###### Abstract  
E-mobility is progressively penetrating the European market with the ever-increasing registration of new battery electric vehicles (BEVs). Although BEVs can significantly contribute to achieving the goal of sustainable road transport, they pose new challenges related to the complexity of managing battery fire events, especially in confined spaces such as road tunnels. In this regard, while fires of BEVs with small-sized batteries (i.e., cars and vans) have been widely studied, the consequences of fires involving battery electric buses (BEBs), which are equipped with larger-capacity traction batteries, have not yet been sufficiently investigated. In this context, 3D computational fluid dynamics (CFD) simulations were performed to quantitatively assess the threat that a BEB might pose to the safety of users and rescue teams when it catches fire in a bi-directional road tunnel. In this respect, a comparison was also carried out with the consequences of the fire of a similar internal combustion engine bus (ICEB). Since the environmental conditions inside a tunnel, apart from its geometry, depend on both the traffic flow and type of ventilation, the safety of the users and rescue teams in the event of a BEB fire was evaluated by considering the tunnel under conditions of congested traffic, as well as natural or longitudinal mechanical ventilation. The results showed that the fire of the BEB, compared to that of its ICEB counterpart, worsened the environmental conditions inside the tunnel, especially in terms of toxic gas concentrations. This caused a reduction in the safety level of the users when considering the scenarios of both the naturally and mechanically ventilated tunnel. Moreover, in the case of natural ventilation, the BEB fire was found to cause a higher reduction in the safety level of the rescue teams. b 202  
compared to 2022, with a total of 32,593 units [4]. Obviously, the fleet of electric vehicles (EVs) on the roads is destined to grow further in the coming years to meet the current policy of greenhouse gas reduction. Moreover, since the sales growth rate of BEVs has mostly been quicker than that of PHEVs [2], BEVs are expected to become the prevalent type of EVs in the future [5]. Although BEVs can significantly contribute to reaching the goal of sustainable road transport, they pose new challenges in the context of fire safety engineering. These challenges are mainly associated with batteries and the complexity of addressing a fire event involving a BEV. Currently, battery electric vehicles are mostly powered by Lithium-Ion Batteries (LIBs), whose thermal runaway (i.e., the phenomena whereby an LIB goes into a state of rapid and uncontrollable self-heating) has been found to be the main cause of BEV fires [5,6]. During the burning of an LIB, the combustion of the electrolyte results in the release of Hydrogen Fluoride (HF), which is a colorless but highly poisonous gas that may pose a serious threat to human health [7]. Apart from the HF emissions, which are absent or reduced in the event of a fire involving conventional internal combustion engine vehicles (ICEVs), the potential reignition of the battery fire, the possible explosion of one or more battery cells with the projectile risk, and the need for an excessive quantity of suppression agent to cool the burning battery make the extinguishment of BEV fires more difficult to address than their ICEV counterparts [5]. In this respect, even more severe consequences and challenging firefighting operations are expected in the event of a BEV fire in confined spaces such as road tunnels, in which the combustion products (e.g., hot gases, toxic substances, and smoke) can quickly fill up the entire structure, making the environmental conditions within it extremely dangerous for the safety of the users and rescue teams engaged in extinguishing the EV fire and/or assisting the tunnel occupants.