## 8 Summary, Conclusions and Discussion for Future Investigations  
The main reasons for justifying the present paper were the need to develop a fluid dynamic modeling capable of reproducing different fire scenarios occurring in a one-way tunnel tube with natural ventilation, used for bi-directional traffic when the adjacent tube is closed for maintenance, repair, or rehabilitation activities; and for assessing the differences in the risk levels to which users are exposed during their evacuation process during the night and the day. To do so, the Fire Dynamics Simulator (FDS) and its evacuation module Evac were used; and the results of simulations were implemented for performing a Quantitative Risk Analysis (QRA) based on a probabilistic approach for evaluation the societal risk expressed in terms of F-N curves. Specifically, four types of fires involving two cars, a bus, and two different types of HGVs were investigated (i.e., characterized by HRR of 8, 30, 50, and 100 MW, respectively). Each fire source was positioned in five different longitudinal locations in the tube (i.e., 145, 280, 420, 570, and 710 m from portal A). The evacuation process in the event of fire within the tube used for bi-directional traffic, due to the direction of the natural ventilation (from portal A to B), was worse for escaping users downstream of the fire in the direction of emergency exit and/or portal B. However, the results showed the positive effects on environmental conditions (in terms of temperature, visibility distance, CO and CO\({}_{2}\) concentration) along the evacuation path when the tunnel tube was used for bi-directional traffic during the night rather than the day (which was justified by lower night traffic volumes). Only for the case of 100 MW fire, the last escaping user was found to be interested by a visibility distance less than 10 m, and a CO concentration higher than 1200 ppm, in the proximity of the portal B. In these circumstances, measures capable of keeping the concentration of smoke below the threshold values should be implemented; alternatively, the presence of rescue teams equipped with emergency services, and with an access time within the tube of less than 10 min, should be foreseen. The results of the quantitative risk analysis, however, showed that: (i) the F-N curve of the tube used for bi-directional traffic with reference to the night always lies below that of the day, (ii) the cumulative frequencies of having a certain number of potential fatalities due to toxic gases during the night is much lower than the day; (iii) the maximum number of potential fatalities during the night is much lower; (iv) the reduction in the risk level is between 80 and 100% for bi-directional night traffic compared to the daytime one. It is to be mentioned that if the closure of the adjacent tube is not scheduled, but is caused suddenly by an incident (e.g., traffic accident or fire), for the use of the tube interested by bi-directional traffic the presence of rescue teams stationed at the two extremities of the twin tube tunnel is recommended for the period of emergency time. Although the results have shown certain benefits on environmental conditions along the evacuation path in the event of fire, when the tunnel tube is used for bi-directional traffic during the night rather than the day, there are still some points worth investigating. For example, the risk level of using an alternative open road when a tube is closed for given activities should also be evaluated. Another important field of future research might be to assess the risk level due to the transit of alternative vehicles, such as the Electric Vehicles (EVs), through tunnels. The battery of EVs might be accompanied by significant releases of toxic gases, in the event of fire, and cause severe negative consequences due to closed spaces. Low flammability road pavements might also be a topic of interest for research in order to limit the emission of toxic substances in tunnels due to burning pavement materials in the event of fire.