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The divergence is usually due to the presence of negative externalities that travelers impose on each other (recurrent and non-recurrent travel delays, schedule delays, crowding) and on society as a whole (air pollution, noise, accidents). Another reason is the multidimensional character of the choices of individuals and firms that ultimately determine the transportation demand for a given link, time and mode.
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Transport markets are challenging subjects of research. One main reason is the frequently found divergence between the user equilibrium and the social optimum. The divergence is usually due to the presence of negative externalities that travelers impose on each other (recurrent and non-recurrent travel delays, schedule delays, crowding) and on society as a whole (air pollution, noise, accidents). Another reason is the multidimensional character of the choices of individuals and firms that ultimately determine the transportation demand for a given link, time and mode. These include (i) short-run choices concerning the trip making, the timing (scheduling), the mode and the route, but also (ii) medium- and long-run choices concerning vehicle ownership and locational decisions of individuals (residence, workplace) and firms (headquarters, distribution centers, subcontractors). Complexity is further aggravated (among others) by the network structure, scheduled transportation, and the existence of market power of service operators and infrastructure suppliers.
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