Figure 4. Monthly Seasonal Factors for Modal Transportation Employment (thousands), January 1990 - July 2010
Figure 4. Monthly Seasonal Factors for Modal Transportation Employment (thousands), January 1990 - July 2010
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Source: U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, special tabulations on data from http://www.bts.gov/publications/key_transportation_indicators/
Another way to observe the seasonal patterns in the modal data, unaffected by the impact of changing long-term trends, is to calculate the average monthly seasonal factors1 for each mode – as shown in Figure 4.2 Seasonality has its greatest negative effect on transit employment in July and August, whereas it has its greatest positive effect on truck employment in August and September. Truck employment falls to its lowest point in January, February, and March.
1 The average monthly seasonal factor is calculated by adjusting the monthly seasonality over time. For example, the adjusted, or "average," seasonal factor for January would be the same in 1990 as it would be for 2010 (or any year in between). By adjusting a month's seasonality over time, the overall impact of that month relative to the other months of the year can be observed.
2 The seasonal factors and trends in transportation employment data are calculated through STAMP (Structural Time Series Analyser, Modeller and Predictor) software. For more information on the software go to: http://www.timerlake.co.uk/