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Appendix E: Abbreviations, Travel Concepts, and Glossary of Terms

Appendix E: Abbreviations, Travel Concepts, and Glossary of Terms


ASCII American standard code for information interchange
ATS American Travel Survey
BTS Bureau of Transportation Statistics
CASRO Council of American Survey Research Organizations
CATI Computer-assisted telephone interviewing
CMSA Consolidated metropolitan statistical area
DOT Department of Transportation
FHWA Federal Highway Administration
FIPS Federal Information Processing Standards
HH Household
ID Identification number
HHM Household member
MPO Metropolitan Planning Organization
MSA Metropolitan statistical area
NHTS National Household Travel Survey
NHTSA National Highway Traffic Safety Administration
NPTS National Personal Transportation Survey
NTS National Travel Survey
PMSA Primary metropolitan statistical area
PMT Person miles of travel
POV Privately owned vehicle
PSU Probability sampling unit
RDD Random digit dialing
SAS Statistical Analysis System
TRC Telephone research center
VMT Vehicle miles of travel

Travel Concepts

Person trip

Definition. A trip by one person in any mode of transportation. This is the most basic and universal measure of personal travel. Each record in the Travel Day file in the NHTS dataset represents one person trip.

Examples. Two household members traveling together in one car are counted as two person trips. Three household members walking to the store together are counted as three person trips.

When to use. The unit of person trips must be used when comparing travel by various modes (e.g., private vehicles, public transportation, walking, school bus, air, etc.). It is the appropriate unit of measure for the movement of people, as opposed to vehicles, e.g., "the High Occupancy Vehicle (HOV) lanes carry 42 percent of all person trips to the central city."

How to compute. Because the person trip is the basic unit of measure on the Travel Day files, to obtain total person trips for useable households, the user should sum the weighted travel day records, i.e., sum WTTRDFIN.

Person miles of travel (PMT)

Definition. The number of miles traveled by each person on a trip.

Examples. If two people traveling together take a six-mile subway trip to the airport, that trip results in 12 person miles of travel. A four-mile van trip with a driver and three passengers counts as 16 person miles of travel (4 people times 4 miles).

When to use. As with person trips, person miles must be used when analyzing travel by the various modes of transport. It is the appropriate measure when the topic of analysis is the miles traveled by people, not vehicles.

Alias. Person miles is often called passenger miles, particularly in the transit and airline industries.

How to compute. Multiply each weighted person trip (WTTRDFIN) by the travel day trip distance in miles (TRPMILES).

Warning. When computing TRPMILES, be sure to exclude entries of
-1 - question not applicable,
-7 - miles refused,
-8 - miles not known, and
-9 - miles not ascertained.

Vehicle trips

Definition. A trip by a single privately operated vehicle (POV) regardless of the number of persons in the vehicle.

Examples. Two people traveling together in a car would be counted as one vehicle trip. Four people going to a restaurant in a van is considered one vehicle trip.

NPTS mode restrictions. To be considered a vehicle trip in NHTS, the trip must have been made in a privately operated vehicle, namely a household-based car, van, sport utility vehicle, pickup truck, other truck, recreational vehicle, motorcycle, or other POV. The vehicle does not need to belong to the household.

Trips made in other highway vehicles, such as buses, streetcars, taxis, and school buses are collected in the NHTS, but these are shown as person trips by those modes. The design of the NHTS is such that it does not serve as a source for vehicle trips in modes such as buses, because there is no way to trace the movement of the bus fleet throughout the day. Those interested in vehicle trips by buses, taxis, etc. need to use a data source that relies on reports from the fleet operators of those vehicles. The National Transit Database of the Federal Transit Administration is one such source.

When to use. The unit of vehicle trips is most appropriately used when considering POV travel, e.g., 20 percent of all POV trips are for commuting to and from work.

How to compute. The variable DRVR_FLG was created to allow the data user to select the vehicle trip records from the travel day file. The typical manner of computing vehicle trips from the NHTS file is to impose two limits on the full universe of Travel Day trips:
- travel mode must be POV (TRPTRANS = 01 -07), and
- only the drivers trip is captured (DRVR_FLG = 01).

The second limitation is to ensure that the trip is counted only once. Remember that the NHTS Travel Day file is a person trip file, so if three household members went somewhere by car, that trip is reflected in three travel day trip records. To ensure that it is only counted once as a vehicle trip, the drivers record is used.

