A vehicle platoon can look orderly until the lead car makes a small speed change. The important question is what happens farther back: does each follower smooth the disturbance, or does each reaction make it larger? String stability names the condition in which the disturbance does not amplify as it moves through the line.
Two platoons encounter the same initial perturbation. In one, successive vehicles brake and accelerate more sharply, creating growing gaps and a stronger backward-moving wave. In the other, follower responses damp the change and preserve a more even stream. The visual contrast shows why safe behavior by each individual vehicle is not enough to guarantee a stable group. Following rules and automated cruise-control settings must also be judged by their collective effect. This matters for both traffic throughput and comfort when many vehicles interact over a long stretch of road.
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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