Geometric Design Issues and Safety Analysis of Two-way Rural Road Tunnels

Rural road tunnels may pose specific issues from several perspectives: mainly environmental, geotechnical/structural (excavation methods), economical. Geometric road design is then an often arduous task, which should be conducted by trying to make trade-offs between the different issues, and at the same time, it should be compliant to relevant standards and regulations. Nevertheless, the safety of road tunnels should be preserved, not only considering technological systems (e.g. ventilation systems, emergency exits and paths), but also regarding traditional road-safety issues related to road geometric features. In fact, the analysis of recent accident data related to two-way two-lane rural road tunnels has revealed that accident frequencies/rates in two-way two-lane rural road tunnels are comparable with those on the corresponding open sections, even with some differences concerning the accident types. Since road safety issues may be exacerbated by geometric design inaccuracies, some relevant road tunnel geometric issues are discussed in this study. These issues, scarcely treated in previous research and not addressed in detail in technical documents, emerge from the match between different design needs. In detail, those relate to: 1) the possible need for variable road tunnel cross-sections in case of lane/shoulder widenings for the sake of improving visibility or for other reasons, or in case of lay-bys, 2) the possible need for climbing/overtaking lanes in case of steep downhill or uphill slopes. An attempt at addressing the mentioned problems is provided, based on relevant existing research and technical documents. In particular, the preliminary design stages leading to a "WES" (Without Enlargement Solution) solution are described. Hence, besides enlarging the research body in this specific field, the study is also potentially deemed useful for practitioners who have to face similar design problems.