Representations of Distance
Three major representations can be used for distance and the friction it imposes on transportation:
  • 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. In a simple form involving only one mode, it is a routing exercise considering the shortest path between two points. In a more complex form, it concerns the set of physical activities related to transportation, such as loading, unloading and transshipment, are considered. On the above figure, the transport distance between locations A and B includes, pickup, travel by mode 1, transshipment, travel by mode 2 and finally, delivery. The same applies to the circulation of people, although the involved activities will be different. For instance, someone using air travel between two locations will require going to an airport, may transit through an intermediate hub airport and will finally need to reach his destination from the airport terminal. Transport distance is jointly expressed in geographical units, in cost and in time.
  • Logistical distance. A complex representation that encompasses all the tasks required so that a movement between two locations can take place. Logistical distance thus includes physical flows, but also a set of activities necessary for the management of these flows. For freight movements, among the most significant tasks are order processing, packing, sorting and inventory management. Geographical distance units are less relevant in this assessment, but the factors of costs and time are very 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. On the above figure, the logistical distance between locations A and B, includes an order from B, which is processed, packed and scheduled to be picked up. At the intermediate transshipment location, sorting and warehousing are performed, and finally, at the destination the delivery will be unpacked and used. For the transportation of passengers, logistical distance also concerns a specific array of tasks. Taking again an air travel example, a ticket would first need to be purchased, commonly several weeks in advance. Other common time and cost tasks concern 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.