When obvious interventions produce non-obvious consequences, there is dynamic complexity. Its overall emergent behavior is difficult to predict, even when subsystem behavior is readily predictable. At the least, developing a model that will predict their performance can be very difficult to do. Joseph M. Sussman, Introduction to Transportation Systems, Artech House Publishers, Inc., Boston, MA, April 2000. Service is provided on complex networks.
Systems are stochastic in nature.
Human decision-makers with complex decision calculi make choices that shape the transportation system. Modeling the entire system is almost unthinkable. Our challenge is to choose relevant subsystems and model them appropriately for the intended purpose, mindfully reflecting the boundary effects of the unmodeled components. It is generally agreed that increasing complexity is at the heart of the most difficult problems facing today's systems of architecting and engineering. In short, Simplify.