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.
The video follows one road from nearly empty conditions into severe congestion. Its two evolving charts show average speed falling as the road fills, while flow first rises and then declines. The key insight is that road capacity has a peak: more vehicles do not always mean more movement. This distinction helps explain bottlenecks, diagnose congestion, and judge when additional demand will make a road perform worse. The curves are measured from an illustrative simulation, so their exact shapes are not universal or calibrated predictions.
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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Traffic can behave differently on the way into congestion than on the way out. Even if travel demand returns to an earlier level, queues and unevenly loaded roads may remain. The present state depends partly on the path taken to reach it.
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