Simulation-Based Performance Analysis of Ventilation and Fire Protection Strategies in Road Tunnel Fires: Review

Simulation-Based Performance Analysis of Ventilation and Fire Protection Strategies in Road Tunnel Fires: Review | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Systematic Review Simulation-Based Performance Analysis of Ventilation and Fire Protection Strategies in Road Tunnel Fires: Review Nakul Mahalle, Rohan Jaiswal, Aayush Dubey, Ravi Shankar Kumar, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7761467/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Tunnel fires are among the most critical hazards in transportation infrastructure due to rapid smoke spread, high heat release, and constrained evacuation opportunities. In recent years, computational simulation has become the primary tool to evaluate tunnel fire safety, enabling detailed analysis of smoke propagation, ventilation effectiveness, suppression systems, and evacuation performance. This review critically synthesizes studies published between 2015 and 2024, with focus on simulation approaches using CFD-based fire modeling and agent-based evacuation tools. Comparative discussion shows that while longitudinal ventilation is effective for moderate fire sizes, its performance in large-scale fires is inconsistent, particularly when coupled with suppression. Water-mist and sprinkler systems demonstrate significant potential in reducing HRR and gas temperatures, though their interaction with strong airflow remains inadequately resolved. Evacuation models highlight the dominant role of visibility and toxic gases in determining egress time, yet often rely on simplified behavior assumptions. The distinct contribution of this paper is its integrated assessment of ventilation, suppression, and evacuation simulations, clarifying contradictions across studies and identifying gaps such as coupled multi-system modeling and validation under extreme fire conditions. The review concludes with recommendations for more realistic, hybrid simulation frameworks to support tunnel fire safety design. Tunnel Fire Fire Dynamics Ventilation Fire Suppression Evacuation PyroSim Figures Figure 1 Figure 2 Figure 3 Introduction Underground tunnels are a critical part of transport infrastructure, offering faster, safer routes through dense urban areas, mountainous regions, and under waterways. However, they also introduce some of the most complex fire safety challenges. Tunnels unlike buildings, possess an enclosed geometry having limited ventilation, limited accessibility for firefighters, and constrained evacuation routes- all of which makes the fire much difficult to contain and manage; consequences being tunnel fires are much severe, not only due to the flame propagation but primarily because of intense smoke propagation, loss of visibility and toxic gas exposure in a short timeframe. Notable incidents such as the Mont Blanc Tunnel fire (1999) and Gotthard Tunnel fire (2001) highlighted that most fatalities in such events are due to smoke inhalation and CO poisoning, not direct flame contact.[ 1 ] These tragedies shifted international attention toward fire safety in tunnels, prompting new technical investigations, full-scale fire tests, and regulatory changes. The European Parliament, through studies like that by Alan N. Beard, has called for a stronger, standardized, and risk-based approach to tunnel safety. Table 1 Major Tunnel Fire Incidents and Lessons Learned Incident Year Country Causes Casualties Key lesson learned Mont Blanc Tunnel 1999 France - Italy Truck carrying flour & margarine caught fire 39 deaths Importance of smoke control & emergency exits Gotthard Tunnel 2001 Switzerland Collision of heavy goods vehicles 11 deaths Need for longitudinal ventilation & quick response Burnley Tunnel 2007 Australia Multi-vehicle accident, HGV fire 3 deaths Fire load underestimated; HRR exceeded 100 MW RunehamarTests 2003–2016 Norway Full-scale HGV fire experiments Experimental Validated deluge & mist systems, airflow interaction Their findings focuses on the urgency of integrating data-driven decision-making, emergency response planning, and ongoing system monitoring, especially for heavy goods vehicle (HGV)-related incidents.[ 2 ] The report emphasizes that traditional prescriptive safety codes, while still useful, are often insufficient for modern, long, or complex tunnels. As tunnels grow in size and traffic volume, there is a growing shift toward performance-based fire safety design, supported by simulation tools, intelligent detection systems, and real-time adaptive ventilation. This shift is resonated in regulatory frameworks like NFPA 502 and PIARC, which now cautiously support the inclusion of active fire suppression technologies and evacuation modelling in tunnel fire strategies.[ 3 ] Tunnel fire safety today requires a unified approach—balancing prescriptive norms with performance based measures like advanced modelling, and ensuring that fire detection, smoke control, suppression, and evacuation planning are not treated in isolation but as components of a coordinated system. Evolution of Tunnel Fire Research 2.1 Early Understanding: Passive Design and Fire load misjudgment Tunnel fire research initially relied on passive protection strategies, with conservative assumptions about fire size. Design fire loads were generally assumed to peak around 20 MW, intensity which was considered by longitudinal ventilation and structural resistance. However, the Burnley Tunnel fire of 2007, alongside research from the University of Edinburgh, demonstrated that heat release rates (HRRs) could exceed 120 MW [ 4 ], especially in fires involving heavy goods vehicles (HGVs). This work fundamentally challenged prior design assumptions, introducing the critical concept of supercritical fires and highlighting throttling effects, where large fires resist longitudinal airflow and require increasing fan capacity to maintain tenability despite stable critical ventilation velocities. 2.2 Shift Toward Active Fire Safety Measures Upon recognizing the limitations related to passive systems, researchers turned to active fire protection method such as water deluge and water mist systems. The Runehamar fire tests (2016) were a landmark in this shift. These tests compared TN-25 and TN-17 nozzles, revealing that larger droplets from TN-25 offered superior flame suppression and fire spread control. However, suppression performance was strongly influenced by several factors like airflow direction, nozzle distance, and activation timing—indicating that such systems must be carefully integrated into overall tunnel design.[ 5 ] 2.3 Regulatory Recognition and Risk-Based Approaches Following catastrophic fires like Mont Blanc and Gotthard, regulatory agencies began revamping tunnel fire safety standards. A pivotal study by the European Parliament, led by Alan N. Beard,[ 2 ] emphasized upon risk-based approaches, proposing that prescriptive rules be supplemented by quantitative models and performance-based designs. The report urged for the integration of active systems, improved ventilation coordination, and real-time system validation. In response, authorities such as NFPA and PIARC revised their frameworks to cautiously support active suppression, stressing the importance of large-scale testing and multi-system synergy.[ 1 ] Table 2 Evolution of Tunnel Fire Safety Approaches Era Focus Typical fire size assumed Strategy Limitation Early Stage (pre-2000) Passive protection (structural resistance, fire load limits) 20 MW Longitudinal ventilation + fire-resistant lining Underestimated real HRR of HGV fires (> 100 MW) Transition (2000–2010) Active protection (deluge, mist, detection systems) 50–100 MW Large-scale fire tests (Runehamar) Suppression–ventilation interactions not fully understood Current (2010–present) Performance-based, risk-informed design 100–200 MW Simulation tools (FDS, PyroSim, Pathfinder) Limited integration of ventilation, suppression & evacuation Smoke Movement and Ventilation Control 3.1 Behaviour of Smoke in Confined Tunnel Geometry Smoke propagation in tunnels differs significantly from open environments due to confined geometry, ventilation influence, and buoyancy effects. In tunnel fires, hot gases form a dense smoke layer near the ceiling, rapidly deteriorating visibility and breathable air. It was observed during a study that a lower longitudinal ventilation velocity (e.g., 0.5 m/s) led to higher peak temperatures directly above the fire, while higher velocities (e.g., 1.9 m/s) reduced local maxima by dispersing heat more widely. These findings justifies that ventilation is not merely a tool for smoke removal but also a determinant of thermal stratification and smoke layering, which are critical for structural resilience and safe evacuation.[ 6 ] 3.2 Ventilation Timing and Direction: A Critical Decision Ventilation activation timing and airflow direction are pivotal in determining the outcome of a tunnel fire scenario. Premature activation of fans can push smoke into occupied regions, the exposure hazard, while delayed activation can lead to thermal buildup and smoke backlayering. Various studies shows that an airflow velocity between 1–1.5 m/s to be maintained during evacuation. Both insufficient and excessive velocities were shown to contribute to either flame propagation or ineffective smoke displacement, respectively.