Seasonal adjustment is the process of estimating and removing movement in a time series caused by regular seasonal variation in activity, e.g., an increase in air travel during summer months. Seasonal movement makes it difficult to see underlying changes in the data. Monthly shifts in data as well as short and long-term trends can be best seen through seasonally-adjusted data.
Seasonally-Adjusted Airline Data
Seasonally-Adjusted Vehicle Miles Traveled
Seasonally-Adjusted Modal Data for the Transportation Services Index (TSI)
The monthly data used to create the TSI are highly seasonal. In order to portray real growth in the modal data, BTS uses a method called X12-ARIMA to seasonally-adjust the data. Whereas the Total, Passenger and Freight TSI indicate the trends of the combined modes, the seasonally-adjusted modal data allow the user to understand which modes are experiencing the changes in trend over time.
Seasonally-adjusted Freight and Passenger TSI Modal Data