Quantifying a Common Safety Target for Tunnels

This article was written in collaboration with WSP Director of Technical Risk Johan Lundin. It was first presented as a part of the 8th edition of the International Symposium on Tunnel Safety and Security (ISTSS) in Borås, Sweden.

This paper focuses on quantifying a common safety target level that can be used to verify safety levels in road tunnels, railway tunnels, and subway systems with regards to life safety. This safety target should provide a risk acceptance level, according to which tunnel projects can be compared to one another while factoring in the benefit of the tunnel in terms of the value of transporting people and goods. In order to ascertain a reasonable safety target, risk acceptance levels of regulations and recent tunnel projects were studied. The estimated risk levels of recent road and railway tunnel projects were also identified, and converted into the same entity of risk. These risk levels were also compared to estimated risk levels of several fictional road and railway tunnels that comply with current standards, with and without the transportation of dangerous goods.

There are large differences in how risk is expressed in road and railway tunnel projects, and none of the projects or regulations studied fully factor in the societal benefit of traffic when formulating safety target as a societal risk profile. Risk-benefit assessment, however, is quite common in the projects studied. The railway tunnel risk acceptance levels used by the Swedish Transport Administration (Trafikverket) are close to incorporate these benefits, as they state risk on the basis of train-km, whereas other bodies and projects only consider the tunnel as a whole or state it in terms of tunnel-km. The analysis of calculated risk levels shows that they can be stated on the basis of risk per person-km, which provides comparable data for the different transport systems and are directly proportional to transport benefits. Risk acceptance criteria for all three transport systems can preferably be measured in deaths per person-km for societal risk profiles. This unit does not state the total risk of each facility; rather, it allows an ALARP zone to be created, which allows a risk-benefit comparison to be performed. In this paper, it is suggested that risk acceptance levels should be set just above those of tunnel projects that have recently been approved by the Swedish Transport Agency (Transportstyrelsen), and that the establishing of ALARP zones should be a requirement in these processes, as this promotes risk assessment and the design of cost-effective tunnel systems.