The transfer of a unit of freight or a passenger between an origin A and a destination B is influenced by the friction of distance, which is the effect that each unit of distance may have on a movement. Spatial constraints such as distance, the physiography (elements of the landscape such as rivers and elevations) or different administrative divisions (notably for international transportation) impede movements. Furthermore, infrastructure must be present and available in order to support a movement.
It is common, although not the rule, for transfer costs to increase proportionally with distance. If costs are prohibitive, a transfer cannot occur or is economically unsound. There is consequently a distance after which a transfer cannot be economically justified, but this varies according to the mode used. Specific transportation modes, because of their performance, have a strong spatial consideration. For instance, given the same amount of time, a pedestrian may cross a D(W) distance, while a cyclist and a car driver would cross a D(C) and a D(D) distance respectively. Different modes have consequently different relationships with space because of their respective frictions of distance.