Basic supply variables for major modes can be categorized by routes, terminals, and vehicles:
- Road. Supply depends on road conditions, traffic (congestion) and the level of control (such as speed limits) and is generally measured in terms of numbers of vehicles per lane per hour. The main supply variables are road width, number of lanes, the capacity of the vehicle (average number of people per vehicle in several North American cities: 1.2), speed, inventory and frequency of service (for mass transit).
- Rail. The main supply variables are the number of tracks, the capacity of stations and railyards, the capacity of the vehicle, and the speed of the vehicle.
- Air. The main supply variables are the capacity of airports, the capacity of aircraft, frequency of services and speed of the vehicle. The capacity of an airline corridor is enormous, but that of airports is not.
- Maritime. The main supply variables are port capacity, the capacity of ships, frequency of services and speed of the ship. The capacity of a maritime route is enormous.