[ 7 ] 3.3 Longitudinal Ventilation Systems and Smoke Flow Patterns Longitudinal ventilation systems are widely adopted due to their simplicity and cost-efficiency, yet they present considerable complexity under fire conditions. In the study, it was revealed that such systems can intensify fire growth, particularly for heavy goods vehicle (HGV) fires. At a ventilation speed of 3 m/s, the fire intensity was observed to increase by 4–5 times; at 10 m/s, the fire could become 10 times more severe due to the enhanced oxygen supply. While low ventilation speeds may limit fire escalation and aid in localized smoke control, higher velocities pose threats to downstream evacuees by accelerating smoke spread.[ 7 ][ 8 ] Table 3 Effect of Ventilation Velocity on Tunnel Fire Behaviour Ventilation Velocity (m/s Observed Effect on Fire/Smoke Risk to Occupants 0.5 m/s (low) High ceiling temperature, localized smoke Poor tenability near fire zone 1–1.5 m/s (moderate) Optimal for evacuation; prevents back layering Recommended range for safe egress 3 m/s (high) Fire intensity increased 4–5x due to oxygen supply Smoke spread downstream faster 10 m/s (very high) Fire intensity 10x higher; suppression less effective Extremely hazardous 3.4 Understanding and Predicting Smoke Backlayering Smoke backlayering—where hot gases move against the airflow—remain a persistent hazard in tunnel fires. The limitations of the cube-root relationship in predicting critical ventilation velocity (CVV), especially for fires exceeding 100 MW or occurring in non-standard tunnel geometries. In order to produce more precise CVV estimates, the researchers suggest updated modelling techniques that take into account obstruction Fig. 1: schematic diagram over a tunnel fire introducing several important terms. effects, tunnel cross-section, and flame geometry. These improvements aid in designing ventilation systems that are responsive to realistic fire scenarios.[ 9 ] 3.5 The Throttling Effect: Resistance to Airflow in Growing Fires An emergent concept in tunnel fire ventilation is the throttling effect, where fires resist incoming ventilation flows as their intensity increases. It was experienced in the experiment of throttling effect in Tunnel Fires, which used CFD simulations via the Fire Dynamics Simulator (FDS) to show that jet fans required for smoke control rise steeply with fire intensity. For fires ≤ 30 MW, 3–4 jet fans sufficed; however, for fires of 60–90 MW, 6–7 fans were needed, and even then, smoke control was not always successful due to disrupted flow patterns. The study highlights the inadequacy of relying solely on CVV and recommends scaling ventilation design based on both tunnel geometry and potential fire growth rates.[ 10 ] Figure 2: Interior view of a road tunnel showcasing prominent jet fans installed near the entrance for longitudinal ventilation. Fire Suppression Strategies This section examines tunnel-specific fire suppression technologies, their configurations, effectiveness, and known limitations. Drawing from full-scale experiments, CFD simulations, and performance reviews detailed in the selected literature, each sub-section offers technical insights aligned with design, application, and integration of suppression systems in tunnel environments. 4.1 Fixed Fire Suppression Systems in Tunnels: Role and Relevance Tunnel environments heavily depend on Fixed Fire Fighting Systems (FFFS) to delay fire growth and allow safe evacuation before emergency services intervene. Unlike in open structures, accessibility is limited, making deluge or mist systems critical. Fixed Fire Protection Systems reduce heat release rates (HRR), prevent fire propagation, and enhance tenability by lowering temperatures and smoke density. However, their effectiveness depends on precise activation logic and integration with ventilation to avoid counterproductive outcomes like stratification breakdown or mist displacement.[ 3 ] 4.2 Deluge Systems: Nozzle Design and Droplet Behaviour Deluge systems are often preferred in tunnels due to their straightforward mechanics and reliability. Performance hinges on droplet size, pressure, spray geometry, and nozzle type. The Runehamar tunnel fire tests [ 3 ] compared multiple nozzles, including TN-17 and TN-25. TN-25 produced larger droplets, achieving better suppression by effectively cooling fuel surfaces and shielding downstream targets. In contrast, TN-17—producing finer droplets—was less effective under strong longitudinal airflow, with fire re-ignition observed after 45 minutes. Thus, nozzle geometry and droplet mass significantly affect system performance in ventilated tunnels.[ 11 ] 4.3 Water Mist Systems: Advantages and Limitations Water mist systems, valued for their lower water demand and infrastructure footprint, generate fine droplets that rapidly evaporate, cooling flames and reducing oxygen. Despite this, their performance deteriorates under strong airflow conditions. Studies, including one from the University of Edinburgh,[ 4 ] indicate that fine mist droplets can be displaced hundreds of meters away by longitudinal ventilation before reaching the combustion zone, making mist systems less compatible with high-velocity airflow tunnels. Mist systems perform best in tunnels with low ventilation velocities or confined geometries, where spray residence time remains high.[ 15 ] 4.4 Suppression Timing: Early vs. Delayed Activation The timing of system activation greatly affects fire development and evacuation safety. The simulations using FDS and Pathfinder showed that early suppression helped delay flashover and maintain tenability. However, if triggered too soon—before proper ventilation is established—it could worsen conditions by condensing smoke and reducing visibility near exits.[ 12 ] This indicates suppression systems should not rely solely on thermal triggers but should integrate dynamic smoke detection for intelligent activation. 4.5 Suppression and Ventilation Interaction: A Design Dilemma One of the most critical challenges in tunnel fire safety is the interaction between ventilation and suppression. As observed in the Runehamar fire experiments [ 3 ] suppression without ventilation adjustment led to reduced system effectiveness due to droplet displacement and altered smoke buoyancy. Additionally, one study introduced the “throttling effect”, where intense fires resist airflow, altering how suppression and ventilation must be coordinated. Mist systems, in particular, become ineffective unless ventilation velocity is specifically controlled during discharge. Designers must coordinate fan speed, air direction, and suppression logic simultaneously.[ 4 ] 4.6 Limitations and Research Gaps While suppression systems reduce HRR and improve evacuation chances, they are not standalone solutions. Their effectiveness hinges on: Appropriate nozzle selection and spacing Integration with longitudinal ventilation systems Smart activation algorithms based on real-time smoke and heat feedback Multiple papers have recommended using hybrid simulations (PyroSim + CFD tools) to predict system behaviour across various tunnel geometries and fire scales. Future work must focus on optimizing suppression-ventilation coupling using probabilistic simulations, refining detection logic, and validating results with full-scale fire tests. Human Behaviour and Evacuation in Tunnel Fires Evacuation during tunnel fires is a high-risk process shaped by environmental and psychological factors. Key determinants such as visibility, heat, smoke spread, signage, and human decision-making under stress dictate evacuation speed and survival rates. The following sub-sections summarize findings from full-scale simulations and behavioral analyses presented in the literature. 5.1 Impact of Visibility and Smoke on Evacuation Time A strong inverse relationship exists between smoke density (visibility) and evacuation speed. It was seen that when visibility drops below 10–15 meters due to smoke, evacuees exhibit delayed or halted movement. Additionally, hesitation increases when exit signage is obscured, or lighting fails.[ 13 ] The same study quantified that evacuation time could increase by up to 60% in tunnels with poor ventilation or without illuminated exit signs—especially when CO concentrations also rise rapidly. 5.2 Exit Spacing and Evacuation Path Layout Figure 3: Conceptual diagram illustrating the integrated components of a comprehensive tunnel fire safety system for coordinated incident response. One study evaluated side exits spaced at 300 m vs. 600 m intervals under different fire growth rates. It concluded that shorter exit intervals enhance survivability, particularly in cases of high HRR fires. Users typically head toward the first visible exit, emphasizing the need for visibility-optimized positioning of egress routes.[ 14 ] Moreover, tunnel slope, pedestrian lighting systems (like flashing floor markers), and exit familiarity were noted to influence evacuee direction and speed. 5.3 Human Behaviour Models and Limitations in Simulation Although tools like Pathfinder and Building EXODUS provide quantitative simulation, they lack in replicating real-world behavioral unpredictability. It was noted that evacuees often exhibit non-optimal behaviours—e.g., returning to entry points, waiting for group members, collecting personal items, or ignoring alarm cues .[ 14 ] These human tendencies introduce deviations between modelled and real evacuation patterns. The study suggested future models should include stochastic behaviour parameters, cultural differences, and delayed responses to better reflect real-world scenarios. 5.4 Integrated Safety Systems and Evacuation Aids Evacuation success improves significantly when multi-layered safety systems are applied. Studies have suggested that the integrated use of audio-visual alarms, dynamic signage, and smoke extraction led to a 35–50% reduction in total evacuation time.