"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: 1) Immediate and continuing impact in air travel 2) Immediate but temporary decline in highway travel 3) No impact on rail travel, and 4) 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: 1) Reduction in the amount of long-distance travel 2) Decrease in the rate of international trip taking 3) Reduction in the rate of personal business travel, and 4) 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 Passenger- Miles were significantly lower than forecast."
Bureau of Transportation Statistics: http://www.bts.gov/