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.
The first two acts use the same demand trajectory in reverse: pressure rises, then falls. The changing traffic indicators do not simply retrace their earlier positions. A final side-by-side comparison selects the same demand level on each leg and exposes two different network states. This loop is traffic hysteresis. It matters because reducing demand after a jam has formed may not restore the original performance immediately. Policies and forecasts based only on current demand can miss the stored congestion already inside the system. The video is a stylized demonstration of that memory effect, not a claim that every road or network will produce the same loop.
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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