## 8 Summary and Conclusions  
The main reason justifying this research lies in the need to quantitatively evaluate the additional risks that a battery electric bus (BEB) might pose--compared to a similar internal combustion engine bus (ICEB)--for the safety of the evacuees and firefighters in the event of a tunnel fire. The investigated bi-directional tunnel is 850 m long and is assumed to be under congested traffic conditions, as well as naturally or mechanically ventilated. The two aforementioned types of buses (BEB or ICEB), which were considered to be located in the middle of the tunnel length when they caught fire, were assumed to have the same geometric characteristics, heat of combustion, CO, and soot yields and differed from each other for both the HRR\({}_{\text{max}}\) and HF release rate: (i) BEB with an HRR\({}_{\text{max}}\) of 34 MW and an HF release rate of 3 g/s; (ii) ICEB with an HRR\({}_{\text{max}}\) of 30 MW and an HF release rate of 1.5 g/s. To achieve the objectives of this study, a 3D computational fluid dynamics (CFD) model was performed, and the FDS tool was used to solve this model. The Evac tool was applied to simulate the consequences on the escape process of the users toward a safe place. The FDS code was preliminary calibrated against a fire involving a battery electric vehicle (BEV) in a confined space, and then the optimal mesh resolution to be used in the numerical modeling was defined by performing a grid sensitivity analysis. In both cases of ventilation investigated, the CFD outcomes revealed that the burning of the BEB, in comparison to that of its ICEB counterpart, significantly worsened the environmental situations for the tunnel occupants while evacuating downstream of the fire, especially in terms of toxic gases concentrations (i.e., HF, CO, and CO\({}_{2}\)), which also led to a significant increase in the values of the FED\({}_{\text{toxic gases}}\) parameter. Regarding the safety of the firefighters entering the tunnel to extinguish the burning bus and/or help users still in the structure, the simulation results related to the naturally ventilated tunnel showed that they might find highly unsustainable environmental conditions upstream of the bus in flames since the HF and CO levels, temperatures, visibility distances, and FED\({}_{\text{toxic gases}}\) were found to exceed the corresponding acceptability limit, especially during the BEB fire. Therefore, the results provided by this study showed that the fire of a BEB in a road tunnel, compared to that of an ICEB, may represent an additional threat to the safety of the evacuees and firefighters. In this respect, although tunnel fires happen less frequently than tunnel traffic accidents, their impact may be more negatively relevant to user safety since a large portion of the tunnel can quickly become engulfed in hot gases, toxic substances, and smoke. Considering this, the evaluation of user safety in the event of a BEB fire in road tunnels is a crucial issue in real life, and it requires particular attention from the authorities at different levels. Although this paper adds knowledge to the field of research concerning the safety of road tunnels in the event of a fire involving a battery electric bus, there are still some points that deserve further investigation. Particularly, experimental tests should be conducted for a more consolidated verification of the results obtained. Moreover, the influence of factors such as stress and panic on users' behavior during their evacuation process toward a safe place is another aspect to be addressed with future studies. Additional investigations should also be made to define a database of traffic accidents involving different categories of EVs (e.g., cars, vans, buses, heavy goods vehicles, dangerous goods vehicles, etc.) with the aim of building a reliable event tree to be used in quantitative risk analysis.