To obtain the total of all vehicle trips, sum all weighted trips that meet the two conditions above, i.e., where DRVR_FLG=1.

Vehicle miles of travel (VMT)

Definition. One vehicle mile of travel is the movement of one privately operated vehicle (POV) for one mile, regardless of the number of people in the vehicle.

Examples. When one person drives her car 12 miles to work, 12 vehicle miles of travel have been made. If two people travel three miles by pickup, three vehicle miles of travel have been made.

Same mode restrictions. For NHTS data, vehicle miles are restricted to the same privately operated vehicles as vehicle trips (see above); that is, a household-based car, van, sport utility vehicle, pickup truck, other truck, recreational vehicle, or other POV.

When to use. Vehicle miles of travel (VMT) are a very commonly used measure of highway travel. This measure is particularly important when analyzing highway capacity, congestion, and air quality.

How to compute. Multiply each weighted vehicle trip by the distance. In terms of NPTS variables, this would look like (DRVR_FLG=1 times WTTRDFIN) times TRPMILES.

Warning. When computing TRPMILES, be sure to exclude entries of
-1 - question not applicable,
-7 - miles refused,
-8 - miles not known, and
-9 - miles not ascertained.

Vehicle occupancy

Definition. For NHTS data, vehicle occupancy is generally computed as person miles of travel per vehicle mile (referred to as the travel method). Note that the other commonly used definition of vehicle occupancy is persons per vehicle trip (referred to as the trip method).

Comments. Because longer trips often have higher occupancies, the travel method generally yields a higher rate than the trip method. The calculation of the travel method requires that trip miles be reported, thus it is calculated on a slightly smaller number of trips than the trip method.

How to compute. The four variables that may be used in the computation are described earlier in this section. Just remember to limit the denominator to person trips or person miles in POVs.


This glossary provides the most common terms used in the NHTS and definitions of those terms. These definitions are provided to assist the user in the interpretation of the NHTS data.


For NHTS, an adult is defined as a person 18 years or older.

Block group

A block group is a subdivision of a Census tract that averages 1000 to 1100 people and approximately 400500 housing units. The source used for the 2001 NHTS was GDT Dynamap 2000 (from Census 2000 TIGER/Line files).

Census region and division

The Census Bureau divides the states into four regions and nine divisions. Note that the divisions are wholly contained within a region; i.e., region lines do not split division lines. The following are Census regions and their component divisions:

Northeast Region

  • Middle Atlantic Division: New Jersey, New York, Pennsylvania

North Central (Midwest) Region


South Region

  • South Atlantic Division: Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia
  • East South Central Division: Alabama, Kentucky, Mississippi, Tennessee
  • West South Central Division: Arkansas, Louisiana, Oklahoma, Texas

West Region

  • Pacific Division: Alaska, California, Hawaii, Oregon, Washington

For the 2001 NHTS the source used for the 2000 Census Region was, and the source used for the 2000 Census Division was

Census tract

A Census tract is a small subdivision of a county, containing approximately 4,000 persons. Tracts can range in population from 2,500 to 8,000. The geographic size of the tract may vary considerably, depending on population density. Tracts were designed to be homogeneous in regard to population characteristics, economic status and living conditions when they were first delineated. Since the first tracts were delineated for the 1890 Census, todays tracts may be far from homogeneous. The source used for the 2001 NHTS was GDT Dynamap 2000 (from Census 2000 TIGER/Line files).

Consolidated metropolitan statistical area (CMSA)

A consolidated metropolitan statistical area is a large metropolitan complex with a population of 1 million or more, containing two or more identifiable component parts designated as primary metropolitan statistical areas (PMSAs). For example, the Boston CMSA is composed of six PMSAs.


For travel day trips, the destination is the point at which there is a break in travel, except if the break is only to change vehicles or means of transport. For travel period trips, the destination is the farthest point of travel from home.




A person is considered employed if (s)he worked for pay, either full time or part time, during the week before the interview.

Education level

Education level is the number of years of regular schooling completed in graded public, private, or parochial schools, or in colleges, universities, or professional schools, whether day school or night school. Regular schooling advances a person toward an elementary or high school diploma, or a college, university, or professional school degree.


A household is a group of persons whose usual place of residence is a specific housing unit; these persons may or may not be related to each other. The total of all U.S. households represents the total civilian non-institutionalized population. A household does not include group quarters (i.e., 10 or more persons living together, none of whom are related).