[ 8 ] By redirecting evacuees away from smoke-affected paths in real-time, the study confirmed that coupling suppression, detection, and intelligent evacuation guidance provides substantial safety gains—making a strong case for holistic system design. Simulation Tools – PyroSim & Pathfinder Simulation has become a cornerstone of modern tunnel fire safety engineering. Real-scale tunnel fire testing is costly, risky, and often impractical. Tools like PyroSim (for fire and smoke modelling) and Pathfinder (for evacuation analysis) allow engineers to recreate tunnel fire conditions with accuracy and flexibility. These tools support proactive safety design, system optimisation, and incident analysis, especially in complex tunnel geometries with constrained ventilation and evacuation options. 6.1 Importance of Simulation in Tunnel Fire Safety Tunnel fire safety involves a complex interplay of geometry, ventilation, fire growth, and human response. Simulation models help engineers explore these variables without relying on full-scale testing. The simulation in a study revealed that the location and activation timing of jet fans significantly affect smoke layering and temperature distribution. Delayed fan activation often led to thermal stratification collapse, which reduces visibility and accelerates untenable conditions.[ 7 ] 6.2 PyroSim for Fire and Smoke Modelling PyroSim, which interfaces with the Fire Dynamics Simulator (FDS), is a CFD-based tool for modelling fire growth, HRR evolution, and smoke transport. It allows input of various tunnel parameters like geometry, fuel load, jet fan layout, and ventilation velocity. In a study, PyroSim was used to simulate a 200 m tunnel with different fan activation timings. It showed that early ventilation helped maintain visibility near exit routes for 3–4 minutes longer and delayed CO rise—thereby extending available safe egress time (ASET). It was also demonstrated that PyroSim can replicate heat distribution across the ceiling and predict critical back-layering conditions, validating its use in performance-based tunnel design.[ 9 ] 6.3 Pathfinder for Evacuation Simulation Pathfinder simulates human movement during fire events using agent-based logic that factors in speed, visibility, CO levels, decision-making behaviour, and group dynamics. When combined with PyroSim data, it allows engineers to evaluate how evacuees behave in real-time tunnel emergencies. From one of the Pathfinder simulations from a study demonstrated that increasing exit frequency and widening doors cut total evacuation time by up to 45%. Bottlenecks typically formed near poorly marked exits or under low-visibility zones, guiding improvements in layout and signage. Pathfinder also showed that introducing dynamic signage and voice alarms redirected evacuees more efficiently away from blocked paths—reducing total evacuation time by over 30% in some scenarios. 6.4 Validation and Reliability of Simulation-Based Studies The reliability of PyroSim and Pathfinder simulations has been confirmed through comparisons with large-scale tunnel fire experiments such as those at Runehamar and Memorial tunnels[ 5 ]. These tools comply with tunnel fire codes including NFPA 502 and PIARC, and their predictions align with known tenability thresholds and back-layering behaviour. It was provided evidence from a study that FDS-based simulations closely matched real incident outcomes in terms of flame spread, CO concentration trends, and temperature profiles. Such validation underscores the reliability of using simulation tools for risk analysis, emergency planning, and tunnel safety system design. Integration of Tunnel Fire Safety Systems: A Path Toward Comprehensive Protection Modern tunnel systems—especially those serving urban interchanges, metro-rail hybrids, or multi-level vehicular corridors—are no longer linear infrastructures. They represent complex, confined, and high-risk environments with dynamic traffic profiles, flammable cargo exposure, and limited evacuation flexibility. Managing fire safety in such scenarios demands an integrated and adaptive approach—where ventilation, suppression, detection, simulation, and evacuation systems work together, not in isolation. Although substantial advances have been made in individual safety domains—such as suppression nozzle optimization, jet fan control algorithms, and behaviour-based evacuation models—current literature reveals a critical lack of integration across these domains. The study of fire simulations accurately captured the effects of smoke layering, exit signage, and pre-movement delays using PyroSim and Pathfinder. However, these simulations typically analyse systems independently, without modelling their real-time interaction or feedback-based coordination. One study highlights that although individual elements like early suppression or longitudinal ventilation performance are well-validated, there is minimal research that synchronizes them into a systemic framework. As observed, even technically efficient systems—such as water mist suppression assisted by jet fans—underperform if airflow direction, occupancy patterns, or detection timing are not appropriately coordinated.[ 8 ][ 7 ] In high-HRR fire events (e.g., > 100 MW) the effectiveness of any single-point solution—like deluge systems or fixed jet fans—depends on timing and interaction. Delayed fan activation, as simulated, resulted in smoke recirculation and reduced evacuation tenability despite suppression working correctly.[ 14 ] These findings underscore the limitations of prescriptive, siloed designs. As tunnel infrastructure grows in scale and functional complexity, a shift toward performance-based design methodologies is essential. These must be supported by multi-system simulations, capable of testing fire detection, smoke movement, suppression reaction, and human behaviour simultaneously under varying conditions. Despite strong evidence advocating integrated design, few studies have gone beyond component-level validation into true system integration modelling. This opens clear opportunities for further research in: Real-time coordination between detection and ventilation logic Feedback-based suppression control linked to HRR and smoke sensors Dynamic evacuation routing tied to environmental sensing Failure-mode testing under hybrid or cascading risk scenarios By leveraging advanced simulation tools not just as diagnostic platforms—but as coordinated design engines—engineers can finally approach tunnel fire safety as a holistic, responsive system rather than a sum of isolated parts. Conclusion Tunnel fire safety has evolved from traditional prescriptive measures to complex, performance-based systems. The literature consistently shows that modern tunnel fires—especially those involving high fire loads and long evacuation paths—demand a comprehensive understanding of smoke dynamics, ventilation strategies, fire suppression technologies, and human evacuation behaviour. Early assumptions of manageable fire sizes have been challenged by real-world incidents and large-scale testing, emphasizing the need for adaptable systems capable of handling fires exceeding 100 MW. Studies highlight how ventilation timing, nozzle design, and exit placement can critically affect survivability, and how even oversights in system coordination minor can lead to cascading failures. Simulation tools like PyroSim and Pathfinder have proven indispensable in bridging theory and real-world behaviour. These tools enable modelling of fire development, toxic gas spread, and crowd dynamics with high accuracy, validating their use in both design and forensic analysis. However, while significant advancements have been made in individual areas, the literature also reveals a notable gap in system-wide integration. While prior reviews have provided useful summaries of tunnel fire safety studies, this paper makes three distinct contributions: Integrated Assessment – Unlike earlier works that treat ventilation, suppression, and evacuation separately, this review examines their combined influence on tunnel fire safety outcomes. Recent Simulation Focus – It consolidates findings from 2015–2024, reflecting the latest simulation-based studies that incorporate advanced CFD modeling, agent-based evacuation tools, and hybrid approaches. Critical Comparative Analysis – Instead of only summarizing, this paper directly compares conflicting results across studies, pointing out methodological differences (e.g., HRR values, ventilation modes, and sprinkler activation times) that lead to contrasting conclusions. To contribute to this evolving field, our further study will focus on analyzing smoke spread behavior under various tunnel fire scenarios and conditions, performing simulation-based evaluations of those scenarios during tunnel fires under forced ventilation, and examining the effectiveness of fire protection system which is sprinkler system in enhancing tenability during road tunnel fire scenarios. This integrated approach aims to support more resilient and responsive tunnel fire safety designs. Declarations Funding: The author(s) received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors for the completion of this research work. Clinical Trial Number: Not applicable Ethics approval: Not Applicable Consent to participate: Not Applicable Consent for publication: Not Applicable References Jonatan Gehandler. Road tunnel fire safety and risk: a review. Fire Science Reviews, Vol. 4, Art. No. 2, 2015. A. N. Beard. Fire safety in tunnels. Fire Safety Journal, Vol. 44, pp. 276–278, 2009. Jack R. Mawhinney. Fixed Fire Protection Systems in Tunnels: Issues and Directions. Fire Technology, Vol. 49, pp. 477–508, 2013. Ricky Carvel. A review of Tunnel Fire Research from Edinburgh. Fire Safety Journal, Vol. 105, pp. 300-306, 2019. Haukur Ingason, Ying Zhen Li. Large scale tunnel