## 8 Conclusions and Discussion for Future Investigations  
This research was mainly motivated by the need to perform a computational fluid dynamics modeling for assessing the effectiveness of a temporary safety measure, which is based on the use of an emergency vehicle equipped with a micronized water system, for contrasting the fire growth before the arrival of the fire brigade in road tunnels that are still involved in refurbishment works necessary for adapting these structures to the provisions of the European Directive [1]. The Fire Dynamics Simulator (FDS) code was used to reproduce the fire of two cars, a van, a bus, and two different types of HGVs (characterized by a HRR\({}_{\text{max}}\) of 8, 15, 30, 50, and 100 MW, respectively). Each burning vehicle was located at five different points along the investigated tunnel (145, 280, 420, 570, and 710 m from portal A). The traffic flow, expressed in terms of peak hour volume (VHP), was assumed to be 1000, 1500, and 2000 vehicles/hour per lane, which corresponds to an Annual Average Daily Traffic (AADT) per lane of 10,000, 15,000, and 20,000 vehicles/day, respectively. The one-way road tunnel investigated is naturally ventilated and was assumed to be affected by refurbishment works (i.e., without hydrants and with the sidewalks and emergency exit that are not available for escaping from the tunnel in the event of fire). The use of a micronized water system was assumed to modify the original fire curves characterized with HRR\({}_{\text{max}}\) of 8, 15, and 30 MW by intercepting the corresponding linear law of the fire growth phase at 8, 6, or 4 MW. This involves that the maximum activation time of the micronized water system should be less than 5 min from the fire's start. With reference to the position of the last user evacuating from the tunnel along the escape route on the road carriageway after 10 min from the fire's start, the FDS simulations showed that with the use of the micronized water system: (i) the tenability limits of temperature, radiant heat flux, and CO\({}_{2}\) concentration are much more verified in all investigated cases; (ii) the CO concentration is within the tenability limit of 1200 ppm also with reference to the fire characterized by HRR\({}_{\text{max}}\) of 30 MW; (iii) the visibility distance increases, however, the last escaping user might still find a visibility distance lower than the threshold value of 10 m, except for the case in which the fire curve with HRR\({}_{\text{max}}\) equal to 8 MW is modified at 4 MW. The exposure to toxic gas concentration expressed in terms of FED\({}_{\text{toxic gases}}\) showed that the number of potential fatalities was significantly reduced when the micronized water system was activated. A Quantitative Risk Analysis (QRA), based on a probabilistic approach, was set up for building the F-N curves through which the risk reduction due to the use of a micronized water system can be quantified in terms of the cumulative frequency of fatalities. For this aim, first of all, the F-N curves developed by the application of the FDS and its evacuation module Evac (i.e., related to the fire of two cars, a van, a bus, and two different types of HGVs) were computed. Then, using the DG-QRAM method, the F-N curves associated with the transport of dangerous goods were calculated. By combining both the mentioned curves, we obtained the final cumulative F-N curves. It was found that: (i) the F-N curves with the use of the micronized water system always lie below those without the mentioned system; (ii) a greater risk reduction was found when the micronized water system is activated at 4 MW of the fire growth phase compared to 6 and 8 MW; (iii) by the activation of the micronized water system, the F-N curves were found to be always within the limits of the ALARP region, while in the absence of this system they may reach or exceed the unacceptable limit.