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 time effect that each unit of distance may have on a movement. Spatial constraints such as distance, physiography (elements of the landscape, such as rivers and elevations), or administrative divisions (notably for international transportation) impede mobility and its duration. Furthermore, modes and infrastructure must be present and available in order to support a movement.
Although not the rule, it is common for transfer costs to increase proportionally with distance. If costs are prohibitive, a transfer cannot occur or is economically unsound. Consequently, a distance after which a transfer cannot be economically justified, but this varies according to the mode used. Because of their performance, specific transportation modes are strongly influenced by spatial considerations. For instance, given the same amount of time (t), a pedestrian may cross a D(W) distance, which involves a friction of distance t/D(W), while a cyclist and a car driver would cross a D(B) and a D(A) distance and have a friction of distance of t/D(B) and t/D(A), respectively. Different modes have consequently different relationships with space because of their respective frictions of distance.