ABSTRACT
Road transport is the source of numerous environmental nuisances and/or negative economic externalities, e.g. fine-particle air pollution, time lost in jams and accidents. Some of these effects can be dealt with, wholly or partially, by considering a vehicle or infrastructure subsystem in isolation. Thus, car manufacturers are constantly improving the safety and energy efficiency of cars. Similarly, both the design and maintenance of road infrastructure benefit from progress made in road engineering research (e.g. size of roads, design of intersections) and road security (e.g. crash barriers, concrete central dividers, surface drainage, etc.). However, some typical road traffic phenomena, particularly traffic congestion, are essentially systemic, in that they naturally result from interactions between system components, and more or less from the individual characteristics of each component. A traffic jam forms when the density of the flow of vehicles present at a given place and time exceeds the infrastructure’s local capacity to clear the flow. This local capacity overshoot (in time and place) may be exceptional and unpredictable when it is itself the result of an exceptional, unpredictable event (e.g. accident, storm). It may also be recurrent and thus predictable. This is the case, for example, for congestion phenomena observed on road networks in major cities during morning and evening rush hours, or on inter-urban networks during major summertime movements.
