Analysing the capacity of a transport network. A general theoretical approach

Knowledge of the capacity offered by a transport network, and of the extent to which this depends on the characteristics of the constituent elements, is important for the purposes of controlling the network (traffic calming measures to reduce existing congestion problems, exploiting reserve capacity, road pricing etc.) and controlling demand (activity times, etc.). It is also important when undertaking the territorial reorganization (or development) of activities and planning measures for adaptation of the infrastructure network. This paper proposes a definition of network capacity (relating to a road network, in particular) that does not rely on knowledge of the current demand and on the envisaged trend of growth in demand: to this end, a systemic approach is adopted, and the concept of network capacity function is introduced. It will be seen that the concept of capacity referred to a transport system or an element of the system, as defined by capacity function, is tied to one or more problems of maximum constraint. By determining the capacity function of a network, it becomes possible to evaluate the performance of the network and thus identify the geometrical parameters that can be modified so as to achieve the biggest increase in terms of capacity, as well as analyzing the sensitivity of the system under analysis to variations of travel demand.