###### Abstract  
We have developed an appropriate Computational Fluid Dynamics (CFD) model for assessing the exposure to risk of tunnel users during their evacuation process in the event of fire. The effects on escaping users, which can be caused by fire from different types of vehicles located in various longitudinal positions within a one-way tunnel with natural ventilation only and length less than 1 km are shown. Simulated fires, in terms of maximum Heat Release Rate (HRR) are: 8, 30, 50, and 100 MW for two cars, a bus, and two types of Heavy Goods Vehicles (HGVs), respectively. With reference to environmental conditions (i.e., temperatures, radiant heat fluxes, visibility distances, and CO and CO\({}_{2}\) concentrations) along the evacuation path, the results prove that these are always within the limits acceptable for user safety. The exposure to toxic gases and heat also confirms that the tunnel users can safely evacuate. The evacuation time was found to be higher when fire was related to the bus, which is due to a major pre-movement time required for leaving the vehicle. The findings show that mechanical ventilation is not necessary in the case of the tunnel investigated. It is to be emphasized that our modeling might represent a reference in investigating the effects of natural ventilation in tunnels. risk analysis; computational fluid dynamics; road tunnels; natural ventilation; user safety +
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restrictions to the passage of vehicles carrying dangerous goods (DGVs) through road tunnels. As a result, tunnels are to be considered free for the transit of all types of vehicles. Therefore, risk analyses are required for tunnels of the TERN with L \(\geq\) 500 m.  
There are several methods for performing risk analysis; however, an analysis based on Computational Fluid Dynamics (CFD) models appears to be the most suitable. CFD modeling can achieve a high level of accuracy by considering the mutual interaction of all the aforementioned factors, which affect user safety in tunnels in the case of fire. By using CFD models, studies available in the literature have prevalently investigated the efficiency of specific safety measures in tunnels such as, in particular, mechanical ventilation. However, in compliance with the European Directive 2004/54/EC, mechanical ventilation is required only for tunnels with lengths over 1000 m. This means that for tunnels with 500 < L < 1000 m, which are very common in Italy, safety conditions are affected by natural ventilation. Unfortunately, the effects of natural ventilation due to vehicle motion (piston effect) hardly appears to have been studied. Some comparisons between natural and mechanical ventilation can be found in [PERSON] and [PERSON] [11]. Also [PERSON] et al. [12], [PERSON] and [PERSON] [13], and [PERSON] et al. [14] deal with the issue of natural ventilation in road tunnels, but their studies are limited to the evaluation of the maximum temperatures and smoke back-layering length as a function of the tunnel slope, without any consideration about the evacuation process from the tunnel. Safety, under natural ventilation conditions of the tunnel in the event of a fire, is instead the main reason for justifying the present paper. Indeed, it is not clear whether natural ventilation is sufficient in all circumstances. This appears to represent a gap in our knowledge. Therefore, a risk analysis must be conducted. In the light of the above considerations, the main objective of the present paper is to investigate the issue of natural ventilation in road tunnels in greater depth. This study may represent an advance in research for providing additional knowledge in the field of safety engineering, and to show whether further technical measures are still necessary to improve tunnel safety with natural ventilation only. For this aim, among the CFD codes currently available for modeling fire scenarios, the Fire Dynamics Simulator (FDS) [15; 16] version 6.7.3 was used. FDS, with associated Evac code (version 2.5.2.