## 6 Analysis of Results
### Environmental Conditions Downstream of the Fire
#### 6.1.6 Extension of the Study  
Generally speaking, it is to be said that the distance between the vehicles stopped in a queue might be also minor than the value of 10 m assumed overnight. Certain references of literature, for example [25], reports that the mean distance between vehicles in the queue might be up to 3 m (with a maximum distance of 7.5 m). However, it is to be said that these studies are related to open roads, and in the event of a traffic disruption that is not due to a fire. This is different from our case in which we are studying the road tunnels in the event of a fire. When a fire occurs in a tunnel, certain warning systems are generally activated (e.g., the red light of traffic signals at the entrance portal, the variable message signs to inform tunnel users of fire, the radio-communications of the emergency, etc.) so that the driving behavior of tunnel users can be different from that on open roads. This plays a crucial role in how the vehicles form a queue and the distance between them. The drivers will become much more aware of the emergency in tunnel, and as a result major distance between the vehicles in the queue might also be expected. [PERSON] et al. [26], for example, showed that the distance between cars stopped follows a Gaussian distribution with mean equal to 8.91 m and standard deviation of 3.11 m. [PERSON] et al. [27] defined a linear dependence between the Annual Average Daily Traffic (AADT) and the distance (d) between vehicles stopped in the queue; in particular, for AADT = 22,000 vehicles/day the distance between vehicles in the queue is 1 m, while it is 10 m for AADT = 4000 vehicles/day.  
Therefore, contrasting results might be found in the literature. However, we have made an extension of our study by considering also a distance of 3 m between the vehicles stopped in the queue overnight. In addition, we have assumed a minor arrival time (2.25 s) of the first vehicle upstream of the burning vehicle (i.e., equal to that of the day).  
Figure 9 shows the longitudinal profiles of temperature, radiant heat flux, visibility distance, CO and CO\({}_{2}\) concentrations, downstream of the fire, in the bi-directional tube for both d = 3 m (overnight) and d = 1 m (peak-hours) after 10 min from the fire start, and at a height of 2 m along the escape route when the burning vehicle is in the middle of the tube length. Figure 9 confirms the results already found in the previous paragraphs, namely that safety issues might arise only in the event of 100 MW fire both during night and day, due to the reduced visibility and the high CO concentration.  
As it is also expected, the longitudinal profiles of the fire-induced threats during the night, by assuming a distance between the vehicles in the queue equal to 3 m, are closer to those of the day (d = 1 m) if compared to the case in which d = 10 m. This is due to the greater number of vehicles stopped in the proximity of the burning vehicle.  
Figure 8: Smoke propagation in the tube used for bi-directional traffic at both night and day, after 10 min from the fire start, when the burning vehicle is in the middle of the tube length and for HRR of: (**a**) 8 MW, (**b**) 30 MW, (**c**) 50 MW, (**d**) 100 MW.