FASE I Mi cuerpo responde a los estímulos

The microsimulation model we have developed is composed of the individual dynamics - the motion - of agents, the behaviour of each agent being largely conditioned by autonomous responses to highly local spatial conditions. We have assumed that the model is calibrated in a traditional manner by scaling up the actions of individual agents into some global pattern which is then matched against some set of observed data. Our assumption is that such global behaviour can be predicted and

possibly controlled through interventions that change the spatial geometry of the system with consequent impacts on individual behaviours. Our approach to date, however, largely ignores the measurement and analysis of individual dynamics using appropriate statistics. We might be more interested in fine-tuning the actual behaviour of each agent rather than in any macro tuning of the agent population. There is even an argument for assuming that the population will take care of itself as long as individual behaviours are modelled correctly. There remains a huge area of investigation only touched upon in the last section, concerning the distributions of different spatial behaviours within the larger population. As yet we have little idea how the statistics of the global patterns we have predicted decompose into individual behaviours but this is an important area for continued exploration in the current set of applications.

Figure 18: Flow Densities Associated with Different Walks of a Single Agent

The aggregation problem for these types of agent-based model involves representing many different classes of agent. So far, all our agents, our walkers, obey the same rules and respond to the same spatial events and patterns. Making such models more realistic must therefore involve defining key classes and types. For example, there is

distinct difference between walkers who are already familiar with local situations and those who are not. Moreover walkers may have different degrees of familiarity with different parts of the same system. Some will be familiar with different neighbourhoods while others will be familiar with different activities such as varieties of retailing and entertainment. Walkers whose goal is shopping or entertainment have different demographic profiles which condition how and what attractions they respond to and this implies that different attractor surfaces must be developed for various categories of walker. This kind of disaggregation suggests that more emphasis be given to different classes of behaviour, something which is limited within the CA software used here where the emphasis is mainly on the way local actions generate global pattern.

The existing model does not contain much agent interaction. In the Tate Gallery example, we switched on the congestion threshold which meant that walkers tend to avoid each other when a congestion threshold is reached. However the positive and negative feedbacks between the number of walkers in a place and the level of spatial attraction was not implemented. Interaction between agents only comes into its own when different classes of agent are introduced. For example, it is well-known that crowds form when individuals see other individuals congregate. When we have walkers who are familiar with a place and know where they are going, other walkers begin to follow. This kind of self-reinforcement also induces a kind of learning in that walkers who do not know a place will find out about it more quickly when they follow others who know it. In fact every time a walker visits a place, something new is learnt even if they are thoroughly familiar with a place. The randomness of motion and the fact that there are differences in the composition of other agents each time a place is visited mean that agents are continually exposed to new information, and this conditions future behaviour. In a sense, this kind of learning can only be embodied in the model if there is a shift to more individually specific modelling software where agents are treated as individuals and where massive amounts of variety can be built into the way agents respond to even quite simple situations. Again a move to more purpose-built software is necessary.

If we develop a framework in which each individual is tracked in detail, then it is likely that the local geometry through which the agents move, must be represented by

something other than continuous attraction surfaces. In fact, locations should be represented by parcels and like objects which in turn can then be endowed with different attributes. The notion of moving through the space can then be articulated as the search for objects of attraction whose attributes match the individual profiles of agents. For example, an agent with a certain set of preferences for a bundle of goods might only find those goods in a small subset of places which in turn are attractive enough to draw the agent. The outputs of the model then become profiles of realised trips which are characterised by the match between what the agent requires - demands - and the way those requirements are met - supplied. The model would thus take on a more explicit economic flavour incorporating elements of choice theory in the manner of disaggregate travel demand modelling (Willumsen and Ortuzar, 1990). In this way, we might begin to build in multi-purpose trips. Finally, if we so disaggregate, we are likely to be able to incorporate the more detailed dynamics of walker behaviour similar to that in the pedestrian models designed by Helbing and Molnar (1995).

This requires us to move to purpose-built software and to relinquish, for a time at least, the powerful graphics and animation capabilities that the current models have. But it also assumes that we begin to build in more detailed profiles of how walkers respond to their local environment and this in turn will require detailed data on their demographic and related behavioural characteristics. This forces us back to ideas in microsimulation but this time in a spatial context where the agents are modelled explicitly and individually according to the behavioural profiles which drive such simulations. These are all elements that we will consider in moving the research to the next level, while at the same time continuing to develop the existing models for Wolverhampton and the Tate Gallery. All these developments will be reported in future papers.