fire tests with different types of large droplet fixed firefighting systems. Fire Safety Journal, Vol. 107, pp. 29–43, 2019. L.H. Hu, R. Huoa, W. Peng, W.K. Chow, R.X. Yang. On the maximum smoke temperature under the ceiling in tunnel fires. Tunnelling and Underground Space Technology, Vol.21, pp. 650–655, 2006. P. Sturm, M. Beyer, M. Rafiei. On the problem of ventilation control in case of a tunnel fire event. Case Studies in Fire Safety, Vol. 7, pp. 36–43, 2017. R.O. Carvel, A.N. Beard, P.W. Jowitt. The influence of longitudinal ventilation systems on fires in tunnels. Tunnelling and Underground Space Technology, Vol. 16, pp. 3-21, 2001. Yasushi Oka, Graham T. Atkinson. Control of Smoke Flow in Tunnel Fires. Fire Safety Journal, Vol. 25, pp. 305-322, 1995. Arnas Vaitkevicius, Ricky Carvel. Investigating the Throttling Effect in Tunnel Fires. Fire Technology, Vol. 52, pp. 1619-1628, 2016. D. Van den Broecke, R. Emberley, T. Sietz, J. L. Torero, C. Maluk. Study on the effectiveness of fire suppression deluge systems in tunnels. Tunnelling and Underground Space Technology, Vol. 108, Art. No. 103764, 2021. Z. Liu, X. Gu, and R. Hong. Fire Protection and Evacuation Analysis in Underground Interchange Tunnels by Integrating BIM and Numerical Simulation. MDPI Fire, Vol. 6, No. 4, pp. 139, 2023. M. Fera, R. Macchiaroli. Use of analytic hierarchy process and fire dynamics simulator to assess the fire protection systems in a tunnel on fire. Int. J. Risk Assessment and Management, Vol. 14, No. 6, pp. 504, 2010. Y. Cui and Z. Liu. Study on Fire Smoke Movement Characteristics and Their Impact on Personal Evacuation in Curved Highway Tunnels. Applied Sciences, Vol. 14, No. 12, Art. No. 6339, 2024. A. Sarvari and S. M. Mazinani. A new tunnel fire detection and suppression system based on camera image processing and water mist jet fans. Heliyon, vol. 5, Issue 6, e0187, 2019. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7761467","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Systematic Review","associatedPublications":[],"authors":[{"id":542168775,"identity":"13f30e0a-973a-4985-ad78-cec399fe1e29","order_by":0,"name":"Nakul Mahalle","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Nakul","middleName":"","lastName":"Mahalle","suffix":""},{"id":542168776,"identity":"4631b53d-6c7c-4323-8b55-09ca3a4a9b50","order_by":1,"name":"Rohan Jaiswal","email":"data:image/png;base64,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","orcid":"","institution":"National Fire Service College","correspondingAuthor":true,"prefix":"","firstName":"Rohan","middleName":"","lastName":"Jaiswal","suffix":""},{"id":542168777,"identity":"0b62f243-2df4-4b8c-9c28-c19966ab171d","order_by":2,"name":"Aayush Dubey","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Aayush","middleName":"","lastName":"Dubey","suffix":""},{"id":542168778,"identity":"81712680-1587-488b-8a00-afed9b5e1378","order_by":3,"name":"Ravi Shankar Kumar","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Ravi","middleName":"Shankar","lastName":"Kumar","suffix":""},{"id":542168779,"identity":"54e155fb-0681-4d13-bc56-9c7614d2f830","order_by":4,"name":"Anuj Goswami","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Anuj","middleName":"","lastName":"Goswami","suffix":""},{"id":542168780,"identity":"e820087d-2a61-47b5-8b47-cdc8e2ac2c15","order_by":5,"name":"Nirmalya Shukla","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Nirmalya","middleName":"","lastName":"Shukla","suffix":""},{"id":542168784,"identity":"4f4a1d9a-307e-472f-ac97-36d2f484c938","order_by":6,"name":"Pyush Chaudhary","email":"","orcid":"","institution":"National Fire Service College","correspondingAuthor":false,"prefix":"","firstName":"Pyush","middleName":"","lastName":"Chaudhary","suffix":""}],"badges":[],"createdAt":"2025-10-01 17:08:25","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7761467/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7761467/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":95582873,"identity":"1dd207bf-9250-484a-b7ea-2ccc99cc21ec","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1755291,"visible":true,"origin":"","legend":"","description":"","filename":"LiteratureReview.docx","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/5e185674ad438c5f3dc75eef.docx"},{"id":95582867,"identity":"43cad37d-396f-4e1e-9d52-19ed09e38428","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8277,"visible":true,"origin":"","legend":"","description":"","filename":"9ff8c16e8b4c418fb70e3169dd16dc73.json","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/12578502905480cc1ba91606.json"},{"id":95654855,"identity":"6dd50ea4-52db-48b3-8228-b95899401234","added_by":"auto","created_at":"2025-11-11 16:13:23","extension":"xml","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":61398,"visible":true,"origin":"","legend":"","description":"","filename":"9ff8c16e8b4c418fb70e3169dd16dc731enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/5ff1482616a2f870ce62cc8c.xml"},{"id":95582870,"identity":"2b8a507e-c0e0-44de-87ac-1f66837a7582","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"jpeg","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1074,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/766a3c4466a67a0fa1ac2e08.jpeg"},{"id":95582868,"identity":"2aa48303-8d45-4d47-b181-3bf608ba62dc","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":935,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/53c6bac8c2a5696c0de5af7d.png"},{"id":95582874,"identity":"707962d9-fd8a-40d8-bc92-afe99e231c13","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"xml","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":59802,"visible":true,"origin":"","legend":"","description":"","filename":"9ff8c16e8b4c418fb70e3169dd16dc731structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/df44790ffaa110886d1dbf13.xml"},{"id":95582872,"identity":"13550a26-80f9-4856-8712-36c828640d5a","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"html","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":67776,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/de52a06637a0548eaf12878f.html"},{"id":95582865,"identity":"77143244-eec3-43c6-bf93-e83902ffddee","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":232994,"visible":true,"origin":"","legend":"\u003cp\u003eschematic diagram over a tunnel fire introducing several important\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/a625f84697b720c1dee1df1c.png"},{"id":95582866,"identity":"286c85b3-8742-46c9-a7f9-c19c8dd7d715","added_by":"auto","created_at":"2025-11-10 21:09:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1415741,"visible":true,"origin":"","legend":"\u003cp\u003eInterior view of a road tunnel showcasing prominent jet fans installed near the entrance for longitudinal ventilation.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/4e8d6aecc66cc6bbe400b0fb.png"},{"id":95656051,"identity":"0650297d-2378-40e7-92f7-81fec5d915f7","added_by":"auto","created_at":"2025-11-11 16:17:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":603047,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual diagram illustrating the integrated components of a comprehensive tunnel fire safety system for coordinated incident response.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/fd70c8600117a5b604427bb8.png"},{"id":96363721,"identity":"de728cbb-1f95-4ad0-859b-d6c74e0bee23","added_by":"auto","created_at":"2025-11-20 10:07:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4341357,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7761467/v1/8486610c-7faf-46c8-b776-cdfcaa8916d9.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Simulation-Based Performance Analysis of Ventilation and Fire Protection Strategies in Road Tunnel Fires: Review","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUnderground tunnels are a critical part of transport infrastructure, offering faster, safer routes through dense urban areas, mountainous regions, and under waterways. However, they also introduce some of the most complex fire safety challenges. Tunnels unlike buildings, possess an enclosed geometry having limited ventilation, limited accessibility for firefighters, and constrained evacuation routes- all of which makes the fire much difficult to contain and manage; consequences being tunnel fires are much severe, not only due to the flame propagation but primarily because of intense smoke propagation, loss of visibility and toxic gas exposure in a short timeframe.\u003c/p\u003e\u003cp\u003eNotable incidents such as the Mont Blanc Tunnel fire (1999) and Gotthard Tunnel fire (2001) highlighted that most fatalities in such events are due to smoke inhalation and CO poisoning, not direct flame contact.[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] These tragedies shifted international attention toward fire safety in tunnels, prompting new technical investigations, full-scale fire tests, and regulatory changes. The European Parliament, through studies like that by Alan N. Beard, has called for a stronger, standardized, and risk-based approach to tunnel safety.