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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Executive Summary

Thursday, December 29, 2011

Executive Summary

The terrorist attacks of September 11, 2001, had an immediate and visible impact on U.S. transportation. While the obvious impacts were temporary, there may have been less obvious yet longer lasting changes in U.S. travel patterns. The Research and Innovative Technology Administration’s Bureau of Transportation Statistics analyzed the impacts in three different ways. All three analyses found these post-9/11 travel trends:

  • Immediate and continuing impact in air travel,
  • Immediate but temporary decline in highway travel,
  • No impact on rail travel, and
  • Travelers switched from air to highway.


1. NHTS Data: A comparison of 2001-2002 National Household Travel Survey (NHTS) long-distance travel data for pre-9/11 and post-9/11 yielded the following initial findings:

  • Reduction in the amount of long-distance travel,
  • Decrease in the rate of international trip taking,
  • Reduction in the rate of personal business travel, and
  • Changes in mode of travel depending on distance traveled.

2. Time Series Analysis: Forecasts using travel data from 1990 to 2001 were compared to what actually took place after 9/11. The comparisons found:

  • Actual Airline Revenue Passenger-Miles began in December 2004 to approach the forecasted values. Otherwise, up to then, Airline Revenue Pas-senger-Miles were significantly lower than forecast.
  • Rail Passenger-Miles showed no evidence of impact from 9/11.
  • Vehicle-Miles Traveled dropped for one month – September 2001 – compared to the expected level. In addition, the actual VMT level for September 2002 – one year later – was significantly lower than expected, while the 11 months between September 2001 and September 2002 did not show any unexpected deviations.

3. Econometric Analysis: A statistical analysis of economic data produced the following conclusions:

  • There was a strong statistical relationship between the events of 9/11 and aviation and highway travel, but not rail travel.
  • Air travel dropped quickly after 9/11 and then continued to drop for the following six months.
  • Highway travel also dropped quickly immediately after 9/11 but then leveled off in the following four months.
  • People switched from air travel to highway travel over the six-month period after 9/1l.

Expected Changes that didn’t Happen

The comparison of pre-9/11 and post 9/11 NHTS data did not find the following expected results:

  • No significant decline in the percent of business travel,
  • No significant decline in overall trips by older Americans,
  • No change in the percent of air trips from individuals living in urban areas.

The Methodology

1. NHTS Data: To assess the near-term impact on travel, we split the 2001-2002 National Household Travel Survey (NHTS) long-distance travel data collection into pre-9/11 and post-9/11 datasets. Each was then weighted to produce an annual estimate of long-distance travel—one based on survey responses before 9/11 and the other based on responses after 9/11. The two new datasets have some limitations that impact our ability to draw comparisons: seasonality effects that are unknown and we do not know how economic changes affected travel behavior after 9/11. Therefore, we conducted additional time-series and econometric data analysis to help assess the before and after 9/11 travel picture.

Based on the entire data collection, there were an estimated 2.6 billion long-distance trips taken in 2001. Privately owned vehicles (POV) accounted for the largest portion of trips, 90 percent, followed by air at 7 percent. Bus was used for only 2 percent of the trips and “other,” which includes trains, ferries, and other transportation means, collectively accounted for only 1 percent.

The NHTS analysis focuses on long-dis-tance trips because this type of travel was most impacted by 9/11 and is where measurable effects would most likely be found. The NHTS is a national survey with data on 45,000 long-distance trips of 50 miles or more from home.

Because the time period of the NHTS overlapped September 11, 2001, the data collected were divided into two files: a pre-9/11 file and a post-9/11 file, with both files weighted to produce estimates of a year’s worth of travel.

2. Time Series Analysis of Long-Distance Passenger Data: Differences due to seasonality could be confounding the measurement of the 9/11 impact in the NHTS data. To understand the seasonality characteristics of the passenger data, we analyzed three different sets of monthly data: air revenue passenger miles (RPM), rail passenger miles (PM), and highway vehicle miles traveled (VMT). To acquire a base from which to measure the 9/11 effect, we forecast these three series beyond September 2001 based on data from January 1990 through August 2001.

3. Econometric Analysis of Travel by Mode: Monthly seasonally adjusted modal data from January 2000 through June 2003 were used to estimate travel equations with a 9/11 dummy variable in order to measure the effects of 9/11 on travel by mode. Air, highway, and rail travel equations were econometrically estimated.

There was a statistically significant substitution relationship over the estimation period between air travel and highway travel, although the cross elasticity is small at -0.041. This cross elasticity was derived from the air travel coefficient and indicates that for any given 10 percent drop in air travel, highway travel will increase at approximately 0.41 percent.