# Design of Adits for People Passing Spacing in High Altitude Highway Tunnels in Cold Regions
## 4 Discussion  
This research into smoke diffusion in highway tunnel fires revealed peculiar patterns of fire characteristics at different altitudes. The increase in altitude may exert a substantial influence on the environment in a tunnel fire, such as reduced air buoyancy, diminished oxygen content, and decreased air density, which have a significant impact on the smoke distribution characteristics. In a tunnel fire, the three main hazardous factors for individuals are increased temperature, decreased visibility, and increased concentrations of harmful gases. The variation in longitudinal temperature, visibility, and CO concentration at a characteristic height were analyzed under different longitudinal ventilation velocities. And based on the results of numerical simulation under critical wind velocity, the available safe egress time was determined. Additionally, the required safe egress time was ascertained based on the results of the Pathfinder model. Based on this, the minimum spacing of adults for people passing at a different altitude was established by ensuring that the required safe egress time should not exceed the available safe egress time. The temperature distribution results are consistent with the findings of previous studies [3; 4; 5; 14; 15; 16; 17; 18]. However, it must be pointed out that we further considered one key factor, the longitudinal ventilation velocity. We conducted a comparative analysis of the  
Figure 11: Comparison of RSET and ASET at different altitudes and the maximum spacing of adults for personal evacuation. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Altitude, m** &
\begin{tabular}{c} **Maximum Adits for People Passing** \\ **Spacing, m** \\ \end{tabular} & **Reduction Factor** \\ \hline
1000 & 250 & 0 \\
2000 & 223 & 0.89 \\
3000 & 137 & 0.55 \\
4000 & 107 & 0.43 \\
5000 & 67 & 0.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Maximum spacing of adults for people passing and reduction factor. impact of varying longitudinal ventilation airflow velocities on smoke dispersion within tunnels situated at different elevations. And the design of Adit for People Passing Spacing was based on the critical velocity, which meets the requirements of the national code [33; 40]. It is essential to underscore that this aspect has not been considered in previous research [20]. We should acknowledge the limitations of our research. Firstly, it is important to emphasize that enough experiments [4; 14; 20] ensure the accuracy of our findings on temperature distribution results. The distribution of human visibility has also been confirmed. while the distribution pattern of CO concentration in high-altitude tunnels still requires experimental verification. In the hypothetical case of this paper, the HRR considers the most unfavorable case, which is 30 MW when a fully loaded truck catches fire. But in an actual situation, affected by the oxygen content decline caused by high altitude and other factors, the HRR of a fully loaded truck will also decrease with the increase in altitude. Usually, in an emergency escape, personnel need to bend down and walk, and the characteristic height of the human eye is 1.6 m. Taking into account the safety margin, this study selects 2 m as the characteristic height for analysis. Furthermore, our selection of simulations has limitations. The simulation does not fully capture the incomplete combustion and other real-world fire scenarios, indicating the necessity for further investigation into the distribution of CO concentration. Meanwhile, although the current codes for safe levels of harmful gases are based on concentration values, new models such as a fractional effective dose have been proposed that can further clarify the impact on personnel evacuation. At the same time, only the impact of panic psychology on the speed of personnel evacuation was considered in the simulation of personnel evacuation. The influence of panic psychology on the selection of a personnel evacuation strategy was not taken into account in this study.