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eMajor Tunnel Fire Incidents and Lessons Learned\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIncident\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eYear\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCountry\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCauses\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eCasualties\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eKey lesson learned\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMont Blanc Tunnel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1999\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFrance - Italy\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTruck carrying flour \u0026amp; margarine caught fire\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e39 deaths\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eImportance of smoke control \u0026amp; emergency exits\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eGotthard Tunnel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSwitzerland\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eCollision of heavy goods vehicles\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e11 deaths\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eNeed for longitudinal ventilation \u0026amp; quick response\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBurnley Tunnel\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2007\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAustralia\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMulti-vehicle accident, HGV fire\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3 deaths\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eFire load underestimated; HRR exceeded 100 MW\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRunehamarTests\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2003\u0026ndash;2016\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNorway\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eFull-scale HGV fire experiments\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eExperimental\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eValidated deluge \u0026amp; mist systems, airflow interaction\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTheir findings focuses on the urgency of integrating data-driven decision-making, emergency response planning, and ongoing system monitoring, especially for heavy goods vehicle (HGV)-related incidents.[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThe report emphasizes that traditional prescriptive safety codes, while still useful, are often insufficient for modern, long, or complex tunnels. As tunnels grow in size and traffic volume, there is a growing shift toward performance-based fire safety design, supported by simulation tools, intelligent detection systems, and real-time adaptive ventilation. This shift is resonated in regulatory frameworks like NFPA 502 and PIARC, which now cautiously support the inclusion of active fire suppression technologies and evacuation modelling in tunnel fire strategies.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eTunnel fire safety today requires a unified approach\u0026mdash;balancing prescriptive norms with performance based measures like advanced modelling, and ensuring that fire detection, smoke control, suppression, and evacuation planning are not treated in isolation but as components of a coordinated system.\u003c/p\u003e"},{"header":"Evolution of Tunnel Fire Research","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Early Understanding: Passive Design and Fire load misjudgment\u003c/h2\u003e\u003cp\u003eTunnel fire research initially relied on passive protection strategies, with conservative assumptions about fire size. Design fire loads were generally assumed to peak around 20 MW, intensity which was considered by longitudinal ventilation and structural resistance. However, the Burnley Tunnel fire of 2007, alongside research from the University of Edinburgh, demonstrated that heat release rates (HRRs) could exceed 120 MW [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], especially in fires involving heavy goods vehicles (HGVs). This work fundamentally challenged prior design assumptions, introducing the critical concept of supercritical fires and highlighting throttling effects, where large fires resist longitudinal airflow and require increasing fan capacity to maintain tenability despite stable critical ventilation velocities.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Shift Toward Active Fire Safety Measures\u003c/h2\u003e\u003cp\u003eUpon recognizing the limitations related to passive systems, researchers turned to active fire protection method such as water deluge and water mist systems. The Runehamar fire tests (2016) were a landmark in this shift. These tests compared TN-25 and TN-17 nozzles, revealing that larger droplets from TN-25 offered superior flame suppression and fire spread control. However, suppression performance was strongly influenced by several factors like airflow direction, nozzle distance, and activation timing\u0026mdash;indicating that such systems must be carefully integrated into overall tunnel design.[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Regulatory Recognition and Risk-Based Approaches\u003c/h2\u003e\u003cp\u003eFollowing catastrophic fires like Mont Blanc and Gotthard, regulatory agencies began revamping tunnel fire safety standards. A pivotal study by the European Parliament, led by Alan N. Beard,[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] emphasized upon risk-based approaches, proposing that prescriptive rules be supplemented by quantitative models and performance-based designs. The report urged for the integration of active systems, improved ventilation coordination, and real-time system validation. In response, authorities such as NFPA and PIARC revised their frameworks to cautiously support active suppression, stressing the importance of large-scale testing and multi-system synergy.[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEvolution of Tunnel Fire Safety Approaches\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEra\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFocus\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTypical fire size assumed\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eStrategy\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eLimitation\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEarly Stage (pre-2000)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePassive protection (structural resistance, fire load limits)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e20 MW\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eLongitudinal ventilation\u0026thinsp;+\u0026thinsp;fire-resistant lining\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUnderestimated real HRR of HGV fires (\u0026gt;\u0026thinsp;100 MW)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTransition (2000\u0026ndash;2010)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eActive protection (deluge, mist, detection systems)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e50\u0026ndash;100 MW\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eLarge-scale fire tests (Runehamar)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eSuppression\u0026ndash;ventilation interactions not fully understood\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCurrent (2010\u0026ndash;present)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePerformance-based, risk-informed design\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100\u0026ndash;200 MW\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSimulation tools (FDS, PyroSim, Pathfinder)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eLimited integration of ventilation, suppression \u0026amp; evacuation\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Smoke Movement and Ventilation Control","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Behaviour of Smoke in Confined Tunnel Geometry\u003c/h2\u003e\u003cp\u003eSmoke propagation in tunnels differs significantly from open environments due to confined geometry, ventilation influence, and buoyancy effects. In tunnel fires, hot gases form a dense smoke layer near the ceiling, rapidly deteriorating visibility and breathable air. It was observed during a study that a lower longitudinal ventilation velocity (e.g., 0.5 m/s) led to higher peak temperatures directly above the fire, while higher velocities (e.g., 1.9 m/s) reduced local maxima by dispersing heat more widely. These findings justifies that ventilation is not merely a tool for smoke removal but also a determinant of thermal stratification and smoke layering, which are critical for structural resilience and safe evacuation.[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Ventilation Timing and Direction: A Critical Decision\u003c/h2\u003e\u003cp\u003eVentilation activation timing and airflow direction are pivotal in determining the outcome of a tunnel fire scenario. Premature activation of fans can push smoke into occupied regions, the exposure hazard, while delayed activation can lead to thermal buildup and smoke backlayering. Various studies shows that an airflow velocity between 1\u0026ndash;1.5 m/s to be maintained during evacuation. Both insufficient and excessive velocities were shown to contribute to either flame propagation or ineffective smoke displacement, respectively.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Longitudinal Ventilation Systems and Smoke Flow Patterns\u003c/h2\u003e\u003cp\u003eLongitudinal ventilation systems are widely adopted due to their simplicity and cost-efficiency, yet they present considerable complexity under fire conditions. In the study, it was revealed that such systems can intensify fire growth, particularly for heavy goods vehicle (HGV) fires. At a ventilation speed of 3 m/s, the fire intensity was observed to increase by 4\u0026ndash;5 times; at 10 m/s, the fire could become 10 times more severe due to the enhanced oxygen supply. While low ventilation speeds may limit fire escalation and aid in localized smoke control, higher velocities pose threats to downstream evacuees by accelerating smoke spread.