The third and last group of main feedback loops concerns the topic of “land availability”. Cities cannot indeed grow indefinitely; above all, it is important to dedicate some space for green areas and parking lots, which complete the viability of the city. The loops belonging to the third group are balancing loops which limit

The Causal Loop Diagram (CLD) developed for the SUSTAIN model is composed by different variables, that represent areas of interest in a general modern urban system at the same time giving emphasis to management issues related to more specific areas/sectors of an urban environment. Stated differently, the model includes both sectors devoted to specific areas of a modern urban environment (e.g., environment, transport, urban planning, waste and water management) and a core sector where “common” variables (e.g., GDP and population) are included and are affected by the decisions taken in other areas of the model. As the CLD (and the subsequently developed Stock and Flow Diagram) was the basis for the development of the rules and elements of the boardgame (which is one of the main outcomes of the SUSTAIN project), the core of the model revolves around one of the most important parameters for deciding who will win the game, i.e. the Attractiveness of the city. This variable is the synthesis of multiple variables that belong to many aspects of the urban system, defining the “wellbeing” of the population who lives in it. The most important effect due to variations in the Attractiveness of the city is a variation of the number of people who live there; this generates many impacts on different urban levels, triggering in turn a certain number of feedback loops. In fact, most of the feedback loops we identified “pass” through the Population variable. It is kind of natural that this happens as, in the end, urban systems exist because of its inhabitants, indeed. Analysing the CLD (see Fig. 4.4), the most important feedback loops were identified and then divided into three main groups. The first group is composed of loops belonging to the “core” of the model, that is constituted by the relation between population, GDP and Industries and Services (Fig. 4.5). The first two reinforcing feedback loops (R1 and R2) trigger when a variation in the Attractiveness of the city causes an increase in Population, which generally has a positive effect on the GDP: the more the GDP, the more the development of industries and services. This generates a twofold positive effect on attractiveness: on one hand, there is the availability of more services and developed industries; on the other hand, more services and industries lead to more jobs for inhabitants. The former phenomenon is limited by a balancing feedback loop (B1), which depicts the saturation of jobs in the city. Finally, GDP and Industries and Services are tied together by a simple reinforcing feedback loop (R3). The second group is composed by loops which belong to the “environmental” part of the model. Water, waste and transport have direct impacts on the total pollution and, in turn, on the Attractiveness of the city. As opposed to the reinforcing loops previously described, there are two balancing loops (B2 and B3) that tend to stabilize the Attractiveness of the city through the possible increase in population, which in turn causes an increase in waste generation and water consumption, with consequences on pollution and water shortage. Another reinforcing feedback (R4) describes how traffic congestion influences the usage of public transport and, in turn, how it impacts pollution. This loop is balanced by two loops (B4 and B5): on one hand, the usage of public transport naturally reduces the problem of traffic congestion; on the other hand, external policies could increase the roads’ capacity and length addressing the same problem The third and last group of main feedback loops concerns the topic of “land availability”. Cities cannot indeed grow indefinitely; above all, it is important to dedicate some space for green areas and parking lots, which complete the viability of the city. The loops belonging to the third group are balancing loops which limit