# Design of Adits for People Passing Spacing in High Altitude Highway Tunnels in Cold Regions
## 1 Introduction  
The key characteristics of high-altitude areas are low temperature, low pressure, and low oxygen levels [1]. The unique and challenging natural environment in these regions further complicates personnel evacuation in high-altitude highway tunnels during fire accidents [2]. These are mainly manifested as an increase in smoke diffusion length and smoke diffusion velocity under the same fire scale. [PERSON] et al. explored the distribution characteristics of temperature and smoke and personal evacuation in the Hsuehshan southbound highway tunnel [3]. [PERSON] et al. conducted full-scale fire tests to study the impact of longitudinal wind velocity on longitudinal distribution of smoke temperature and maximum ceiling temperature [4; 5]. [PERSON] et al. explored the relationship between the heat release rate (HRR) and the width of the tunnel and the ventilation wind velocity during the tunnel fire, and found that fires in narrow tunnels have a higher heat release rate than in wide tunnels [6]. [PERSON] et al. proved the reliability of the numerical simulation by comparing the results of FDS numerical simulation with results from experiment in a 1/3 scale tunnel under the condition of lower and higher than critical wind velocity [7]. [PERSON] et al. pointed out that the longitudinal slope can have an obvious impact on the smoke diffusion by conducting a numerical investigation to analyze the thermal smoke behavior and the distribution of the ceiling temperature in a sloping tunnel with the slope varying from 0% to 15% [8]. [PERSON] et al. researched the temperature and CO concentration regulation under the impact of longitudinal ventilation velocity, and found that personal evacuation downstream of a fire is only advantageouswhen the longitudinal ventilation velocity surpasses the critical ventilation velocity [9; 10]. [PERSON] et al. studied the behavior and reaction of people in a tunnel fire and pointed out that social influence, etc., is crucial to the decision to leave the vehicle and the choice of exit [11]. In 2013, [PERSON] et al. explored the theory that the evacuation time primarily relies on the promptness of evacuation actions, while being minimally influenced by the lead time for evacuation [12]. And an appropriate Computational Fluid Dynamics model was developed to evaluate the exposure to risk of tunnel users during their evacuation process in a fire in 2020 [13]. However, most research on fire smoke diffusion and personnel evacuation in highway tunnels mainly focuses on low-altitude tunnels with a normal temperature, regular ambient atmospheric pressure and normal oxygen levels. The temperature, pressure, and oxygen levels in many of the tunnels in western China are below the normal value. [PERSON] et al. conducted investigated heat release rate, temperature distribution and smoke diffusion characteristics in a high-altitude tunnel by conducting six full-scale fire tests at an altitude of 4100 m. The impact of altitude on personnel evacuation behavior was also investigated through onsite random evacuation tests [14; 15]. [PERSON] et al. analyzed temperature and concentration distribution characteristics of fire smoke in high-altitude tunnel by conducting model tests in the Guanjiao tunnel [16; 17; 18]. [PERSON] et al. conducted tunnel fire tests at various altitudes that revealed the impact of altitude on the heat release rate, smoke temperature distribution, smoke diffusion, and toxicity distribution in ultra-high-altitude tunnels [19; 20; 21; 22]. [PERSON] et al. revealed that the maximum temperature beneath the tunnel ceiling and the longitudinal temperature distribution increase as the ambient pressure decreases; this was mainly affected by reduced heat loss due to reduced air density [23]. [PERSON] et al. conducted multivariant analyses of different fire development scenarios and evacuation in a highway tunnel based on sound assumptions [24]. [PERSON] et al.