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e][\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEffect of Ventilation Velocity on Tunnel Fire Behaviour\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eVentilation Velocity (m/s\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eObserved Effect on Fire/Smoke\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRisk to Occupants\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e0.5 m/s (low)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHigh ceiling temperature, localized smoke\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePoor tenability near fire zone\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u0026ndash;1.5 m/s (moderate)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOptimal for evacuation; prevents back layering\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRecommended range for safe egress\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3 m/s (high)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFire intensity increased 4\u0026ndash;5x due to oxygen supply\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSmoke spread downstream faster\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e10 m/s (very high)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFire intensity 10x higher; suppression less effective\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eExtremely hazardous\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Understanding and Predicting Smoke Backlayering\u003c/h2\u003e\u003cp\u003eSmoke backlayering\u0026mdash;where hot gases move against the airflow\u0026mdash;remain a persistent hazard in tunnel fires. The limitations of the cube-root relationship in predicting critical ventilation velocity (CVV), especially for fires exceeding 100 MW or occurring in non-standard tunnel geometries. In order to produce more precise CVV estimates, the researchers suggest updated modelling techniques that take into account obstruction Fig.\u0026nbsp;1: schematic diagram over a tunnel fire introducing several important terms.\u003c/p\u003e\u003cp\u003eeffects, tunnel cross-section, and flame geometry. These improvements aid in designing ventilation systems that are responsive to realistic fire scenarios.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.5 The Throttling Effect: Resistance to Airflow in Growing Fires\u003c/h2\u003e\u003cp\u003eAn emergent concept in tunnel fire ventilation is the throttling effect, where fires resist incoming ventilation flows as their intensity increases. It was experienced in the experiment of throttling effect in Tunnel Fires, which used CFD simulations via the Fire Dynamics Simulator (FDS) to show that jet fans required for smoke control rise steeply with fire intensity. For fires\u0026thinsp;\u0026le;\u0026thinsp;30 MW, 3\u0026ndash;4 jet fans sufficed; however, for fires of 60\u0026ndash;90 MW, 6\u0026ndash;7 fans were needed, and even then, smoke control was not always successful due to disrupted flow patterns. The study highlights the inadequacy of relying solely on CVV and recommends scaling ventilation design based on both tunnel geometry and potential fire growth rates.[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eFigure 2: Interior view of a road tunnel showcasing prominent jet fans installed near the entrance for longitudinal ventilation.\u003c/p\u003e\u003c/div\u003e"},{"header":"Fire Suppression Strategies","content":"\u003cp\u003eThis section examines tunnel-specific fire suppression technologies, their configurations, effectiveness, and known limitations. Drawing from full-scale experiments, CFD simulations, and performance reviews detailed in the selected literature, each sub-section offers technical insights aligned with design, application, and integration of suppression systems in tunnel environments.\u003c/p\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Fixed Fire Suppression Systems in Tunnels: Role and Relevance\u003c/h2\u003e\u003cp\u003eTunnel environments heavily depend on Fixed Fire Fighting Systems (FFFS) to delay fire growth and allow safe evacuation before emergency services intervene. Unlike in open structures, accessibility is limited, making deluge or mist systems critical.\u003c/p\u003e\u003cp\u003eFixed Fire Protection Systems reduce heat release rates (HRR), prevent fire propagation, and enhance tenability by lowering temperatures and smoke density. However, their effectiveness depends on precise activation logic and integration with ventilation to avoid counterproductive outcomes like stratification breakdown or mist displacement.[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e4.2 Deluge Systems: Nozzle Design and Droplet Behaviour\u003c/h2\u003e\u003cp\u003eDeluge systems are often preferred in tunnels due to their straightforward mechanics and reliability. Performance hinges on droplet size, pressure, spray geometry, and nozzle type.\u003c/p\u003e\u003cp\u003eThe Runehamar tunnel fire tests [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] compared multiple nozzles, including TN-17 and TN-25. TN-25 produced larger droplets, achieving better suppression by effectively cooling fuel surfaces and shielding downstream targets. In contrast, TN-17\u0026mdash;producing finer droplets\u0026mdash;was less effective under strong longitudinal airflow, with fire re-ignition observed after 45 minutes.\u003c/p\u003e\u003cp\u003eThus, nozzle geometry and droplet mass significantly affect system performance in ventilated tunnels.[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e4.3 Water Mist Systems: Advantages and Limitations\u003c/h2\u003e\u003cp\u003eWater mist systems, valued for their lower water demand and infrastructure footprint, generate fine droplets that rapidly evaporate, cooling flames and reducing oxygen. Despite this, their performance deteriorates under strong airflow conditions.\u003c/p\u003e\u003cp\u003eStudies, including one from the University of Edinburgh,[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] indicate that fine mist droplets can be displaced hundreds of meters away by longitudinal ventilation before reaching the combustion zone, making mist systems less compatible with high-velocity airflow tunnels.\u003c/p\u003e\u003cp\u003eMist systems perform best in tunnels with low ventilation velocities or confined geometries, where spray residence time remains high.[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e4.4 Suppression Timing: Early vs. Delayed Activation\u003c/h2\u003e\u003cp\u003eThe timing of system activation greatly affects fire development and evacuation safety. The simulations using FDS and Pathfinder showed that early suppression helped delay flashover and maintain tenability. However, if triggered too soon\u0026mdash;before proper ventilation is established\u0026mdash;it could worsen conditions by condensing smoke and reducing visibility near exits.[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThis indicates suppression systems should not rely solely on thermal triggers but should integrate dynamic smoke detection for intelligent activation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e4.5 Suppression and Ventilation Interaction: A Design Dilemma\u003c/h2\u003e\u003cp\u003eOne of the most critical challenges in tunnel fire safety is the interaction between ventilation and suppression. As observed in the Runehamar fire experiments [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] suppression without ventilation adjustment led to reduced system effectiveness due to droplet displacement and altered smoke buoyancy.\u003c/p\u003e\u003cp\u003eAdditionally, one study introduced the \u0026ldquo;throttling effect\u0026rdquo;, where intense fires resist airflow, altering how suppression and ventilation must be coordinated. Mist systems, in particular, become ineffective unless ventilation velocity is specifically controlled during discharge. Designers must coordinate fan speed, air direction, and suppression logic simultaneously.[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e4.6 Limitations and Research Gaps\u003c/h2\u003e\u003cp\u003eWhile suppression systems reduce HRR and improve evacuation chances, they are not standalone solutions. Their effectiveness hinges on:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eAppropriate nozzle selection and spacing\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eIntegration with longitudinal ventilation systems\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eSmart activation algorithms based on real-time smoke and heat feedback\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eMultiple papers have recommended using hybrid simulations (PyroSim\u0026thinsp;+\u0026thinsp;CFD tools) to predict system behaviour across various tunnel geometries and fire scales.\u003c/p\u003e\u003cp\u003eFuture work must focus on optimizing suppression-ventilation coupling using probabilistic simulations, refining detection logic, and validating results with full-scale fire tests.\u003c/p\u003e\u003c/div\u003e"},{"header":"Human Behaviour and Evacuation in Tunnel Fires","content":"\u003cp\u003eEvacuation during tunnel fires is a high-risk process shaped by environmental and psychological factors. Key determinants such as visibility, heat, smoke spread, signage, and human decision-making under stress dictate evacuation speed and survival rates. The following sub-sections summarize findings from full-scale simulations and behavioral analyses presented in the literature.\u003c/p\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003e5.1 Impact of Visibility and Smoke on Evacuation Time\u003c/h2\u003e\u003cp\u003eA strong inverse relationship exists between smoke density (visibility) and evacuation speed. It was seen that when visibility drops below 10\u0026ndash;15 meters due to smoke, evacuees exhibit delayed or halted movement. Additionally, hesitation increases when exit signage is obscured, or lighting fails.