Using a structural analysis, we show that any journey requires a reliable mode, a sufficiently meshed network, a competent user and a common set of rules, all forming a modal system. The first two components correspond to the technical transport system studied by the engineers. The last two represent the human and social dimension of mobility, which is of particular interest to sociologists. Each constituent of this system generates a particular effect: a club effect, a park effect, a network effect or a safety effect. For a modal system develops, all its components must progress together, in order to avoid the emergence of mismatchies that slow down the process. Progress is accelerated when a certain coherence is found. The history of each transport mode can therefore be analysed as a search for a balance between all these components. The competition between modes explains to a large extent how the evolution of a modal system is initiated or halted. All aspects of mobility can be analysed through this quadriptych. For example, the four-stage traffic model cover the four identified components. Road safety mobilises the triptych vehicle-infrastructure-driver, but some specialists deplore the fact that the social dimension is forgotten. A modal shift is only possible if all components of the new modal system are stimulated. The negative externalities of transport converge towards four major indirect impacts: degradation of human health, disaffection for non-motorised modes (i.e. dependence on motorised modes), reduction in neighbourhood relations (and an increase in distant relations) and degradation of the living environment.