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Many countries, in particular those with a long tradition in operating road tunnels, have developed a framework of guidelines and regulations for the design, the construction and the operation of road tunnels. Although safety has always been an important issue, guidelines often focus on technical design specifications in order to establish a certain level of standardization and to guarantee an adequate performance of the various technical systems. However, this traditional prescriptive approach does not take into account the effectiveness of prescribed measures in a particular tunnel. Hence the resulting safety level might differ from tunnel to tunnel. Furthermore, even if a tunnel fulfils all regulatory requirements, there are residual risks which in the traditional prescriptive approach to safety are not obvious and not specifically addressed ^[1].