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Abstract Fire safety is an important aspect of underground road tunnels’ operation. Undertaking high traffic volumes, they are one of the most critical infrastructure for the daily operation of modern urban networks. Aiming to guarantee their safety, risk assessment has been established as a valuable tool. However, the deterministic approach of existing methods has weaknesses in addressing the embedded uncertainty included in important parameters of the system. This paper, applying a stochastic-based approach, proposes a novel quantitative risk assessment method, named SIREN. Initially, the system’s parameters are investigated and the ones which should be treated as stochastic are identified. Subsequently, the method considers both tunnel airflows and trapped-users’ evacuation in order to estimate their potential losses. Finally, the system’s level of safety is provided through the distribution of the trapped-users losses, which occurred by accumulating the results that derived from the Monte Carlo Simulation. The proposed method is illustrated through the case of an urban underground road tunnel during rush hour. The outcome highlights a significant proportion of scenarios that exceed the number of losses estimated by the traditional methods. Meanwhile, the method examines the parameters’ criticality supporting, thus, safety analysts in selecting additional to standard safety measures, if needed. Furthermore, the proposed method aids analysts to act consistently with the as low as reasonable practicable principle. [ABSTRACT FROM AUTHOR]