Modelling safety-related driving behaviour: impact of parameter values

Traffic micro-simulation models make assumptions about the safety-related behaviour of drivers. The question is whether these assumptions should reflect safe behaviour or actual behaviour. It could be considered unethical to design a scheme based on a tool which assumes unsafe behaviour if this could lead to the adoption of designs which are known to be unsafe. Vehicle interactions and drivers' behaviour are generally represented in microsimulation models through car-following, gap-acceptance, lane-changing models. The consequences of changing the value of safety-related parameters are discussed in relation to drivers using inadequate headways and the resultant increase in traffic flow and the capacity of a road network. The DRACULA traffic micro-simulation model was used to explore the impact that changes in key behavioural parameters might have on various model estimates of system performance. Data are given for traffic modelling for UK Heathrow Airport Terminal 5. It is considered that, depending on how the model predictions are being used, use of realistic-but-unsafe parameter values could result in the adoption of unsafe designs. However, it is noted that this problem can be overcome by paying attention to the safety aspects of designs and that, in general, the use of realistic values is to be preferred. The possibility of using traffic simulation models to produce estimates of accident potential and the difficulties involved in doing so are discussed. For the covering abstract see ITRD E126595.