Air carrier - Certificated provider of scheduled and nonscheduled services.

Class I railroad - A freight railroad with an annual gross operating revenue in excess of $250 million (based on 1991 dollars).

Commuter rail - Urban/suburban passenger train service for short-distance travel between a central city and adjacent suburbs run on tracks of a traditional railroad system. Does not include heavy- or light-rail service.

Directional route-miles - The sum of the mileage in each direction over which transit vehicles travel while in revenue service.

Fatality - For purposes of compiling DOT safety statistics, any injury that results in death within 30 days of a transportation crash, accident, or incident.

General aviation - All civil aviation operations other than those air carriers holding a Certificate of Public Convenience and Necessity. Types of aircraft used in general aviation range from corporate, multi-engine jets piloted by a professional crew to amateur-built, single-engine, piston-driven, acrobatic planes.

Heavy-rail transit - High-speed passenger rail operated on rights-of-way that exclude all other vehicles and pedestrians.

Hub - A geographic area based on the percentage of total enplaned passengers in that area. A hub may have more than one airport in it. This definition should not be confused with the definition used by airlines in describing their "hub and spoke" route structures.

Light-rail transit - Urban transit rail operated on a reserved right-of-way that may be crossed by roads used by motor vehicles and pedestrians.

Nonself-propelled vessels - Includes dry cargo and tank barges and railroad car floats that operate in U.S. ports and waterways.

Other 2-axle, 4-tire vehicles - Includes vans, pickup trucks, and sport utility vehicles. Does not include passenger cars.

Passenger-mile - One passenger transported one mile. One vehicle traveling 3 miles carrying 5 passengers generates 15 passenger-miles.

Personal-use vehicles - Cars, pickup trucks, or vans; other small trucks; rental cars, trucks, or vans; recreational vehicles or motor homes; or motorcycles or mopeds.

Self-propelled vessels - Includes dry cargo vessels, tankers, and offshore supply vessels, tugboats, pushboats, and passenger vessels, such as excursion/sightseeing boats, combination passenger and dry cargo vessels, and ferries.

Ton-miles - A unit of measure equal to the movement of one ton over one mile.

Single unit - A large truck on a single frame with at least 2 axles and 6 tires. Excludes "other 2-axle, 4-tire vehicles" noted above.
Combination - A power unit (truck or truck tractor) and one or more trailing units.

Vehicle-mile - One vehicle traveling one mile.