Quantification of a Safety Target for an Underground CNG Bus Terminal in Stockholm

This article was first presented as a part of the 8^th edition of the International Symposium on Tunnel Safety and Security (ISTSS) in Borås, Sweden.

There are currently no detailed regulations or guidelines for designing the safety concept for an underground terminal for buses powered by compressed natural gas (CNG). Neither are there any explicit safety targets for this type of facility in legislation or practice. A dilemma arises both in the land-use planning process and building design process when evaluating whether the safety of such a bus terminal is sufficient. However, there are internationally accepted principles in other sectors that may be useful in defining a safety target in this case. 

In this paper, it is proposed that such a safety target can be quantified using risk acceptance criteria expressed in terms of individual and societal risk. The method developed in this study is based on comparisons with risk acceptance criteria applied in other types of facilities and activities, both nationally and internationally, and required an extensive inventory of these. The method also takes into account the fact that people’s perceptions of risks affect their acceptance. 

 The proposed safety target is presented in terms of an upper and a lower F-N curve, and includes the ALARP principle. In addition to this a maximum average risk is specified for the facility. A plausibility check was carried out indicating that the risk level defined by the safety target is lower than, or of the same order of magnitude as, many other corresponding risks in society, e.g. in other transport systems.