Traffic flow

Traffic flow is the movement of vehicles and other road users through roads and intersections. Its changing speed, density, and interactions determine how much a network can carry.

Traffic flow theory: The study of how traffic volume, speed, and density vary across roads and time. It models the relationships that govern traffic movement and congestion.
Traffic engineering: The application of engineering principles to the planning, design, and operation of road transport systems. It uses traffic-flow measurements to design and manage roads.
Traffic congestion: A condition in which traffic demand approaches or exceeds the available road capacity, causing slower travel or queues. Congestion is a degraded traffic state, not simply the movement of traffic.
Traffic prediction: The estimation of future traffic conditions from observed data and models. Reliable forecasts must anticipate changing demand and sudden flow disruptions.
Traffic density: The number of vehicles occupying a unit length of road at a given time. Density helps determine speed and the onset of congestion.
Traffic signal coordination: The timing of multiple traffic signals to regulate progression along a corridor. Coordinated signals can reduce stops and manage arterial traffic flow.
Traffic breakdown: A transition from relatively stable traffic to sustained congested flow. Breakdown can occur near capacity, but its timing is not perfectly predictable.
Traffic oscillation: A repeated variation in vehicle speed or spacing along a stream of traffic. Researchers study how disturbances arise and persist without an obvious obstacle.
Traffic capacity: The maximum traffic rate a road or traffic facility can sustain under specified conditions. Capacity marks the approximate flow limit before queues grow.
Ramp metering: The use of traffic signals to regulate vehicles entering a freeway. Controlling entry rates can prevent merging demand from triggering breakdown.

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