Modelling safety-related driving behaviour--impact of parameter values

Traffic simulation models make assumptions about the safety-related behaviour of drivers. These assumptions may or may not replicate the real behaviour of those drivers who adopt seemingly unsafe behaviour, for example running red lights at signalised intersections or too closely following the vehicles in front. Such behaviour results in the performance of the system that we observe but will often result in conflicts and very occasionally in accidents. The question is whether these models should reflect safe behaviour or actual behaviour. Good design should seek to enhance safety, but is the safety of a design necessarily enhanced by making unrealistically optimistic assumptions about the safety of drivers' behaviour? This paper explores the questions associated with the choice of values for safety-related parameters in simulation models. The paper identifies the key parameters of traffic simulation models and notes that several of them have been derived from theory or informed guesswork rather than observation of real behaviour and that, even where they are based on observations, these may have been conducted in circumstances quite different to those which now apply. Tests with the micro-simulation model DRACULA demonstrate the sensitivity of model predictions--and perhaps policy decisions--to the value of some of the key parameters. It is concluded that, in general, it is better to use values that are realistic-but-unsafe than values that are safe-but-unrealistic. Although the use of realistic-but-unsafe parameter values could result in the adoption of unsafe designs, this problem can be overcome by paying attention to the safety aspects of designs. The possibility of using traffic simulation models to produce estimates of accident potential and the difficulties involved in doing so are discussed.