Household income

Household income is the money earned by all family members in a household, including those temporarily absent. Annual income consisted of the income earned 12 months preceding the interview. Household income includes monies from all sources, such as wages and salary, commissions, tips, cash bonuses, income from a business or farm, pensions, dividends, interest, unemployment or workmens compensation, social security, veterans payments, rent received from owned property (minus the operating costs), public assistance payments, regular gifts of money from friends or relatives not living in the household, alimony, child support, and other kinds of periodic money income other than earnings. Household income excludes in-kind income such as room and board, insurance payments, lump-sum inheritances, occasional gifts of money from persons not living in the same household, withdrawal of savings from banks, tax refunds, and the proceeds of the sale of ones house, car, or other personal property.

Household income has been provided in two variables. HHINCTTL is the variable that reflects the basic household income (HHFAMINC) plus the income of household members that were not reported in HHFAMINC but instead reported separately (INC_Pn, INCM_Pn). The purpose of creating HHINCTTL is to provide a more accurate level of household income for those households where some members reported separate income. HHFAMINC was the main income variable in the earlier NPTS series. When comparing 2001 data to the earlier surveys, conducted in 1969, 1977, 1983, 1990 and 1995, use HHFAMINC. Appendix G provides details on how these variables were created.

Household members

Household members include all people, whether present or temporarily absent, whose usual place of residence is in the sample unit. Household members also include people staying in the sample unit who have no other usual place of residence elsewhere.

Household vehicle

A household vehicle is a motorized vehicle that is owned, leased, rented or company owned and available to be used regularly by household members. Household vehicles include vehicles used solely for business purposes or business-owned vehicles, so long as they are driven home and can be used for the home to work trip, (e.g., taxicabs, police cars, etc.). Household vehicles include all vehicles that were owned or available for use by members of the household during the travel period, even though a vehicle may have been sold before the interview. Vehicles excluded from household vehicles are those that were not working and were not expected to be working and vehicles that were purchased or received after the designated travel day.

Means of transportation

A means of transportation is a mode of travel used for going from one place (origin) to another (destination). This includes private and public modes as well as walking.

The following transportation modes, grouped by major mode, are included in the NHTS data. The numbers correspond to the code for the mode in the NHTS questionnaire (see Appendix J).

Private vehicle

1. Car. A privately owned and/or operated licensed motorized vehicle including cars and station wagons. Leased and rented cars are included if they are privately operated and not used for picking up passengers in return for fare.
2. Van. A privately owned and/or operated van or minivan designed to carry 5 to 13 passengers or to haul cargo.
3. Sport utility vehicle. A privately owned and/or operated vehicle that is a hybrid of design elements from a van, a pickup truck, and a station wagon. Examples include a Chevrolet Blazer, Ford Bronco, Jeep Cherokee, or Nissan Pathfinder.
4. Pickup truck. A pickup truck is a motorized vehicle, privately owned and/or operated, with an enclosed cab that usually accommodates 2-3 passengers, and an open cargo area in the rear. Later model pickups often have a back seat that allows for total seating of 4-6 passengers. Pickup trucks usually have the same size of wheel-base as a full-size station wagon. This category also includes pickups with campers.
5. Other truck. This category consists of all trucks other than pickup trucks (i.e., dump trucks, trailer trucks, etc.).
6. RV or motor home. An RV or motor home includes a self-powered recreational vehicle that is operated as a unit without being towed by another vehicle (e.g., a Winnebago motor home).
7. Motorcycle. This category includes large, medium, and small motorcycles and mopeds.

Public transportation

10. Local public transit buses. Mass transit buses that are available to the general public.
11. Commuter buses.
16. Commuter train.
17. Subway/elevated rail. Also know as rail rapid transit is a high capacity system operated on a fixed rail or guide way system on a private right of way.
18. Street car/trolley. Vehicles that run on a fixed rail system powered by electricity obtained from an overhead power distribution system.

Other modes

8. Commercial/charter airplane. Airplanes that are available for use by the general public in exchange for a fare.
9. Private/corporate airplanes.
12. School buses.
13. Charter/tour buses. Privately owned buses that are either rented by a group or are available to the public for a fee for sightseeing.
14. City to city buses. Buses that run from one urban center to the other.
15. Amtrak/intercity train. Heavy passenger rail that runs form one urban center to another.
19. Ship/cruise ships.
20. Passenger line/ferry.
21. Sailboat/motorboat/yacht.
22. Taxicab. Includes the use of a taxicab by a passenger for fare. The taxi category does not include rental cars if they are privately operated.
23. Limousine. Includes the use of a limousine by passenger for fare. The limousine category does not include rental cars if they are privately operated.
24. Hotel/airport shuttle. Includes privately operated shuttle buses that are operated between a limited number of points for a fare.
25. Bicycle. Includes bicycles of all speeds and sizes that do not have a motor.
26. Walk. Includes walking and jogging.
91. Other. Includes any types of transportation not previously listed, e.g. skateboards.