[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThe same study quantified that evacuation time could increase by up to 60% in tunnels with poor ventilation or without illuminated exit signs\u0026mdash;especially when CO concentrations also rise rapidly.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003e5.2 Exit Spacing and Evacuation Path Layout\u003c/h2\u003e\u003cp\u003eFigure 3: Conceptual diagram illustrating the integrated components of a comprehensive tunnel fire safety system for coordinated incident response.\u003c/p\u003e\u003cp\u003eOne study evaluated side exits spaced at 300 m vs. 600 m intervals under different fire growth rates. It concluded that shorter exit intervals enhance survivability, particularly in cases of high HRR fires. Users typically head toward the first visible exit, emphasizing the need for visibility-optimized positioning of egress routes.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eMoreover, tunnel slope, pedestrian lighting systems (like flashing floor markers), and exit familiarity were noted to influence evacuee direction and speed.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003e5.3 Human Behaviour Models and Limitations in Simulation\u003c/h2\u003e\u003cp\u003eAlthough tools like Pathfinder and Building EXODUS provide quantitative simulation, they lack in replicating real-world behavioral unpredictability. It was noted that evacuees often exhibit non-optimal behaviours\u0026mdash;e.g., returning to entry points, waiting for group members, collecting personal items, or ignoring alarm cues .[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThese human tendencies introduce deviations between modelled and real evacuation patterns. The study suggested future models should include stochastic behaviour parameters, cultural differences, and delayed responses to better reflect real-world scenarios.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec23\" class=\"Section2\"\u003e\u003ch2\u003e5.4 Integrated Safety Systems and Evacuation Aids\u003c/h2\u003e\u003cp\u003eEvacuation success improves significantly when multi-layered safety systems are applied. Studies have suggested that the integrated use of audio-visual alarms, dynamic signage, and smoke extraction led to a 35\u0026ndash;50% reduction in total evacuation time.[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eBy redirecting evacuees away from smoke-affected paths in real-time, the study confirmed that coupling suppression, detection, and intelligent evacuation guidance provides substantial safety gains\u0026mdash;making a strong case for holistic system design.\u003c/p\u003e\u003c/div\u003e"},{"header":"Simulation Tools – PyroSim \u0026 Pathfinder","content":"\u003cp\u003eSimulation has become a cornerstone of modern tunnel fire safety engineering. Real-scale tunnel fire testing is costly, risky, and often impractical. Tools like PyroSim (for fire and smoke modelling) and Pathfinder (for evacuation analysis) allow engineers to recreate tunnel fire conditions with accuracy and flexibility. These tools support proactive safety design, system optimisation, and incident analysis, especially in complex tunnel geometries with constrained ventilation and evacuation options.\u003c/p\u003e\u003cdiv id=\"Sec25\" class=\"Section2\"\u003e\u003ch2\u003e6.1 Importance of Simulation in Tunnel Fire Safety\u003c/h2\u003e\u003cp\u003eTunnel fire safety involves a complex interplay of geometry, ventilation, fire growth, and human response. Simulation models help engineers explore these variables without relying on full-scale testing.\u003c/p\u003e\u003cp\u003eThe simulation in a study revealed that the location and activation timing of jet fans significantly affect smoke layering and temperature distribution. Delayed fan activation often led to thermal stratification collapse, which reduces visibility and accelerates untenable conditions.[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec26\" class=\"Section2\"\u003e\u003ch2\u003e6.2 PyroSim for Fire and Smoke Modelling\u003c/h2\u003e\u003cp\u003ePyroSim, which interfaces with the Fire Dynamics Simulator (FDS), is a CFD-based tool for modelling fire growth, HRR evolution, and smoke transport. It allows input of various tunnel parameters like geometry, fuel load, jet fan layout, and ventilation velocity.\u003c/p\u003e\u003cp\u003eIn a study, PyroSim was used to simulate a 200 m tunnel with different fan activation timings. It showed that early ventilation helped maintain visibility near exit routes for 3\u0026ndash;4 minutes longer and delayed CO rise\u0026mdash;thereby extending available safe egress time (ASET).\u003c/p\u003e\u003cp\u003eIt was also demonstrated that PyroSim can replicate heat distribution across the ceiling and predict critical back-layering conditions, validating its use in performance-based tunnel design.[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec27\" class=\"Section2\"\u003e\u003ch2\u003e6.3 Pathfinder for Evacuation Simulation\u003c/h2\u003e\u003cp\u003ePathfinder simulates human movement during fire events using agent-based logic that factors in speed, visibility, CO levels, decision-making behaviour, and group dynamics. When combined with PyroSim data, it allows engineers to evaluate how evacuees behave in real-time tunnel emergencies.\u003c/p\u003e\u003cp\u003eFrom one of the Pathfinder simulations from a study demonstrated that increasing exit frequency and widening doors cut total evacuation time by up to 45%. Bottlenecks typically formed near poorly marked exits or under low-visibility zones, guiding improvements in layout and signage.\u003c/p\u003e\u003cp\u003ePathfinder also showed that introducing dynamic signage and voice alarms redirected evacuees more efficiently away from blocked paths\u0026mdash;reducing total evacuation time by over 30% in some scenarios.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec28\" class=\"Section2\"\u003e\u003ch2\u003e6.4 Validation and Reliability of Simulation-Based Studies\u003c/h2\u003e\u003cp\u003eThe reliability of PyroSim and Pathfinder simulations has been confirmed through comparisons with large-scale tunnel fire experiments such as those at Runehamar and Memorial tunnels[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. These tools comply with tunnel fire codes including NFPA 502 and PIARC, and their predictions align with known tenability thresholds and back-layering behaviour.\u003c/p\u003e\u003cp\u003eIt was provided evidence from a study that FDS-based simulations closely matched real incident outcomes in terms of flame spread, CO concentration trends, and temperature profiles. Such validation underscores the reliability of using simulation tools for risk analysis, emergency planning, and tunnel safety system design.\u003c/p\u003e\u003c/div\u003e"},{"header":"Integration of Tunnel Fire Safety Systems: A Path Toward Comprehensive Protection","content":"\u003cp\u003eModern tunnel systems\u0026mdash;especially those serving urban interchanges, metro-rail hybrids, or multi-level vehicular corridors\u0026mdash;are no longer linear infrastructures. They represent complex, confined, and high-risk environments with dynamic traffic profiles, flammable cargo exposure, and limited evacuation flexibility. Managing fire safety in such scenarios demands an integrated and adaptive approach\u0026mdash;where ventilation, suppression, detection, simulation, and evacuation systems work together, not in isolation.\u003c/p\u003e\u003cp\u003eAlthough substantial advances have been made in individual safety domains\u0026mdash;such as suppression nozzle optimization, jet fan control algorithms, and behaviour-based evacuation models\u0026mdash;current literature reveals a critical lack of integration across these domains. The study of fire simulations accurately captured the effects of smoke layering, exit signage, and pre-movement delays using PyroSim and Pathfinder. However, these simulations typically analyse systems independently, without modelling their real-time interaction or feedback-based coordination.\u003c/p\u003e\u003cp\u003eOne study highlights that although individual elements like early suppression or longitudinal ventilation performance are well-validated, there is minimal research that synchronizes them into a systemic framework. As observed, even technically efficient systems\u0026mdash;such as water mist suppression assisted by jet fans\u0026mdash;underperform if airflow direction, occupancy patterns, or detection timing are not appropriately coordinated.[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e][\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eIn high-HRR fire events (e.g., \u0026gt;\u0026thinsp;100 MW) the effectiveness of any single-point solution\u0026mdash;like deluge systems or fixed jet fans\u0026mdash;depends on timing and interaction. Delayed fan activation, as simulated, resulted in smoke recirculation and reduced evacuation tenability despite suppression working correctly.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThese findings underscore the limitations of prescriptive, siloed designs. As tunnel infrastructure grows in scale and functional complexity, a shift toward performance-based design methodologies is essential. These must be supported by multi-system simulations, capable of testing fire detection, smoke movement, suppression reaction, and human behaviour simultaneously under varying conditions.