## 8 Summary and Conclusions  
This research was motivated in particular by the need to develop a CFD modeling for simulating several fire scenarios in a one-way road tunnel with natural ventilation, and for assessing the exposure to risk of users during their evacuation process. To this aim, the FDS with the associated Evac code was set up, and then used for a tunnel with L < 1 km for which mechanical ventilation is not required according to the European Directive 2004/54/EC. Four fire scenarios corresponding to different types of burning vehicles were considered (car, bus, and two HGVs). Fire scenarios, in terms of maximum HRR, for the mentioned vehicles were assumed to be: 8, 30, 50, and 100 MW, respectively. Each burning  
Figure 9: Contours of smoke propagation within the tunnel investigated, after 10 min from the start of fire, when the burning vehicle is in the middle of the tunnel length: (a) 8 MW; (b) 30 MW; (c) 50 MW; (d) 100 MW. vehicle was assumed to be located along the tunnel investigated in five different positions from the entrance portal. The evacuation times were found to be higher when fire was related to a bus, which is due to a major pre-movement time needed so that the last person occupying the bus may also leave the vehicle. However, the results of the fluid dynamic analysis, interpreted in terms of environmental conditions along the evacuation path (i.e., temperatures, radiant heat fluxes, visibility distances, and CO and CO\({}_{2}\) concentrations) are all contained within the limits of acceptable safety for tunnel users. The results of the exposure to toxic gases and heat along the evacuation path, expressed in terms respectively of \(\text{FED}_{\text{toxic gases}}\) and \(\text{FED}_{\text{heat}}\), also confirm that the tunnel users can safely evacuate. The present paper has shown that mechanical ventilation is not necessary in the case of the tunnel investigated. However, engineers must always conduct a risk analysis of tunnels with a length less than 1 km that demonstrates the efficiency of natural ventilation. The CFD modeling presented in the present paper has a general validity. Therefore, the results obtained might be extended to other tunnels naturally ventilated with similar geometric and traffic characteristics. Although the authors are confident that they have performed an appropriate analysis--based on the optimization of mesh in FDS and the use of a suitable convergence criterion in Evac--which contributes towards increasing our knowledge, there are still certain points that should be investigated. Risk analysis should be done also neglecting natural ventilation due to piston effect or in consideration of adverse wind situations (i.e., wind blowing in a direction opposite to the traffic flow). The effects of using a one-way tunnel for bidirectional traffic in specific emergency circumstances should also be investigated. Therefore, additional studies are needed for making further developments possible in assessing the risk of users in road tunnels that are not mechanically ventilated. Conceptualization, C.C., G.G., and I.R. ; methodology, C.C., G.G., and I.R. ; software, C.C., G.G., and I.R. ; validation, C.C., G.G., and I.R. ; formal analysis, C.C., G.G., and I.R. ; investigation, C.C., G.G., and I.R. ; data curation, C.C., G.G., and I.R. ; writing-original draft preparation, C.C., G.G., and I.R. ; writing-review and editing, C.C., G.G., and I.R. ; visualization, C.C., G.G., and I.R. ; supervision, C.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. **Institutional Review Board Statement**: Not applicable. **Informed Consent Statement**: Informed consent was obtained from all subjects involved in the study. **Data Availability Statement**: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions. **Conflicts of Interest**: The authors declare no conflict of interest.