Air carrierCertificated provider of scheduled and nonscheduled services.

Chained dollarsA method to measure real changes in dollar values between years that uses chain-type indices, rather than constant dollars. The method first calculates the real changes between adjacent years. Annual rates of real changes are then chained (multiplied) together to obtain the rate of real changes between nonadjacent years.

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

Commercial waterway facilitiesWaterway facilities as counted by the U.S. Army Corps of Engineers are piers, wharves, and docks. Not included are those facilities used exclusively for recreational or active military craft and generally those providing nonmaritime use.

Commuter railUrban/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 transit service.

Demand-responsive transitA nonfixed-route, nonfixed-schedule form of transportation that operates in response to calls from passengers or their agents to the transit operator or dispatcher.

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

General aviationAll 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 transit rail operated on rights-of-way that exclude all other vehicles and pedestrians.

HubA 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 or other definitions of hubs used by the Federal Aviation Administration focusing on traffic at individual airports.

Large certificated air carrierCarriers operating aircraft with a maximum passenger capacity of more than 60 seats or a maximum payload of more than 18,000 pounds. These carriers are also grouped by annual operating revenues: 1) majorsmore than $1 billion; 2) nationalsbetween $100 million and $1 billion; 3) large regionalsbetween $20 million and $99,999,999; and 4) medium regionalsless than $20 million.

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.

Light truckTrucks of 10,000 pounds gross vehicle weight rating or less, including pickups, vans, truck-based station wagons, and sport utility vehicles.

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

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

Passenger-mileOne passenger transported one mile. For example, one vehicle traveling 3 miles carrying 5 passengers generates 15 passenger-miles.

Self-propelled vesselsIncludes 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-milesA unit of measure equal to the movement of one ton over one mile.

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

Vehicle-mileOne vehicle traveling one mile.