We've all found ourselves stuck in traffic and wondered what could have caused the queue. Sometimes it's the fault of an unfortunate traffic light. Other times roadworks, accidents, double-parked cars, or other impediments that reduce the roadway or slow down traffic. Even more often, however, the reason does not exist. Or rather, it is inherent to the very presence of many vehicles on the road. In these cases we talk about “phantom queues”, a phenomenon studied by science, which emerges from the complex system made up of cars and their drivers, without the need for external interference.
A real phenomenon
A Japanese study in 2008 experimentally observed the phenomenon of phantom queues. Researchers at Nagoya University had 22 volunteers drive along a 230-meter-long ring road at a constant speed and distance. In the first few minutes, traffic proceeded smoothly without any hitches. But then the researchers saw a queue forming that propagated backwards: a phantom queue, in fact, and a “jamiton”, that is, the wave of traffic traveling backwards between the cars on the road.
The studies carried out on the topic have made it possible to understand that it is a spontaneous phenomenon, which mathematics can easily explain by approaching car traffic as a fluid dynamics problem. Considering road traffic as a fluid, and cars as particles of the fluid, it has in fact been demonstrated that once a critical density threshold (the number of cars present on the road) is exceeded, the phantom queue invariably occurs, without the need for errors or behaviors incorrect or careless on the part of the pilots.
The phantom queue mechanism
The theory says that vehicles traveling on a road are subjected to two contrasting effects: a stabilizing effect linked to the proactive action of the drivers, who try to predict the evolution of traffic and adapt the speed of their vehicle, and conversely , a destabilizing effect, linked to the fact that drivers have to decelerate when the density of vehicles on the road increases, and that this change in speed is not instantaneous. When the density of cars on the road exceeds the critical threshold, the stabilizing effect of proactive driving no longer balances the destabilizing effect caused by speed changes, and a phantom queue is created, which travels backwards along the line of cars on the road until until it reaches a sufficiently clear area of the road, and dissipates naturally.
Imagine it like this: at a particularly busy point, all it takes is for any driver to slow down even a little for the driver following him to slow down in the same way, and then the next one, and so on, the slowdowns continue to propagate backwards progressively getting worse because the drivers' reflexes are not perfect and every small delay only forces those behind us to slow down a little more, until the slowdown is such that it begins to force the cars to stop.
Since this is a phenomenon that emerges naturally from the density of cars on the road, unfortunately there are no solutions for phantom queues, at least as long as we human beings are driving, with all our imperfections. Self-driving cars, on the other hand, would have little trouble coordinating with each other, and reacting quickly enough to prevent queues from forming. If they ever actually arrive on our roads, therefore, traffic, barring accidents and roadworks, will probably become a thing of the past.