This article further elaborates the findings by Reniers et al. in 2007 and 2008. A discrete-time approximation is presented to determine the severity of a major accident threat triggering immediate evacuation and its expected resulting costs. By implementing the proposed mathematical model, precautionary evacuation decision problems can be tackled in a realistic way, i.e., allowing for major accident threats with limited duration. Furthermore, the model is moulded into a working procedure which was used to develop software to solve the suggested algorithms. A case-study is provided and the results obtained by application of the methodology are discussed. Using a (realistic) discrete-time approximation computer simulation, we found that ignoring option characteristics may produce suboptimal intervention decisions in shutdown settings. [Copyright &y& Elsevier]