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Chapter 1. The NHTS and 9/11

Chapter 1. The NHTS and 9/11

Pre- and Post-9/11 Data Files

The 2001 National Household Travel Survey (NHTS) was conducted from March 2001 through May 2002. Because the data collection time period includes 9/11/2001, there has been considerable interest in using this data set to assess the effect of the events of 9/11 on travel behavior, especially for long-distance trips. This document discusses long-distance trip data set files only.

However, the 2001 NHTS was not designed to assess the effects of 9/11 on long-distance travel and several factors preclude the direct comparison of pre-and post-9/11 travel data. Travel is influenced by seasonality, economic conditions, and other factors.  Therefore, the differences in travel volume and patterns in the pre- and post-9/11 data sets cannot be attributed solely to the impact of the terrorist attacks.

The pre-9/11 period of 2001 NHTS covers March 2001 to September 2001, a period of over 5 months, and includes the summer season in which a large proportion of long-distance trips are taken. There were approximately 22,000 persons responding about travel prior to 9/11. On the other hand, the post-9/11 period of the survey covers September 2001 to May 2002, a period of roughly 8 months, and includes Thanksgiving and Christmas and other winter holiday travel a traditionally heavy season for long-distance trips. This survey had responses from approximately 38,000 persons on their long-distance trips after 9/11. The demographic, spatial, and temporal composition of persons who took long-distance trips prior to 9/11 and those who took long-distance trips following 9/11 were not the same.

To facilitate assessment of the effect of 9/11, one approach is to make each of the two groups a nationally representative sample. This is achieved by constructing new weights using statistically sound methods.

The pre-9/11 and post-9/11 data files were created for both person-level and long-distance trip level by dividing the NHTS 2001 public use person data file and long-distance trip data file, respectively, into two parts organized into four different data files: Pre-9/11 Person, Post-9/11 Person, Pre-9/11 Long-Distance Trip, and Post-9/11 Long-Distance Trip. Note that at this point, because of interest, the daily trips level file has not been divided for pre/post- 9/11 comparison. The new full sample weights and replicates for each of these four data files were created by applying statistical adjustment on the original weights to obtain valid annual estimates and to estimate the statistical significance of the estimates. The weight calculation for Pre-9/11 and Post-9/11 data only applied for the national usable level, so there is only one level weight for each data set.

How the data, weights, and replicates were created and how to use them are described in detail in Chapter 2 of this documentation.

NHTS 2001--Overview

The National Household Travel Survey is a U.S. Department of Transportation (DOT) effort sponsored by the Bureau of Transportation Statistics (BTS) and the Federal Highway Administration (FHWA) to collect data on both long-distance and daily travel by the American public. The joint survey gathers trip-related data such as mode of transportation, duration, distance, and purpose of trip. The Survey also collects demographic, geographic, and economic data for analysis purposes. Policy makers, individual state DOTs, metropolitan planning organizations, industry professionals, and academic researchers use the data to gauge the extent and patterns of travel, to plan new investments, and to better understand the implications of travel trends on the nations transportation infrastructure.

The 2001 NHTS updates information gathered by two earlier series of travel surveysthe Nationwide Personal Transportation Survey (NPTS) conducted in 1969, 1977, 1983, 1990, and 1995, and the American Travel Survey (ATS) conducted in 1977 and 1995. It is a nationally representative survey of the daily and long-distance travel patterns of the nation using travel behavior data collected from all household residents in roughly 26,000 households.

The survey was conducted from March 2001 through May 2002. The first telephone call to recruit a household was made on March 19, 2001, and the last telephone call was made on May 4, 2002. Each household was sent a survey form and asked to report all travel by household members on a randomly assigned travel dayInterviewers followed up with a phone call and asked respondents about their travel on the travel day and the preceding 27 days. This four-week period is called the travel period. Travel days for daily-travel trip reporting were assigned for all seven days of the week, including all holidays. The first travel day assigned was March 29, 2001, and the last travel day assigned was May 4, 2002. The last travel period assigned was April 7 through May 4, 2002. The respondents were asked to report on trips 50 miles or more, referred to as along-distance trips or travel-period trips taken by household members during the travel period, including the travel day.

Public-use national data from the 2001 NHTS is organized into five different data files:

  1. households,
  2. persons,
  3. vehicles,
  4. daily travel, and
  5. long-distance travel.

Weights and replicates were provided for each of these five data files. This information is used to extrapolate the entire populations annual travel patterns from this representative survey of residents travel patterns. Weights and replicates match the studied variables of the sample size to the same information of the population at large.

The data files contain two kinds of weights: one is from "usable" households in which person interviews were completed with at least 50 percent of adults in the household (26,038 households in the sample), and another is from"100 percent" households in which person interviews were completed with all adults in the household (22,178 households in the sample).

For more information about weights see Person-Level Weight.

For more information about long-distance and daily trip data in NHTS 2001, please refer to the National Household Travel Survey (NHTS)National Data and Analysis Tool CD.