Advances in Complex Systems

In this preface we propose a synthetic survey of the main ideas which are in our aim at the origin of the "Physics and the City" Symposium (Bologna–Italy, 15–17 December 2005). These ideas are developed from different point of view in the following collected papers, also with different scientific languages, from the sociological or planning to the mathematical or physical ones. Very schematically we think that the present proceedings could prefigure the emergence of a new "science of the city", really multidisciplinary and with possible spin-off on every single science concerned. Moreover with a large range of possible applications to the actual social life, in order to contribute to a better quality of life. Surely we are at the beginning and the topology of this "science of the city" it is not still well defined, but as H. Poincaré said: Pour obtenir un résultat qui ait une valeur réelle, il ne suffit pas de mettre les choses en ordre; ce n'est pas seulement l'ordre, c'est l'ordre inattendu qui vaut quelque chose.

We illustrate a physicist's point of view on the rise of Complexity Science. We analyze the change of meaning of the word "prediction" in the passage from Laplace's determinism to Statistical Mechanics. We show that an analogous change in the meaning of prediction is at the base of the Physics of complex systems.

This paper makes an attempt to deal with the description, and possibly the explanation, of the profound transformation of the city, and of the changes in urban experience and practices, connected with the shape of contemporary urbanization. I stress the term description, because as it occurs in all periods of fast social changes, our conceptual tools tend to become blunt before we realize that normal science keeps failing us. The city is a complex and ambiguous object, as it is constituted by two parts, or orders of facts, inextricably bound. One is visible, i.e. observable through physical wavelengths: the other is not physically visible and can be grasped only by intellectual tools. Arrangements in the second order of facts, though, are responsible for arrangements in the first one, in the sense of producing them, and these in turn affect the former ones, although in ways and in degrees that are far from being clear. In general, decoding between the two order of facts is hazardous and has not produced a set of rules widely consented upon. This is why I remain very skeptical and in some cases outright critical of models stressing continuities in the concepts of civic organization, by referring to traditional urban forms. Reminiscent of Anthony Gidden's sharp statement that the city is one of those social forms that display "a specious continuity with pre-existing social orders" (Giddens, (1990) The Consequences of Modernity, Stanford University Press, Stanford, p. 6) I will try to etch analytical tools capable to clarify at least a few processes that are shaping civic life in contemporary metropolis.

We consider the problem of modeling urban mobility using the complex systems key of lecture. We discuss the properties of the Auto-mobilis model, a virtual simulator for private car mobility based on the individual citizen propensities and the existence of "chronotopoi", which are the primary causes of citizens' mobility requests. Auto-mobilis is a microscopic agent-based model that takes into account both the physical interactions and the cognitive dynamics. We present the current status of the project and discuss the applications to real situations. The model assumptions are justified by a comparison with GPS observational data on single vehicle trajectories.

Based on suitable video recordings of interactive pedestrian motion and improved tracking software, we apply an evolutionary optimization algorithm to determine optimal parameter specifications for the social force model. The calibrated model is then used for large-scale pedestrian simulations of evacuation scenarios, pilgrimage, and urban environments.

This paper provides an introduction to multiagent traffic simulation. It includes some description of where we are with respect to the implementation of a real-world Berlin scenario. It turns out that the difficulties there, because of data availability and because of the special situation of Berlin after the reunification, are considerably larger than in previous scenarios that we have treated.

The planning process is a multiactor, multilevel process, and the techniques we need for it must be suitable for all the protagonists involved. Moreover, the difficulty in dealing with the complexity of urban systems and the related difficulty of analyzing and forecasting are twofold: one kind of difficulty lies in the complexity of the system itself, and the other is due to the actions of actors, which are "acts of freedom." Correspondingly, the process of urban planning requires a set of techniques and models that have proved to be of great potential for management of communication, participation, consensus-building and system' simulation. In this paper, we describe the peculiarity of the articulated set of tools that should be used to support planning, allowing the setting up and management of processes of participation and communication tailored to the needs of specific projects — a set of friendly tools that make the relationship between technicians, clients and users effective and efficacious. Following this discussion we present a very flexible software environment based on cellular automata (Cellular Automata General Environment — CAGE) that can be used to simulate planning decisions and can be coupled with other modules dealing with the "social side" of complexity.

Urban safety management remains a field of action in cities that is unanimously considered as being a necessity as well as being difficult to apprehend in its theoretical, disciplinary and spatial dimensions. The analysis frameworks that are today consensual are based on system approaches and on analyses of the complexity of urban dynamics and dysfunctional situations.

We propose a panorama of models that can be used for action and combined for effective interventions. Notably, the analysis of microregulation carried out in real time in driving situations uses psychoergonomic and cognitive psychology models, while the analysis of traffic accidents uses models developed for the more general diagnosis of safety.

Macroregulation of the travel system, carried out with a time delay by network managers, should work in favor of research for consistency among the various actions on the one hand, and the various levels of intervention on space on the other. Safety actions thus require a good understanding of the cognitive models at work in public policies.

Transportation systems are "internally" complex systems, made up of many elements influencing each other both directly and indirectly, often nonlinearly, with many feedback cycles. Apart from the internal complexity, transportation systems are closely interrelated with other systems, which can be defined as external. Transport projects may have implications for the economy, the location and intensity of the activities in a given area, the environment, the quality of life and social cohesion. Actually, today, in order to have a better view of the urban problems and mainly for better governance of mobility, the urban system per se is no longer studied, but rather the whole metropolitan area surrounding it. Transport and land use are clearly integrated. Each technological innovation in transport has encouraged both people and industry to change their location to take advantage of the improved mobility; similarly, each change in land development (e.g. houses, factories, offices and shops) has influenced the demand for travel and ultimately the provision of new transport infrastructures and services. An example of integrated land use and transport planning is the Regional Metro System (RMS) project, carried out in Campania, which is finally reported.

In the introduction a distinction is made between the macroscopic approach, which the spatially explicit method is connected with, and the microscopic approach. The spatially explicit method is considered in relation to the interactions between located variables: without and with transport of matter. It is compared with the microscopic approach and the main difference is highlighted: the long range interactions of the latter instead of the neighborhood-based interactions of the former. Finally, a comprehensive approach based on agents, goods and markets, considered as main classes, is proposed. In this approach, the basic actions — buy, produce and sell — are able to connect the main classes and to generate an economic system and an urban spatial pattern.

We develop a collision-avoiding mechanism for a system of individual agents (pedestrians) that move in a crowd trying to reach their different goal points. The agents avoid collisions on the basis of a model of the other agents' behavior, a "theory of mind," which is realized at different levels through an iterative process (the first level, or level 0, corresponds to ignoring the other agents' behavior, level 1 to assuming that the other agents will ignore each other, and so on).

The model is conceived in order to perform an evolutionary simulation of some basic parameters that determine the agent's sensorial, cognitive and behavioral system (the perception of the agent's own size, the attraction to the goal, the radius and angle of the field of view and the level of the theory of mind).

In this preliminary work we present our model, show that it reproduces some of the simplest organized behaviors of a system of pedestrians, and focus on some features of the theory of mind, as the difference between odd and even levels.

Street networks play a crucial role in the emergence of vehicular traffic. Here we present an experimental analysis of the street networks of several European cities (London, Paris, Rome, Modena). This analysis evidences the existence of universal properties in the morphology of different cities, which is captured by a model for the dynamics of city growth. Then, we discuss the implications of our findings for the development of city evacuation plans, whose efficiency is of paramount importance for cities threatened by natural hazards, such as volcanoes.