Humans move in three distinct (but not always separate) conditions: land, air and water. (DUKW ("Duck") boats allow travel on water and land without changing vehicles.) Within these three conditions, four main modes of travel exist: the airplane, the ship (or boat), the train and the car. Within each of these modes, various sub-modes exist, each with their own specific circulatory needs.
The diagram above shows the different basic hub types for each mode of transportation (right column). The left column shows how fundamentally these hubs are linked to one another.
The Airplane
The airplane's hub is the airport. When in the sky, the airplane's movement limited to a series of planes (as in, a horizontal axis) which it moves between gradually and carefully. Within that plane (axis), the aircraft's movement is only restricted by the capacity of its tail. Still, this only describes an aircraft's movement from one hub to another. The airport is most important to the human, as it is the primary place of interaction. At the airport, we get off and on the aircraft, or change between one aircraft to another. Aircraft are not capable of exchanging passengers mid-air; thus the airport facilitates the transition. The aircraft moved in and out of the airport by the runway which, compared to the craft's movement in the air, is much more restricted, as there is only one way in and out.
LaGuardia Airport in New York City has two runways, but an aircraft can only use one runway at a time. The craft enters at one end, and leaves from the other. The entrance and exit are precise and rigidly directional. The airplane is the anomaly in that it uses both land and air in its movement.
The Ship/Boat
Ships move about on the water much as an airplane does in the air, but without the freedom of vertical movement (unless aided by a lock). Without physical obstruction to conform their movement, ships may travel in any (horizontal) direction they please, as enabled by their rudder.
The dock, or pier, is more forgiving than the airport, but it also implies a "correct" way of entering and exiting.
The nature of the pier implies that ships pull up alongside, paralleling their edges. A ship may enter backwards or forwards, but one long side must touch that of the pier.
The Train
Compared to the previous two modes, the movement of the train is incredibly rigid. A train may only move on a track. A train's path may change elevation, but the train car itself cannot move up or down.
The train station is where we get on and off and move between trains.
As they approach the station, the tracks diverge to individual platforms where each train exchanges passengers. Again, the movement of the train car is specifically prescribed to the track and the platform.
The Car
The car moves in a way that is only minimally prescribed by the vehicle itself. Cars move in a way determined by roads and legislation. We move in and out of them by bringing them to rest at a designated location. These locations, while apparent and distinct, are often merged quite closely to the places of movement.
Two conditions are present in the image of Giant Eagle Squirrel Hill. The parking lot, which is the vehicle destination, is set away from the street, but has designated paths of circulation within. Second is the street parking which lines flanks the lanes of moving traffic. Here the parking is secondary to the street as circulation, but still highly visible.
