Three major representations of the friction that distance imposes on transportation can be considered:
- Euclidean distance. The most basic manner to represent distance as a simple function of a straight line between two locations, where distance is expressed in geographical units such as kilometers. Commonly used to provide an approximation of distance, but rarely has a practical use.
- Transport distance. A more complex representation that accounts for the existing structure of the transport network. A simple form involving only one mode is a routing exercise that considers the shortest path between two points, navigating the network. In a more complex form, it concerns the set of physical activities related to transportation, such as loading, unloading, and transshipment. In the above figure, the transport distance between locations A and B includes pickup, travel by mode 1, transshipment, travel by mode 2, and delivery. The same applies to the movement of people, though the activities involved will differ. For instance, air travel between two locations requires going to an airport, potentially transiting through an intermediate hub airport, and finally reaching the destination from the airport terminal. Transport distance is jointly expressed in geographical units, cost, and time.
- Logistical distance. A complex representation that encompasses all the tasks required for a movement to take place between two locations. Logistical distance includes the physical flows and activities necessary to manage them. Among the most significant tasks for freight movements are order processing, packing, sorting, and inventory management. Geographical distance units are less relevant to this assessment, but cost and time factors are significant. Time not only involves the delay related to management and circulation but also how it is used to service the transport demand, namely the scheduling of pickups and deliveries. In the figure above, the logistical distance between locations A and B includes an order from B that is processed, packed, and scheduled for pickup. At the intermediate transshipment location, sorting and warehousing are performed, and finally, at the destination, the delivery will be unpacked and used. For passenger transportation, the logistical distance also encompasses a range of tasks. Using air travel as an example, a ticket would first need to be purchased, commonly several weeks in advance, which requires advance planning. Other common time- and cost-related tasks include checking in, security checks, boarding and disembarking, and picking up luggage. Thus, a three-hour flight often requires to be planned several weeks in advance, and its full realization can take twice as much time if all the related logistical activities are considered.