# Design of Adits for People Passing Spacing in High Altitude Highway Tunnels in Cold Regions
## 2 Materials and Methods
### Personal Evacuation Scenarios Analysis
#### 3.1.2 Visibility  
Figure 7 presents the variation in longitudinal visibility at a characteristic height for 300 s. In the initial stage of the fire, due to the thermal updraft generated by the fire source, smoke primarily accumulates in the tunnel vault. This has a minimal impact on visibility at a characteristic height and there remains no significant change in the temperature rise or smoke concentration. As the fire progresses, smoke continues to vertically disperse throughout the tunnel, resulting in a continuous decline in visibility at a characteristic height away from the fire source. However, as distance increases, human eye visibility gradually improves. For instance, visibility at a distance of 300 m from the fire source in working conditions 1, 4, 7, 11, 14 and 17 remains within the safe range at 300 s. However, the visibility within the area 400 m away from the fire source is lower than 5 m at 360 s.  
No matter what the longitudinal wind velocity is, visibility becomes a disaster factor when a tunnel fire occurs, but its influence range and action time differ. Under the conditions of no longitudinal wind in the tunnel, the visibility at the characteristic height of the tunnel is greater than 10 m within 240 s, which is conducive to personnel evacuation. Affected by the low buoyancy at a high altitude, the visibility at a characteristic height is partly less than 10 m at an altitude of 4000 m and 5000 m (test 13 and test 16). However, with the further accumulation of smoke, the visibility at the characteristic height of the tunnel is gradually affected and has become a key disaster factor at 300 s. The visibility near the fire source is always in the safe range, mainly due to the effect of the fire plume generated by the heated air continuously rising under the action of buoyancy. However, the key disaster factor in this area is temperature. When considering the longitudinal wind velocity, during the initial stage of fire development, the presence of longitudinal wind exerts a certain inhibitory effect on smoke diffusion at the characteristic height of the tunnel. The higher the wind velocity is, the smaller the overall reduction in visibility and the shorter the affected length will be. Simultaneously, the introduction of cold air by longitudinal wind partially dilutes smoke concentration and improves visibility. However, an increase in longitudinal wind velocity also accelerates smoke spread along its direction. As the fire progresses, downwind areas gradually become affected and this is directly proportional to the longitudinal wind velocity. The higher the longitudinal wind velocity is, the faster the same position enters hazardous visibility, and the longer the affected area of visibility danger persists. When comparing tunnels at different altitudes under identical longitudinal ventilation wind velocities, we see that higher altitudes have a greater impact on visibility. Hazardous visibility is reached earlier from the fire source at the same position, and the duration of visibility danger is prolonged. The visibility from the same position of the fire source reaches the hazardous standard earlier, and the length of the hazardous range is longer. Considering that air density and oxygen content decrease with altitude, both the flame height and smoke elevation are reduced. Consequently, buoyancy for high-temperature smoke under the influence of longitudinal wind spreading along the tunnel gradually diminishes. In most cases, compared to longitudinal temperature changes at a characteristic height, smoke-induced visibility decline serves as a key disaster factor in fires and should be considered as a main element in escape design. It should be noted that, at 360 s, human visibility within 20-80 m upstream of the fire source of test 11, 14, and 17 (altitude of 3000 m, 4000 m, and 5000 m) is affected by back-layering due to the low air buoyancy at a high altitude when the vertical ventilation velocity is 2 m/s.