Frequently Asked Questions (FAQ)
About the NHTS
- What are the NHTS and NPTS?
- How is NHTS data used?
- What is an NHTS add-on?
- How was the NHTS conducted?
- When was data collected for the 2009 NHTS?
- Who is included in the survey?
- Which daily trips are included?
Notes for Data Users
- How do I identify weekday vs. weekend travel?
- How do I use trip count variables?
- Why does a variable have negative values, such as -1 or -9?
- How do I obtain information on trip purpose?
- Which variable(s) do I use to determine urban/rural status?
- How can I cite NHTS data in my research?
- How can I determine what values of a specific variable mean?
- How do I compute statistics such as person trips per person and vehicle miles of travel per household?
- At what geographic levels is it statistically valid to use NHTS data?
- What is the difference between standard errors and margins of error?
About the NHTS
What are the NHTS and NPTS?
The NHTS provides information to assist transportation planners and policy makers who need comprehensive data on travel and transportation patterns in the United States. The 2009 NHTS updates information gathered in the 2001 NHTS and in prior Nationwide Personal Transportation Surveys (NPTS) conducted in 1969, 1977, 1983, 1990, and 1995.
How is NHTS data used?
NHTS data has been used to explore topics including traffic safety, congestion, the environment, energy consumption, demographic trends, bicycle and pedestrian studies, and transit planning.
For detailed examples, see the NHTS Compendium of Uses.
What is an NHTS add-on?
An NHTS add-on refers to a state or MPO that purchases additional samples for their region. This allows for, at the discretion of the state/MPO, estimates to be computed at lower levels of geography, more precision for a general area, or the addition of a handful of additional questions to be asked that are not part of the entire national sample.
For more information, see the About the Add-on Program page.
How was the NHTS conducted?
The 2009 NHTS was conducted using Computer-Assisted Telephone Interviewing (CATI) technology. Each household in the sample was assigned a specific 24-hour "Travel Day" and kept diaries to record all travel by all household members for the assigned day.
When was data collected for the 2009 NHTS?
The 2009 NHTS was conducted from March 2008 to April 2009.
Who is included in the survey?
The 2009 NHTS collected travel data from a national sample of the civilian, non-institutionalized population of the United States. As such, an eligible household excludes telephones in motels, hotels, group quarters, such as nursing homes, prisons, barracks, convents or monasteries and any living quarters with 10 or more unrelated roommates.
There are approximately a total of 150,000 households in the final 2009 NHTS dataset. About 25,000 households are in the national sample, while the remaining 125,000 households are from twenty add-on areas.
Which daily trips are included?
All trips reported by eligible household members in eligible households in the U.S. were recorded. The designated 24-hour travel day starts at 4:00 AM of the day assigned and continues until 3:59 AM of the following day. On a typical day, 4 AM represents the time when the fewest number of people are traveling, thus allowing collection of more coherent data on trips. On weekends the travel day begins on Friday at 6PM and ends on Sunday at midnight.
Notes for Data Users
How do I identify weekday vs. weekend travel?
Households were assigned a specific travel day and instructed to record all their travel for that day. The variable, TRAVDAY, identifies the assigned travel day of each household. The TRAVDAY variable is in all four data files (household, person, travel day, and vehicle).
For more information, see the writeup Weekday vs. Weekend Travel [PDF].
How do I use trip count variables?
The count variables included in the household and person files should be used with caution (e.g., CNTTDHH in the household file and CNTTDTR in the person file). These variables were added to the dataset to make it easier for the user to categorize the number of trips per household and per person. They are especially useful to separate zero-trip persons or households for special analysis. Examples of the correct use of these variables would be to construct a table showing the distribution of households by household size and number of travel day trips (0, 1, 2, 3, 4, etc.), or a distribution of persons by age/gender category and number of travel day trips.
Do NOT use these "count" variables to generate estimated trips for the travel day. The weights on the household and person files, which are applied to the "count" variables, do not fully represent the travel for the sample day. The only legitimate use of these variables is to generate distributions of households or persons by number of trip category, such as zero travel day trips, 1-2, 3-4, 5 or more.
For more information, see the writeup A Special Note on Count Variables [PDF].
Why does a variable have negative values, such as -1 or -9?
When a survey respondent did not answer a specific question, whether via refusal or a legitimate survey skip (e.g., a question on school safety would not be asked of elderly respondents), the value in the dataset is typically negative. For variables such as trip distance (TRPMILES), such values will skew computed statistics downward. Thus, one should remove such values when computing sums or averages.
For more information, see the writeup on Negative Values [PDF].
How do I obtain information on trip purpose?
Three different types of trip purpose data are found in the most recent NHTS. These types include historical trip purpose definitions that are consistent with past surveys back to 1983 (WHYTRP90), expanded trip purpose definitions that include "To Home" as a purpose (WHYTRP95/WHYTO), and trip purpose for urban planners that include home-based and not home-based coding (TRIPPURP).
For information on historical trip purposes, view the NHTS Academy module Historical Trip Purpose Development (1983-2009) [WHYTRP90].
For information on expanded trip purpose definitions, view the NHTS Academy module Expanded Trip Purpose Definitions Including Trips to Home (1995-2009) [WHYTRP95/WHYTO].
For more information on trip purpose for urban planners, see the writeup TRIPPURP Definitions [PDF] or view the NHTS Academy module Trip Purpose for Urban Planners: Home-Based and Non Home-based Codes [TRIPPURP].
Which variable(s) do I use to determine urban/rural status?
The variable URBAN contains information such as whether a surveyed household is in an urban cluster, in an urban area, surrounded by urban areas, or is not in an urban area. The variable URBRUR condenses this information into urban (in an urban cluster, in an urban area, surrounded by urban areas) vs. rural (not in an urban area).
For more information, see the writeup URBAN-RURAL Designations [PDF].
How can I cite NHTS data in my research?
To recognize the valuable role of National Household Travel Survey (NHTS) data in the transportation research process and to facilitate repeatability of the research, users of NHTS data are asked to formally acknowledge the data source. Where possible, this acknowledgment should take place in the form of a formal citation, such as when writing a research report, planning document, on-line article, and other publications. The citation can be formatted as follows:
U.S. Department of Transportation, Federal Highway Administration, 2009 National Household Travel Survey. URL: http://nhts.ornl.gov.
How do I compute statistics such as person trips per person and vehicle miles of travel per household?
First, compute total trips using the class variable(s) of your choice. Then, compute total persons or total households using those same class variables. Then divide total trips by persons or households for each respective class level for annual trips or miles per person or household. For daily numbers, divide the annual number by 365. One common mistake is trying to calculate VMT per household using only the person file, but since households and persons/trips are weighted differently, one needs to use both the household and travel day files and then divide through using the respective totals.
At what geographic levels is it statistically valid to use NHTS data?
In the 2009 NHTS, estimates are valid down to the state level. However, there are several things to note. First, several states and MPOs purchased add-on samples for their areas, so estimates in those areas may be (but are not necessarily) valid to smaller levels of geography. Second, standard errors (or margins or error in Table Designer) should generally be used, particularly when looking at estimates at geographies smaller than the national level. Third, previous surveys were sampled at even higher levels of geography, meaning samples were not valid down to the state level. For example, in 2001 estimates are valid down to the Census division level, while in 1995 and before, estimates are only valid at the Census region level.
What is the difference between standard errors and margins of error?
Standard errors in the NHTS can be computed to measure sampling error. In general terms, a statistic can be estimated to within 95% confidence within two standard errors. The margin of error makes this 95% confidence computation easy, as it is simply two times the standard error.