The concept of friction of distance is based on the notion that distance usually requires some amount of effort (energy) and/or money to overcome. Because of this "friction", spatial interactions, especially transport and the particular case of human migration, will tend to take place more often over shorter distances; amount and intensity of interaction will decay with distance. Even though no physical friction is involved, price and effort are metaphorically assumed to correspond to forces counteracting transport movements.
Friction may be a continuous, smooth function of distance or may contain jumps; it may be constant, linear or nonlinear or even a combination of jumps and proportional behaviour as with Intermodal freight transport and Intermodal passenger transport.