Bridging the user equilibrium and the system optimum in static traffic assignment: how the cooperation among drivers can solve the congestion problem in city networks
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Solving the road congestion problem is one of the most pressing issues in moderncities since it causes time wasting, pollution, higher industrial costs and huge roadmaintenance costs. Advances in ITS technologies and the advent of autonomousvehicles are changing mobility dramatically. They enable the implementation of acoordination mechanism, called coordinated traffic assignment, among the sat-navdevices aiming at assigning paths to drivers to eliminate congestion and to re-duce the total travel time in traffic networks. Among possible congestion avoidance methods, coordinated traffic assignment is a valuable choice since it does not involvehuge investments to expand the road network. Traffic assignments are traditionally devoted to two main perspectives on which the well-known Wardropian principlesare inspired: the user equilibrium and the system optimum. User equilibrium is a user-driven traffic assignment in which each user chooses the most convenientpath selfishly. It guarantees that fairness among users is respected since, whenthe equilibrium is reached, all users sharing the same origin and destination willexperience the same travel time. The main drawback in a user equilibrium is thatthe system total travel time is not minimized and, hence, the so-called Price ofAnarchy is paid. On the other hand, the system optimum is an efficient system-wide traffic assignment in which drivers are routed on the network in such a waythe total travel time is minimized but users might experience travel times that arehigher than the other users travelling from the same origin to the same destina-tion affecting the compliance. Thus, drawbacks in implementing one of the two assignments can be overcome by hybridizing the two approaches aiming at bridging users' fairness to system-wide efficiency. The survey reviews the state-of-the-art of these trade-off approaches
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