A slow vehicle can reduce the usable capacity of a road even though the vehicle itself keeps moving. Cars catch it, bunch behind it, and create a disturbance that travels back through the traffic stream. The location of the obstruction and the location of its resulting congestion need not remain together.
In the video, a slow vehicle joins a steady flow from an entry ramp and later exits at another ramp. The long-road display reveals a growing upstream wave, while close-up views show the vehicles caught in and around the disruption. A smaller pocket of congestion near the ramp section fades after the slow vehicle leaves; the larger wave behind it continues to propagate. This separation is the core lesson. Removing a moving bottleneck stops the immediate restriction, but does not instantly erase the queue it has already generated. Understanding that delayed effect helps explain why congestion can outlive the incident or vehicle that started it.
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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