NHTS Frequently Asked Questions

1. Why can't I use 2001 NHTS data for my city or state?
2. Will I be able to use the 2001 NHTS data to estimate travel in my state or my planning area?
3. What is an add-on area?
4. What are the similarities and differences between the 1995 NPTS and the 2001 NHTS?
5. Why does a household member have his/her information in the household file but not in the person file?
6. What was the rule in designating whether a sample household was a complete household?

• NHTS Study Participants, Important Information

1) Why can't I use 2001 NHTS data for my city or state?

In order for the 2001 NHTS to provide statistically accurate estimates for your state or your MPO, a sufficient number of households in your state or your MPO must be interviewed in this survey. Unfortunately, limited resources have prohibited this level of sampling. As such, the 2001 NHTS data can not be used to calculate trip rates and travel statistics specific to your state or MPO. The only exception to this are the add-ons.

That said, it is possible for you to use the 2001 NHTS data to estimate trip rates and travel statistics for your state or MPO (albeit not with the statistical rigor of the add-ons). Please see FAQ on "Will I be able to use the 2001 NHTS data to estimate travel in my state or planning area?" for details.  

2) Will I be able to use the 2001 NHTS data to estimate travel in my state or my planning area?

The 2001 NHTS was designed to provide statistically valid estimates of trips rates and travel statistics at the national level, specifically at the nine Census Division level, and at the seven area-type levels. The nine Census Divisions are:

1. New England,
2. Middle Atlantic,
3. East North Central,
4. West North Central,
5. South Atlantic,
6. East South Central,
7. West South Central,
8. Mountain, and
9. Pacific.

The seven area types are with respect to population size and the availability of transit services:

1. An area with more than three million people with heavy transit,
2. An area with more than three million people without heavy transit,
3. An area with more than one million but less than three million people,
4. An area with more than 500,000 and less than 1 million people,
5. An area with more than 250,000 but less than 500,000 people,
6. An area with less than 250,000 people, and
7. An area not in an MSA or an CMSA.

If you are not an add-on area, you could use the 2001 NHTS data to estimate trip rates for your state or MPO by assuming that the travel behavior in your area is similar to that of the sampled households in states within the same Census Division as yours, and in areas with a population size similar to that of your MPO. To identify sampled households in an area that is similar to yours with respect to Census Division, population size, and the availability of transit services, please use variable CDIVMSAR.

There is another approach that you can use to estimate travel for your state or planning area. Please see Transferability of National Data for Local Estimates for details.  

3) What is an add-on area?

An add-on area is either a state or a planning organization that purchased additional sampling of households in its state/area to be surveyed. The rationale is to increase the number of sample households in the state/area so that trip rates and travel statistics can be reliably estimated at that geographic level.

Four states and five planning areas purchased additional samples in the 2001 survey, adding over 40,000 sample households to the NHTS. The add-ons and the target sample sizes were:

1. State of Wisconsin: 16,000 households
2. State of New York: 10,884 households
3. State of Texas: 3,500 households
4. State of Hawaii: 1,500 households
5. Kentucky 4 county area: 1,000 households
6. Baltimore, MD: 3,471 households
7. Des Moines MPO, IA: 1,200 households
8. Lancaster MPO, PA: 1,000 households

The final sample size for the 2001 NHTS was targeted at 65,055 = (25,000 for the national sample + 40,055 for the add-on samples) households.

Statistically developed sample weights (i.e., factors that expand data collected from the sample households to represent the entire nation) do not allow add-on samples to overpower the national sample and to bias the national level estimates.

4) What are the similarities and differences between the 1995 NPTS and the 2001 NHTS?

There are a number of improvements and changes in the 2001 NHTS:

1. Travel taken by persons younger than five years old are enumerated,
2. More emphasis on walk and bike trips by prompting specifically for these trips,
3. If one did not travel on the designated travel day, then information on the most recent trip was collected,
4. If one did not travel during the designated travel period (the 28-day period when data on long distance trips were collected), then information on the most recent long distance trip was collected,
5. Information on access and egress to the transit station was explicitly collected,
6. The travel period was a 28-day period in 2001 but a two-week period in 1995, and
7. Long distance trips in 2001 were those with the farthest destination 50 miles away from home while the criterion in 1995 was 75 miles

In addition, a number of questions were added to the 2001 NHTS to cover emerging trends pertinent to personal travel behavior:

At the Household Level.

• Cell-phone ownership
• Number of phone lines owned and how they were used (voice, fax, modem)
• Vehicle fuel consumption and annual fuel cost

At the Person level.

• Internet access and frequency and location of use
• Travel disability and its effect on mobility
• Primary activities during .last week .
• Explicit coding of multiple jobs
• Broad categories of occupation
• Immigrant status
• Frequency of walk & bike trips during the week prior to the interview day

At the Individual Daily Trip level.

• More detail on trip purpose
• Access and egress modes to transit stations

At the Individual Long distance Trip Level.

• Types of lodging used
• Access and egress modes to airport, train station, etc.
• Overnight stops & purpose of stops
• All modes used at the final destination

5) Why does a household member have his/her information in the household file but not in the person file?

The 2001 NHTS data collection consisted of two main phases:

1. Household Interview, also known as the "Screener Survey". It collected information about the household (including its mailing address), the household members and their driving status, and the vehicles available to the household. It was conducted once per household. The inventory of all household members is on the household file regardless of whether all or some of the household members were interviewed.
2. Person Interview, also known as the "Extended Survey". It collected detailed information about a household member with respect to six major categories:
   1. Demographics (e..g, race, education, employment status)
   2. Commute patterns during the week prior to the interview day,
   3. Trips taken on the designated travel day,
   4. Long distance travel during the designated travel period,
   5. Vehicles available to the household, and
   6. Assessment of the travel services, impediments to travel, and other factors impacting travel behavior.

It was conducted once per household member.

If a household member did not complete the person-level interview and if no proxy completed this interview for him/her , then his/her information would NOT be included in the person file. Note that as long as a household member (or a proxy) completed his/her personal level interview, there will be a record for him/her in the person file regardless of whether he/she took any trips on the designated travel day.

6) What was the rule in designating whether a sample household was a "complete" household?

The concept of a "complete" household is used in the context of survey responses. The NHTS used a fifty-percent rule in deciding whether a household was a complete household.

The fifty-percent rule states that if fifty or more percent of the adults (18 years or older) within a household completed the Person Interview (or Extended Survey), then this household was considered a complete household. The person weights were adjusted to account for non-interviewed persons within an interviewed household. The fifty-percent rule was adopted to address a concern that excluding households where some of the household members did not respond to the survey might bias the survey results. This concern was confirmed in a Federal Highway Administration-sponsored report where low-income households and large households (with four or more members) were found more likely not to respond to surveys than other households.

In the NHTS, the household file contains information on all members of the household (such as age, gender, and employment and drivers license status) regardless of whether all of the members responded to the Person Interview (or Extended Survey).