Traffic congestion

Traffic congestion occurs when demand for road space exceeds its capacity, slowing vehicles and creating queues or delays.

Traffic flow theory: The study of how vehicles move through roads and interact with one another. It models the relationships between traffic density, speed, and flow that shape congestion.
Congestion pricing: A policy that charges road users for travel in crowded places or at busy times. Pricing can reduce peak demand by making scarce road space more costly to use.
Travel time reliability: The consistency of travel times across trips made under comparable conditions. Congestion makes arrival times less predictable, even when average delays are known.
Traffic incident: An unexpected event on or near a road that disrupts normal traffic movement. Incidents can cause congestion, but congestion can also arise from recurring demand and limited capacity.
Traffic shockwave: A moving boundary between traffic states, such as free flow and a queue. A braking disturbance can propagate backward through traffic as a shockwave.
Ramp metering: A traffic-control method that regulates vehicles entering a freeway using signals on entrance ramps. Limiting entry can keep freeway flow below the point where breakdown occurs.
Value of time: The monetary value assigned to time saved or lost in travel and other activities. It is used to estimate the economic cost of delay caused by congestion.
Gridlock: A traffic condition in which intersecting queues block one another and movement nearly stops. It is a severe form of congestion in which vehicles cannot clear the network.
Bottleneck: A restriction in a system that limits the rate at which the whole system can operate. A road segment with less capacity than its approaches can trigger recurring queues.
High-occupancy vehicle lane: A lane reserved for vehicles carrying a minimum number of occupants, sometimes with exceptions for eligible vehicles. It encourages shared travel and allocates road space differently during busy periods.

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