Modellingthe construction of a virtual representation of a physical situation or processis now an essential component of most projects. Models allow engineers to forecast likely outcomes based on past experience, and then design appropriate mitigating measures. Just as importantly, models allow engineers to experiment in a 'risk-free' environment. Of course, in order for engineers to have confidence in a model it is essential that the output is validated to ensure that it correlates with the observed physical outcomes. Modelling is the common theme running through all the papers in this issue of Transport. Applications range from a high-speed rail network in Spain, by way of highways in Brazil to internal air routes in Iran.

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