Fundamental diagram of traffic flow

Fundamental diagram of traffic flow

The Fundamental Diagram of traffic flow is a diagram that gives a relation between the traffic flux (cars/hour) and the traffic density (cars/km). A macroscopic traffic model involving traffic flux, traffic density and velocity forms the basis of the fundamental diagram. It can be used to predict the capability of a road system, or its behaviour when applying inflow regulation or speed limits.

Basic Statements

  • There is a connection between traffic density and vehicle velocity: The more vehicles are on a road, the slower their velocity will be.
  • To prevent congestion and to keep traffic flow stable, the number of vehicles entering the control zone has to be smaller or equal to the number of vehicles leaving the zone in the same time.
  • At a critical traffic density and a corresponding critical velocity the state of flow will change from stable to unstable.
  • If one of the vehicles brakes in unstable flow regime the flow will collapse.

The fundamental diagram is of major importance for the design of road systems and traffic management systems.

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