The Simulation of Organised Car Sharing (2) – The Simulation Models and their Calibration.

This paper is one of a series describing the methods and findings of a TRRL sponsored project to simulate organised car sharing. This volume describes the simulation model and its calibration. The model requires as input a description of each individual tripmaker within the system (there are 180,000 such individuals in the study area under investigation). Using these individuals as actors, the model simulates, within a calibrated choice model, the decision-of each of them in turn whether or not to apply to join a hypothesised car sharing scheme. The matching of such applicants to form potential car pools is then effected in a special routine. A second calibrated choice model then simulates the decisions by each of the applicants whether to enter an arrangement with any of the potential partners with whom they have been provisionally matched. A mutual evaluation of utilities then determines which arrangements will actually come to fruition. The microsimulation approach to transport modelling involves consideration of choice options by the fundamental actors within the system - the individual tripmakers. The approach is inherently attractive but only recently have decreasing computer costs made microsimulation a viable branch of travel demand modelling. The choice models were calibrated on the basis of a series of field surveys which were designed to mirror exactly the simulation models - respondents were invited to make decisions and to evaluate proposals drawn from the simulation models themselves. Thus the respondents effectively became actors within the simulation and their reactions to the choices available to them were used to calibrate the models. Previous investigations of car sharing have concentrated either on the behavioural and psychological aspects of the mode with little attempt to estimate the global consequences of these or have concentrated on the probable demand for the mode with little attempt to consider the interaction between potential matchees . The microsimulation approach adopted here has successfully combined a calibrated model of demand for the mode with an accurate rendition of the supply side - the matching of individual trip makers with compatible requirements and, finally, a calibrated model of the decision whether or not to enter an arrangement with a specified individual. This approach has been particularly appropriate to the modelling of organised car sharing but can obviously provide the basis for a whole range of behaviourally orientated planning models.