18
2 Complexity Time Bomb
in networks, and systemic interdependencies. And I will illustrate these problems
through a large variety of examples such as traffic jams, electrical blackouts, financial
crises, crime, wars and revolutions.
2.1 Phantom Traffic Jams
Complex systems can be found all around us and include phenomena such as turbulent
flows in our global weather system, decision-making processes, opinion formation in
groups, financial and economic markets, and the evolution and spread of languages.
However, we must carefully distinguish complex systems from complicated ones.
While a car, which consists of thousands of parts, is complicated, it is easy to control
nevertheless (when it works properly). Traffic flow, on the other hand, depends on the
dynamical interactions of many cars, and forms a complex dynamical system. These
interactions produce counter-intuitive phenomena such as “phantom traffic jams”
which appear to have no cause. Such “emergent” phenomena cannot be understood
from the properties of the single parts of the system in isolation, here the drivervehicle units. While many traffic jams occur for specific, identifiable reasons, such
as accidents or road works, almost everyone has also encountered situations where
a queue of vehicles seems to form “out of nothing” and where there is no visible
cause.
2
To explore the true reasons for these “phantom traffic jams”, Yuki Sugiyama and
his colleagues at Nagoya University in Japan carried out an experiment in which they
asked many people to drive around a circular track.
3 The task sounds simple, and
the vehicles did in fact flow smoothly for some time. Then, however, one of the cars
caused a minor variation in the traffic flow, which triggered stop-and-go traffic—a
traffic jam that moved backwards around the track.
While we often blame poor driving skills of others for such “phantom traffic jams”,
studies in complexity science have shown that they are actually an emergent collective
phenomenon, which is the inevitable result of the interaction between vehicles. A
detailed analysis demonstrates that, if the traffic density exceeds a certain “critical”
threshold—that is, if the average separation of the vehicles is smaller than a certain
value—then even the slightest variation in the speed of cars can eventually cause a
disruption of the traffic flow through an amplification effect. As the next driver in
line needs some time to adjust to a change in the speed of the vehicle ahead, he or she
will have to brake a bit harder to compensate for the delay. The then following driver
will have to break even harder, and so on. The resulting chain reaction amplifies the
initially small variation in a vehicle’s speed and this eventually produces a traffic jam
which, of course, every single driver tried to avoid.
2 See https://traffic-simulation.de/ring.html.
3 See https://www.youtube.com/watch?v=7wm-pZp_mi0 and Sugiyama et al. [1].
2 Complexity Time Bomb
in networks, and systemic interdependencies. And I will illustrate these problems
through a large variety of examples such as traffic jams, electrical blackouts, financial
crises, crime, wars and revolutions.
2.1 Phantom Traffic Jams
Complex systems can be found all around us and include phenomena such as turbulent
flows in our global weather system, decision-making processes, opinion formation in
groups, financial and economic markets, and the evolution and spread of languages.
However, we must carefully distinguish complex systems from complicated ones.
While a car, which consists of thousands of parts, is complicated, it is easy to control
nevertheless (when it works properly). Traffic flow, on the other hand, depends on the
dynamical interactions of many cars, and forms a complex dynamical system. These
interactions produce counter-intuitive phenomena such as “phantom traffic jams”
which appear to have no cause. Such “emergent” phenomena cannot be understood
from the properties of the single parts of the system in isolation, here the drivervehicle units. While many traffic jams occur for specific, identifiable reasons, such
as accidents or road works, almost everyone has also encountered situations where
a queue of vehicles seems to form “out of nothing” and where there is no visible
cause.
2
To explore the true reasons for these “phantom traffic jams”, Yuki Sugiyama and
his colleagues at Nagoya University in Japan carried out an experiment in which they
asked many people to drive around a circular track.
3 The task sounds simple, and
the vehicles did in fact flow smoothly for some time. Then, however, one of the cars
caused a minor variation in the traffic flow, which triggered stop-and-go traffic—a
traffic jam that moved backwards around the track.
While we often blame poor driving skills of others for such “phantom traffic jams”,
studies in complexity science have shown that they are actually an emergent collective
phenomenon, which is the inevitable result of the interaction between vehicles. A
detailed analysis demonstrates that, if the traffic density exceeds a certain “critical”
threshold—that is, if the average separation of the vehicles is smaller than a certain
value—then even the slightest variation in the speed of cars can eventually cause a
disruption of the traffic flow through an amplification effect. As the next driver in
line needs some time to adjust to a change in the speed of the vehicle ahead, he or she
will have to brake a bit harder to compensate for the delay. The then following driver
will have to break even harder, and so on. The resulting chain reaction amplifies the
initially small variation in a vehicle’s speed and this eventually produces a traffic jam
which, of course, every single driver tried to avoid.
2 See https://traffic-simulation.de/ring.html.
3 See https://www.youtube.com/watch?v=7wm-pZp_mi0 and Sugiyama et al. [1].