation into a single indicator. A similar passenger index measures passenger miles and trips by plane, train and public transportation. Both measures have been found to change ahead of the wider economy.
"A very large portion of freight volume consists of raw materials and other intermediate goods, which may be ordered in anticipation of growing activity in the manufacturing sector," BTS analysts wrote in their last historical review of the data.
Conversely, as downstream demand begins to falter, freight shipments also decline. While passenger travel is a consumer service and may be expected to lag or change with the economy, personal and business travel also seem to respond to economic confidence and sometimes lead other indicators.
By combining multiple types of weighted transportation data in the two indexes, the TSI captures overall economic changes rather than simple shifts in market conditions between the various transportation providers (such as a change from using trains to using trucks, for example).