## 7 Summary and Conclusions  
The present study was mainly motivated by the need to simultaneously couple user safety with resilience in order to identify the most appropriate arrangement to be adopted in the event of a traffic accident in a tube of a twin-tube road tunnel. In this respect, we investigated different scenarios by assuming that during the partial or complete closure of  
Figure 12: Resilience index and expected value (EV) of risk for each scenario investigated. Scenario 0: ordinary conditions of functionality of the tunnel system (i.e., both lanes of each tube are used for one driving direction) with a fire that occurs in the north tube; Scenario Ia: partial closure of the north tube (i.e., only the right lane is closed due to a traffic accident) with a fire that might occur on the left lane of the north tube; Scenario Ib: similar to Scenario Ia, but with the HGVs traveling towards the north tube diverted to an alternative itinerary; Scenario IIa: complete closure of the north tube (i.e., both lanes are closed due to a traffic accident) with a fire that might occur on the lane of the adjacent tube that is used by traffic traveling towards the north; Scenario IIb: similar to Scenario IIa, but with the HGVs traveling towards the north tube diverted to an alternative itinerary. a tube, because of the occurrence of a traffic accident, a fire might simultaneously occur on the undisrupted remaining lane, or in the parallel tube used for two-way traffic in the case of the partial or complete closure of the disrupted tube, respectively. For this purpose, we developed a computational fluid dynamics (CFD) model, and simulated the corresponding egress process, to evaluate the risk level of tunnel users in the event of a fire. The Fire Dynamics Simulator (FDS) code, as well as its evacuation module (Evac), were implemented, and their output were used to carry out a quantitative risk analysis (QRA) based on a probabilistic approach. The effects of five different types of burning vehicles were investigated. Each burning vehicle was assumed to be located at five different positions along the remaining undisrupted lane, or in the parallel tube used for two-way traffic in the case of the partial or complete closure of the tube affected by the traffic accident. The burning vehicles were assumed to be two cars, a van, a bus, and two different types of HGVs capable of developing a maximum heat release rate (HRR\({}_{\text{max}}\)) of 8, 15, 30, 50, and 100 MW, respectively. With reference to the last user escaping from the tunnel tube in the event of a fire, the findings of the CFD simulations, in terms of temperature, radiant heat flux, visibility distance, and toxic gases concentration at breathing height along the escape route, showed that: (i) better environmental conditions were found with partial closure of the tube rather than complete closure; (ii) additional benefits can be found by activating variable message signs (VMSs) that suggest an alternative itinerary for HGVs only; (iii) safety issues for human health may arise only in the case of a 100 MW fire, occurring during the complete closure of the north tube and the use of the parallel one for bi-directional traffic, due to exceeding the acceptable tenability limits of both visibility distance and CO concentration. The results of the QRA were found to be consistent with those obtained by CFD modeling, as follows: (i) the partial closure of the tube presents a lower risk level than that associated with the tube in its ordinary conditions of functionality; (ii) the complete closure of a tube and the use of the parallel tube for two-way traffic leads to a higher risk level compared to that of the ordinary conditions of functionality of the tunnel; (iii) additional reductions in risk can be found by activating VMSs to divert only HGVs towards an alternative itinerary.