Data Review

Transportation Services Index

INTRODUCTION

In 2002, a team of academics, under a research grant from the Bureau of Transportation Statistics (BTS), developed the Transportation Services Index (TSI) to measure the state of the transportation sector and its contribution to the economy. The TSI, which consists of three seasonally adjusted monthly indexes, reflects the changes in the output of services for the passenger, freight, and total transportation sector (figure 1 and table 1 and table 2). The Journal of Transportation and Statisticspublished the results of the research project in 2003 (Lahiri et al. 2003).

CALCULATION OF INDEX

BTS calculates three monthly transportation output indexes: the total TSI, the freight TSI, and the passenger TSI. Collectively, the three indexes use eight sources of component data that reflect output in terms of passenger-miles or ton-miles (or some proxy thereof).

The TSI freight index, calculated using tons or ton-miles depending on the modal data source, includes for-hire trucking, freight railroad services (including intermodal shipments), inland waterway traffic, pipeline movements, and air freight. The freight index does not include international or coastal steamship movements, private trucking, courier services, or the United States Postal Service.

The TSI passenger index, calculated using passenger-miles, includes local mass transit, intercity passenger rail, and passenger air transportation. The passenger index does not include intercity bus, sightseeing services, taxi service, private automobile usage, or bicycling and other nonmotorized forms means of transportation.

The process of converting the component data into the final indexes includes forecasting missing values, deseasonalizing raw data, indexing values, weighting, and chaining final modal estimates.

• Forecasting is used to estimate data that are not reported in a timely manner.
• All data used in the TSI are deseasonalized to remove the impact of seasonal patterns. Transportation data are highly seasonal, masking true changes from month to month and long-term changes through the years.
• All modal data are indexed to a base year, currently set to 2000.
• Indexed modal data are weighted based on the value added by each mode to the transportation service sector of Gross Domestic Product (GDP), provided by the Bureau of Economic Analysis in the National Income Product Accounts.
• Using the chained Fisher Ideal Index and the GDP weights, the modal data are aggregated into the three composite indexes.

DATA TRENDS

Since its inception, the TSI has experienced an overall upward trend. During 1990, the TSI's first year of historical data, the total TSI ranged from 67.6 to 69.4. In comparison, during 2004, the total TSI ranged from 104.3 to 110.3.¹The freight TSI and passenger TSI, as components of the total TSI, experienced similar growth.

Minor directional changes in the TSI are frequent, with no period of more than three consecutive months of decline over the history of the index. Sustained and larger-than-normal change in the trend of the index occurs around the time of major events. Most noticeably, the events of September 11, 2001, caused a major change in all three indexes. Between August and September 2001, the total TSI declined 9.3%, the freight TSI declined 3.3%, and the passenger TSI declined 23.2%. However, the total and freight TSI returned to pre-September 2001 levels within a year; the passenger index took several years to return to pre-September 2001 levels.

BTS is currently researching the relationship of the TSI to the general economy to ascertain if the cycle of transportation output turns at about the same time as the economy turns (which would make this a coincident index) or if transportation turns in advance of the turns in the economy (a leading index). However, due to the limited length of the TSI data series, it has only been possible to observe the TSI during one entire recession.²

RELEASE OF TSI

BTS recognized the benefits of producing the TSI on a monthly basis and released the first monthly estimates in March 2004, with an historic data series beginning January 1990. Currently, TSI is the only combined, multimodal, seasonally adjusted measure of transportation services made available on a monthly basis.

On approximately the 6th of each month, new and updated TSI numbers are released on the BTS website. The newest three months are preliminary numbers. Each month BTS releases the latest preliminary TSI, and replaces the oldest preliminary TSI with a revised TSI. After a number is revised, it will not change until the annual update is released in mid-year when most of the prior year data have been finalized.

To obtain the most current TSI data, as well as further documentation on the indexes, visit the BTS website: http://www.bts.gov/xml/tsi/src/index.xml.

REFERENCE

Lahiri, K., W. Yao, H. Stekler, and P. Young. 2003. Monthly Output Index for the U.S. Transportation Sector. Journal of Transportation and Statistics6(2/3):1–27.

END NOTES

1. All numbers are indexed to a base year of 2000 (2000 = 100).

2. The National Bureau of Economic Research designated March 2001 through November 2001 a period of recession.

Review by: Jennifer Brady, Analyst, Bureau of Transportation Statistics, Research and Innovative Technology Administration, U.S. Department of Transportation, 400 Seventh St. SW, Room 3430, Washington, DC 20590. Email address: jennifer.brady@dot.gov.