## 1 Introduction  
Tunnels are complex infrastructures, usually of semicircular or rectangular cross-sections, constructed to establish connections between different sections of a road or railroad network. The growing reports of indoor fire accidents with high population mobility, such as tunnels, have created the need to consider the prevailing conditions in such cases as well as the best evacuation plan to ensure human life [1]. The consequences after a tunnel fire can be much more serious than an outdoor fire because of high smoke concentrations and confined movement space [2, 3]. Accidents involving fire and hazardous material displacement can be studied for different urban scales and settings. Although the fire poses a threat near its vicinity, toxic substances can travel along the flow of the air [4]. After a fire accident, high temperatures and toxic fumes are produced, which have harmful consequences for passengers who are trapped inside the tunnel. In 2003, an arson fire in the subway tunnel in Daegu, South Korea resulted in 198 deaths. In 2017, the Jinji Road Tunnel Fire in Shanxi, China, 2017, caused 31 deaths [5]. A wrong evacuation plan can lead people in to the fire or smoke direction, with a negative effect on their health due to the presence of toxic gases, heat, and radiation [6, 7]. To minimize human exposure to smoke during a tunnel evacuation, various types of ventilation systems are used to remove the smoke from the tunnel or to keep the smoke in the tunnel above the head [8, 9]. These can be either mechanical or natural ventilation systems [10]. Forced ventilation systems use fans to extract or push smoke out of the tunnel. Natural ventilation with vertical wells in the roof of the tunnel take advantage of the buoyancy of hot smoke, which escapes through the shafts due to the stack effect and piston effect due to the movement of vehicles or trains inside the tunnel. This approach generally does not require mechanical fans, resulting in space-saving at the higher level of the tunnel, and it is suitable for overground [11]. One of the key criteria for the passenger's safety after a fire accident is the number of toxic products of combustion that the individual inhales during the event and the evacuation process. This is calculated using the FED (Fractional Effective Doses) index. The FED index was first introduced by [PERSON] [12] and has since been adopted by various organizations, such as the NFPA [13]. The basic idea is that a person who inhales a certain dose of toxic products for a period of time is considered unconscious or dead. To assess how a person's life is threatened, FED is the main factor considered [1], alongside the thermal exposure and the low visibility due to smoke (hence the difficulty of accessing emergency exits). Evacuation in underground public transport systems can be complicated and the outcome of an evacuation scenario depends on many different parameters. These parameters can be the people's walking speed, the visibility, the design of the train and tunnel, or the physical condition of the people [14]. Tunnels are designed with elevated platforms to assist a safe passenger evacuation in case of a fire accident. The reduced height difference between the level of the train and the platform, compared to the evacuation of a train directly to the rail area, is a method of increasing the safety of the population. The difficulty of evacuating a train to the rails, especially for people with mobility difficulties, has been analyzed in the METRO project of [PERSON] et al. [15]  
The purpose of a tunnel ventilation system, besides for maintaining air quality, is to keep the longest part of a tunnel free of smoke for as long as possible. In the case of a fire accident inside a tunnel, passengers are exposed to smoke, even though the ventilation system exists. In such a situation, the activation of the ventilation system must be delayed slightly or operated at a slower speed so as not to disturb the natural stratification of the air and smoke layers [16; 17].