Anyone who has ever travelled on a long motorway will have come across one at least once. tail in spots that seems to form out of nowhere. No accidents, no narrowing of the lane, yet traffic slows down, stops, and then starts again for no apparent reason. This phenomenon, commonly known as “ghost tails”is a tangible manifestation of the complex dynamics that govern traffic flow, often influenced by individual driver choices. But how they are formed exactly these queues, and what are the implications for those travelling?
How Piecewise Queues Form: The Dynamics
The spotty queues are mainly formed due to small disturbances in the traffic flow that propagate backwards, creating a chain reaction. Consider a busy but smooth road: vehicles proceed at a constant speed while maintaining a safe distance. However, if a car suddenly brakes, even slightly, the vehicle following will have to brake in turn, and so on for all subsequent vehicles. Each car, however, will brake more intensely compared to the one preceding it, as the safety distance is progressively reduced. This mechanism creates aslowing wave propagating backwards for miles, causing a traffic jam without any visible obstacle. Restarting is also a slower process than braking, causing a build-up of vehicles that further fuels the queue.
The unpredictability of traffic and the challenge of fluid dynamics
While the fluids they move according to well-defined physical laws, car traffic is subject to a high variability due to individual decisions of drivers. When the vehicle density exceeds a certain critical thresholdtraffic flow becomes extremely sensitive to small disturbances, such as risky overtaking or sudden slowdowns. In these cases, a certain stretch of road is said to have reached maximum capacity, thus becoming a saturation zone. Even in the absence of distractions, if traffic is dense enough, queues form spontaneously.
A experiment conducted with 22 vehicles in a circular closed circuit showed that, although drivers tried to maintain a constant speed of 30 km/hafter only After 10 minutes queues had already formed at times. This happens because it is difficult for man maintain a constant speed and maintain the safety distancesthat is, react precisely to small changes in speed of the vehicle in front. Simply put, it is not within the capabilities of humans to have such precision, it is like asking man to become an automaton. And in fact what is the solution? Precisely the self-driving cars.
The Solution: Self-Driving Vehicles
The answer to the problem of ghost queues seems to lie in automation. self-driving vehiclesthanks to their ability to maintain a constant speed and thanks to GPS they can communicate in real time with surrounding vehicles, they can eliminate most of the disturbances that cause the queues.
In an experiment similar to the one previously described, but with autonomous vehicles, no traffic jams occurred. These vehicles are able to quickly adapt to changes in traffic and maintain the safety distances ideal, thus preventing the formation of slowdown waves.
A common behavior that makes the situation worse: abusing the passing lane
Another common cause of phantom tails is theImproper use of the overtaking lane. Many drivers tend to stay in the leftmost lane even when they are not passing, ignoring the fact that this limits the ability of the other lanes to absorb the flow of traffic. This behavior creates a “invisible funnel” which slows down the entire traffic, causing queues at times that could be avoided simply by respecting the traffic rules. In fact, the Highway Code establishes that overtaking must take place on the left, and always remaining in the overtaking lane prevents other vehicles from correcting the disturbances in the traffic flow.