Metropolitan statistical area (MSA)

Except in the New England States, a metropolitan statistical area is a county or group of contiguous counties which contains at least one city of 50,000 inhabitants or more, or twin cities with a combined population of at least 50,000. In addition, contiguous counties are included in an MSA if, according to certain criteria, they are socially and economically integrated with the central city. In the New England States, MSAs consist of towns and cities instead of counties. The source used for the 2001 NHTS was 1999 Metropolitan Areas: Cartographic Boundary Files. File ma99_99.shp from

Motorized vehicle

Motorized vehicles are all vehicles that are licensed for highway driving.


Occupancy is the number of persons, including driver and passenger(s) in a vehicle. NHTS occupancy rates are generally calculated as person miles divided by vehicle miles.


Origin is the starting point of a trip.

Overlap trip

An overlap trip is a travel period trip that occurs on the travel day and is thus collected in both portions of the NHTS questionnaire. To ensure that this trip is not counted twice, eliminate overlap trips from travel day data when travel day and travel period data will be added together.


For a specific trip, a passenger is any occupant of a motorized vehicle other than the driver.

Person miles of travel (PMT)

PMT is a primary measure of person travel. When one person travels one mile, the result is one person mile of travel. Where two or more persons travel together in the same vehicle, each person makes the same number of person miles as the vehicle miles. Therefore, four persons traveling 5 miles in the same vehicle results in 20 person miles (4 x 5 = 20).

Person trip

A person trip is a trip by one or more persons in any mode of transportation. Each person is considered as making one person trip. For example, four persons traveling together in one auto are counted as four person trips.

Privately owned vehicle (POV)

A privately owned vehicle or privately operated vehicle. Either way, the intent here is that this is not a vehicle available to the public for a fee, such as a bus, subway, taxi, etc.

Travel day

A travel day is a 24-hour period from 4:00a.m. to 3:59 a.m. designated as the reference period for studying trips and travel by members of a sampled household.

Travel period

A travel period consists of a four-week period ending with the travel day.

Travel day trip

A travel day trip is defined as any time the respondent went from one address to another by private motor vehicle, public transportation, bicycle, walking, or other means. However, a separate trip is not counted in two instances in the following cases:

1. When the sole purpose for the trip is to get to another vehicle or mode of transportation in order to continue to the destination.
2. Travel within a shopping center, mall or shopping areas of 4-5 blocks is to be considered as travel to one destination.

Travel period trip

A travel period trip is a trip where the farthest destination is at least 50 miles from home. The outgoing portion of this trip can take place at any time, but the return must be within the four-week travel period. The four-week travel period ends on and includes the assigned travel day.

Travel day trip purpose

A trip purpose is the main reason that motivates a trip. There are 36 travel day trip purposes used in the 2001 NHTS.

For the 2001 Survey, trip purposes were collected using a from-to approach. For each trip, the origin and destination are on the file in generic terms, e.g. from work to shopping. The 36 trip reasons are defined as follows:

1. To Home. Travel to home after leaving for some reason.
2. Go to Work. The first trip to the work location on travel day.
3. Return to Work. A trip to work that is not the first trip to work on the travel day.
4. Attend Business Meeting/Trip. A work-related trip whose purpose is to attend a business meeting.
5. Other Work Related. A work-related trip whose purpose is not specifically to attend a business meeting.
6. Go to School as a Student. A trip whose purpose is to go to school as a student.
7. Go to Religious Activity. A trip whose purpose is to go to a place to attend a religious activity.
8. Go to Library, School Related. A trip whose purpose is to go to the library as part of a school-related activity.
9. Go to Daycare. A trip whose purpose is to attend day care.
10. Other School/Religious Activity. School and religious activities not covered by categories 6 through 8 above.
11. Medical/Dental Services. A trip made for medical, dental, or mental health treatment, or other related professional services.
12. Buy Goods, (e.g., groceries/clothing/hardware store). A shopping trip whose purpose is to purchase commodities for use or consumption elsewhere. This purpose also includes window shopping and trip made to shop even if nothing is purchased.
13. Buy Services, (e.g., video rentals/dry cleaning/post office/car service/bank). The category includes the purchase of services other than medical/dental or other professional services.
14. Buy Gas. A trip made specifically to get gas.
15. Shopping/Errands. Shopping/errand trips not covered by categories 12 through 14 above.
16. Go to the Gym/Exercise/Play Sports. A trip made for exercise or to participate in a sport.
17. Rest or Relaxation/Vacation.
18. Visit Friends/Relatives. The social/recreational trip whose purpose is to visit with family and friends.
19. Go out/Hang out, Entertainment/Theater/Sports Event/Go to Bar. The purpose of the trip is entertainment or hanging out with friends.
20. Visit Public Place, Historical Site/Museum/Park/Library.
21. Social/Recreational. Includes social and recreational trips not covered by categories 16 through 20 above.
22. Use Professional Services, Attorney/Accountant. A trip made for professional services other than for medical/dental purposes.
23. Attend Funeral/Wedding. A personal trip to attend a funeral or a wedding.
24. Use Personal Services, Grooming/Haircut/Nails. A trip for personal services such as to a hairdresser.
25. Pet Care, Walk the dog/vet visits.
26. Attend Meeting, PTA/Home Owners Association/Local Government. The purpose of the trip is to attend a non-work-related meeting, such as a community meeting.
27. Family Personal Business/Obligations. A trip for personal business not covered by categories 22 through 26 above.
28. Pickup Someone.
29. Take and Wait. A trip made to take someone to a destination and then wait with them at the destination and return together.
30. Drop Someone Off.
31. Transport Someone. Trips with a passenger that are related to picking up or dropping off someone but not covered by categories 28 through 30.
32. Social Event. A trip whose purpose is to eat a meal at a social event.
33. Get/Eat Meal. A trip whose purpose is to get and eat a meal but not at a social event.
34. Coffee/Ice Cream/Snacks. A trip whose purpose is to get/eat a snack or drink, something less than a meal.
35. Meals. A trip whose purpose is to eat or get a meal but not covered by categories 32 through 34 above.
36. Other. A trip purpose not covered by categories 1 through 36 above.

For more on trip purpose coding and variables, see Appendix M.

Travel period trip purpose

A trip purpose is the main reason that motivates a trip. There were 18 travel period trip purposes in the 2001 NHTS. The main reason and all other reasons for the trip were collected.

Urbanized area

An urbanized area consists of the built up area surrounding a central core (or central city), with a population density of at least 1,000 persons per square mile. Urbanized areas do not follow jurisdictional boundaries thus it is common for the urbanized area boundary to divide a county.

For the 2001 NHTS, Urban Areas were calculated two ways.

  • Variable URBAN uses the 2000 Urbanized Areas: Cartographic Boundary Files. File ua00_d00.shp from Four codes are used. 1 = in Urban Cluster, 2 = in Urban Area, 3 = in area surrounded by urban areas. 4 = not in Urban Area.
  • Variable URBRUR uses the 2000 Urbanized Areas: Cartographic Boundary Files. File ua00_d00.shp from Two codes are used: 1 = in Urban Area, 2 = Rural (Not in Urban Area).


The 2001 NHTS, the term vehicle includes autos, passenger vans, sport utility vehicles, pickups and other light trucks, RVs, and motorcycles and mopeds owned or available to the household.

Vehicle miles of travel (VMT)


Vehicle occupancy

Vehicle occupancy is the number of persons, including driver and passenger(s) in a vehicle; also includes persons who did not complete a whole trip. NHTS occupancy rates are generally calculated as person miles divided by vehicle miles.

Vehicle trip

A vehicle trip is a trip by a single privately operated vehicle (POV) regardless of the number of persons in the vehicle.

Vehicle type

For purposes of the 2001 NHTS, vehicle type is one of the following:

1. Automobile (including station wagon)
2. Van
3. Sport utility vehicle
4. Pickup truck (including pickup with camper)
5. Other truck
6. RV or motor home
7. Motorcycle
8. Other

See Means of Transportation for definitions of these vehicle types. For NHTS, vehicle types are limited to privately operated vehicles (POV) because other vehicles that the respondent may have ridden in (e.g., bus) were not tracked throughout the day, as was the case with household vehicles.


See Employed.