## 5 CFD Results
### CO\({}_{2}\) Concentration  
Figure 9 shows the longitudinal profiles of CO\({}_{2}\) concentration predicted at 2 m height along the escape path after 10 min from the start of the 100 MW (i.e., Scenarios 0, Ia, and IIa) or 15 MW (i.e., Scenarios Ib and IIb) fire located in the middle of the tunnel length. Figure 9 shows how the last escaping user is always exposed to a CO\({}_{2}\) concentration below the tenability limit of 40,000 ppm (i.e., dotted line) [51]. Figure 7: Longitudinal profiles of visibility distance predicted at 2 m height along the escape path after 10 min from the start of the 100 MW (i.e., Scenarios 0, Ia, and IIa) or 15 MW (i.e., Scenarios Ib and IIb) fire located in the middle of the tunnel length. 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. Summing up the results above, with reference to the last evacuating user: (i) better environmental conditions were observed to occur with partial closure of the tube rather than complete closure (i.e., Scenarios Ia and Ib against Scenarios IIa and IIb); (ii) additional benefits can be found by activating VMSs that suggest an alternative itinerary for HGVs only (i.e., Scenario Ib versus Scenario Ia, and Scenario IIb against Scenario IIa); this is due to the fact that when the HGVs are diverted to the alternative route, the most potentially dangerous event is represented only by a van fire (i.e., \(\text{HRR}_{\text{max}}\) of 15 MW); (iii) safety issues for human health may arise only in the event of 100 MW fire occurring during the complete closure of the north tube and the use of the parallel one for two-way traffic (i.e., Scenario IIa) due to exceeding the acceptable limit of 10 m and 1200 ppm for the visibility distance and CO concentration, respectively. It is worth highlighting that similar evacuation conditions were also found for the remaining locations of the burning vehicle (i.e., 145, 280, 570, 710 m from portal A of the north tube for Scenarios 0, Ia, and Ib, or from portal D of the adjacent tube used for bi-directional traffic for Scenarios IIa and IIb). With reference to the remaining \(\text{HRR}_{\text{max}}\) investigated (i.e., 8, 15, 30, and 50 MW fires for Scenarios 0, Ia, and IIa; or 8 MW fire for Scenarios Ib and IIb) and locations of the fire in the tunnel tube, we did not find any unsafe conditions for the users evacuating the tube. However, it needs to be stressed that, in order to save space, in the present paper, we have reported only the results of the worst-case scenarios that were found to be the burning vehicles with \(\text{HRR}_{\text{max}}=100\) MW located in the middle of the tunnel tube length for Scenarios 0, Ia, and IIa, and for the fire of 15 MW in Scenarios Ib and IIb. Figure 8: Profiles of longitudinal CO concentration predicted at 2 m height along the escape path after 10 min from the start of the 100 MW (i.e., Scenarios 0, Ia, and IIa) or 15 MW (i.e., Scenarios Ib and IIb) fire located in the middle of the tunnel length.