Vehicle-miles traveled (otherwise known as VMT) by cars, trucks and buses on public roads are used as a key measure of roadway use. This measure of mobility is used as the primary measure of travel activity on the highway system. When viewed monthly (as seen in Figure 1), the consistent seasonal variation makes it difficult for the reader to study the longer-term trend under that seasonality. The purpose of this analysis is to separate the seasonality component from that trend behavior of VMT.
A cursory review of the graph in Figure 1 gives the impression that, underneath the regular seasonal repetition, there exists an upward trend in the growth of VMT. However, the more recent data (e.g., the last two years) may be indicating a slow-down in that growth. Does that slowdown exist, and, if so, when did it start?
To look at the underlying trend, the data series is statistically decomposed to allow the seasonal component to be removed. Once decomposed, the seasonal as well as the trend components can be viewed separately. Figure 2 provides a plot of the seasonal component over time. As mentioned previously, the seasonal behavior is relatively consistent over time, with the exception of a further dip in the winter of 2001.
Since the seasonality is repetitive, it would be appropriate to average the same months over time to show the average monthly variation, which is shown in Figure 3. As can be seen in the graph, the summer travel months of May through August result in higher than average VMT values, whereas the winter months, November through February, result in lower than average VMT values.
The statistical component that represents the underlying trend is shown in Figure 4. As can be seen in the graph, the trend shows a strong upward growth, with an average increase of approximately 440 million VMT per month, until the beginning of 2000. Around July 2000, this trend shifts downward, and then resumes the same upward growth rate.