Not every traffic jam begins at a crash, lane closure, or visible road defect. On a busy road, one driver's brief slowdown can force the next driver to brake a little more, and that reaction can repeat from car to car. A self-sustaining queue may then appear without a permanent obstacle.
Near the downstream end of a long road, one vehicle creates a small disturbance. Colored conditions on the full corridor show the slow region expanding and traveling upstream. The close-up camera periodically stops at successive locations, so viewers can see individual cars still moving forward while the congested wave moves backward through them. That opposing motion is the essential insight behind a phantom jam. Near capacity, even a local and temporary behavior can become a network-scale delay. The video makes the mechanism visible rather than implying that every real congestion event has the same origin.
How does congestion build up on a road, and what makes it disappear? Traffic conservation provides the most basic answer: vehicles cannot accumulate or vanish without crossing the boundaries of a road segment. If more vehicles enter than leave, the segment fills. If more leave than enter, it empties. When the two flows balance, the number of vehicles stays steady.
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Following every car is one way to simulate traffic, but large roads and networks often need a simpler description. The cell transmission model divides a road into short pieces and advances traffic in time steps, moving vehicles from one piece to the next only when upstream demand and downstream space allow it.
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How can a road carry less traffic even as more vehicles try to use it? Fundamental diagrams describe how the number of vehicles occupying a road relates to their average speed and to the rate at which traffic moves through it. When traffic is sparse, vehicles can travel quickly, but few are present. As the road fills, it initially carries more traffic. Beyond a critical level, vehicles interfere with one another and queues develop; adding still more vehicles can slow movement enough to reduce the flow.
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