General principles of tunnel safety assessment

In the past in many countries road tunnel safety to a great extent was based upon regulations and guidelines for the design, the construction and the operation of road tunnels: if the applicable prescriptions of relevant guidelines were fulfilled the tunnel was regarded as safe. These guidelines had been developed over decades and were mainly based on the experience of everyday operation, including incidents and accidents.

However, this prescriptive approach has some shortcomings which are particularly evident in incidents exceeding the range of existing operational experience:

Technical design specifications defined in guidelines are able to establish a certain level of standardization and to guarantee an adequate performance of the various technical systems; but this approach does not take into account the effectiveness of specific measures, which may be dependent on the specific conditions of an individual tunnel.

Further, even if a tunnel fulfils all regulative requirements it has a residual risk which is not obvious and not specifically addressed.

Hence, in addition to the traditional prescriptive approach, especially for complex systems a supplement is needed which specifically addresses emergency situations: the risk-based approach. Risk-based approaches allow a structured, harmonised and transparent assessment of risks for an individual tunnel, including the consideration of local conditions in terms of relevant influence factors, their interrelations and possible consequences of incidents. Moreover, risk-based approaches make it possible to propose relevant additional safety measures for the purpose of risk mitigation. Thus risk assessment can be the basis for decision-making considering cost-effectiveness in order to assure the optimum use of limited financial resources.

Therefore modern safety standards also take into account the evaluation of the effectiveness of safety measures based on risk assessment. In the European Union, for instance, the EC Directive 2004/54/EC on minimum safety requirements for road tunnels in article 13 introduces risk assessment as a practical tool for the evaluation of tunnel safety.

When performing risk assessment, it is important to realise that decision-making about risks is complex. Whereas risk analysis is a scientific process of assessment and/or quantification of probabilities and the expected consequences of identified risks, risk evaluation is a socio-political process in which judgments are made about the acceptability of those risks. Certain risk criteria have to be established to be able to evaluate the results of a risk analysis. In Chapter 4 of the technical report 2012 R23  “Current practice for risk evaluation for road tunnels” practically applicable risk evaluation strategies are presented.