\u003c/p\u003e\u003cp\u003eDespite strong evidence advocating integrated design, few studies have gone beyond component-level validation into true system integration modelling. This opens clear opportunities for further research in:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eReal-time coordination between detection and ventilation logic\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eFeedback-based suppression control linked to HRR and smoke sensors\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eDynamic evacuation routing tied to environmental sensing\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eFailure-mode testing under hybrid or cascading risk scenarios\u003c/p\u003e\u003cp\u003eBy leveraging advanced simulation tools not just as diagnostic platforms\u0026mdash;but as coordinated design engines\u0026mdash;engineers can finally approach tunnel fire safety as a holistic, responsive system rather than a sum of isolated parts.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eTunnel fire safety has evolved from traditional prescriptive measures to complex, performance-based systems. The literature consistently shows that modern tunnel fires\u0026mdash;especially those involving high fire loads and long evacuation paths\u0026mdash;demand a comprehensive understanding of smoke dynamics, ventilation strategies, fire suppression technologies, and human evacuation behaviour. Early assumptions of manageable fire sizes have been challenged by real-world incidents and large-scale testing, emphasizing the need for adaptable systems capable of handling fires exceeding 100 MW. Studies highlight how ventilation timing, nozzle design, and exit placement can critically affect survivability, and how even oversights in system coordination minor can lead to cascading failures.\u003c/p\u003e\u003cp\u003eSimulation tools like PyroSim and Pathfinder have proven indispensable in bridging theory and real-world behaviour. These tools enable modelling of fire development, toxic gas spread, and crowd dynamics with high accuracy, validating their use in both design and forensic analysis. However, while significant advancements have been made in individual areas, the literature also reveals a notable gap in system-wide integration.\u003c/p\u003e\u003cp\u003eWhile prior reviews have provided useful summaries of tunnel fire safety studies, this paper makes three distinct contributions:\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eIntegrated Assessment\u003c/b\u003e \u0026ndash; Unlike earlier works that treat ventilation, suppression, and evacuation separately, this review examines their combined influence on tunnel fire safety outcomes.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eRecent Simulation Focus\u003c/b\u003e \u0026ndash; It consolidates findings from 2015\u0026ndash;2024, reflecting the latest simulation-based studies that incorporate advanced CFD modeling, agent-based evacuation tools, and hybrid approaches.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003eCritical Comparative Analysis\u003c/b\u003e \u0026ndash; Instead of only summarizing, this paper directly compares conflicting results across studies, pointing out methodological differences (e.g., HRR values, ventilation modes, and sprinkler activation times) that lead to contrasting conclusions.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e\u003cp\u003eTo contribute to this evolving field, our further study will focus on analyzing smoke spread behavior under various tunnel fire scenarios and conditions, performing simulation-based evaluations of those scenarios during tunnel fires under forced ventilation, and examining the effectiveness of fire protection system which is sprinkler system in enhancing tenability during road tunnel fire scenarios. This integrated approach aims to support more resilient and responsive tunnel fire safety designs.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThe author(s) received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors for the completion of this research work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;Clinical Trial Number: Not applicable\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval: Not Applicable\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate: Not Applicable\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication: Not Applicable\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJonatan Gehandler. Road tunnel fire safety and risk: a review. Fire Science Reviews, Vol. 4, Art. No. 2, 2015.\u003c/li\u003e\n\u003cli\u003eA. N. Beard. Fire safety in tunnels. Fire Safety Journal, Vol. 44, pp. 276\u0026ndash;278, 2009.\u003c/li\u003e\n\u003cli\u003eJack R. Mawhinney. Fixed Fire Protection Systems in Tunnels: Issues and Directions. Fire Technology, Vol. 49, pp. 477\u0026ndash;508, 2013.\u003c/li\u003e\n\u003cli\u003eRicky Carvel. A review of Tunnel Fire Research from Edinburgh. Fire Safety Journal, Vol. 105, pp. 300-306, 2019.\u003c/li\u003e\n\u003cli\u003eHaukur Ingason, Ying Zhen Li. Large scale tunnel fire tests with different types of large droplet fixed firefighting systems. Fire Safety Journal, Vol. 107, pp. 29\u0026ndash;43, 2019.\u003c/li\u003e\n\u003cli\u003eL.H. Hu, R. Huoa, W. Peng, W.K. Chow, R.X. Yang. On the maximum smoke temperature under the ceiling in tunnel fires. Tunnelling and Underground Space Technology, Vol.21, pp. 650\u0026ndash;655, 2006.\u003c/li\u003e\n\u003cli\u003eP. Sturm, M. Beyer, M. Rafiei. On the problem of ventilation control in case of a tunnel fire event. Case Studies in Fire Safety, Vol. 7, pp. 36\u0026ndash;43, 2017.\u003c/li\u003e\n\u003cli\u003eR.O. Carvel, A.N. Beard, P.W. Jowitt. The influence of longitudinal ventilation systems on fires in tunnels. Tunnelling and Underground Space Technology, Vol. 16, pp. 3-21, 2001.\u003c/li\u003e\n\u003cli\u003eYasushi Oka, Graham T. Atkinson. Control of Smoke Flow in Tunnel Fires. Fire Safety Journal, Vol. 25, pp. 305-322, 1995.\u003c/li\u003e\n\u003cli\u003eArnas Vaitkevicius, Ricky Carvel. Investigating the Throttling Effect in Tunnel Fires. Fire Technology, Vol. 52, pp. 1619-1628, 2016.\u003c/li\u003e\n\u003cli\u003eD. Van den Broecke, R. Emberley, T. Sietz, J. L. Torero, C. Maluk. Study on the effectiveness of fire suppression deluge systems in tunnels. Tunnelling and Underground Space Technology, Vol. 108, Art. No. 103764, 2021.\u003c/li\u003e\n\u003cli\u003eZ. Liu, X. Gu, and R. Hong. Fire Protection and Evacuation Analysis in Underground Interchange Tunnels by Integrating BIM and Numerical Simulation. MDPI Fire, Vol. 6, No. 4, pp. 139, 2023.\u003c/li\u003e\n\u003cli\u003eM. Fera, R. Macchiaroli. Use of analytic hierarchy process and fire dynamics simulator to assess the fire protection systems in a tunnel on fire. Int. J. Risk Assessment and Management, Vol. 14, No. 6, pp. 504, 2010.\u003c/li\u003e\n\u003cli\u003eY. Cui and Z. Liu. Study on Fire Smoke Movement Characteristics and Their Impact on Personal Evacuation in Curved Highway Tunnels. Applied Sciences, Vol. 14, No. 12, Art. No. 6339, 2024.\u003c/li\u003e\n\u003cli\u003eA. Sarvari and S. M. Mazinani. A new tunnel fire detection and suppression system based on camera image processing and water mist jet fans. Heliyon, vol. 5, Issue 6, e0187, 2019.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Tunnel Fire, Fire Dynamics, Ventilation, Fire Suppression, Evacuation, PyroSim","lastPublishedDoi":"10.21203/rs.3.rs-7761467/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7761467/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTunnel fires are among the most critical hazards in transportation infrastructure due to rapid smoke spread, high heat release, and constrained evacuation opportunities. In recent years, computational simulation has become the primary tool to evaluate tunnel fire safety, enabling detailed analysis of smoke propagation, ventilation effectiveness, suppression systems, and evacuation performance. This review critically synthesizes studies published between 2015 and 2024, with focus on simulation approaches using CFD-based fire modeling and agent-based evacuation tools. Comparative discussion shows that while longitudinal ventilation is effective for moderate fire sizes, its performance in large-scale fires is inconsistent, particularly when coupled with suppression. Water-mist and sprinkler systems demonstrate significant potential in reducing HRR and gas temperatures, though their interaction with strong airflow remains inadequately resolved. Evacuation models highlight the dominant role of visibility and toxic gases in determining egress time, yet often rely on simplified behavior assumptions. The distinct contribution of this paper is its integrated assessment of ventilation, suppression, and evacuation simulations, clarifying contradictions across studies and identifying gaps such as coupled multi-system modeling and validation under extreme fire conditions. The review concludes with recommendations for more realistic, hybrid simulation frameworks to support tunnel fire safety design.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e","manuscriptTitle":"Simulation-Based Performance Analysis of Ventilation and Fire Protection Strategies in Road Tunnel Fires: Review","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-10 21:08:59","doi":"10.21203/rs.3.rs-7761467/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2d8cea7d-2534-44cf-b9f4-daf8823a13d7","owner":[],"postedDate":"November 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-19T09:24:00+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-10 21:08:59","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7761467","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7761467","identity":"rs-7761467","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}