#critical velocity

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Tunnel fire scene inside a long alpine road tunnel at night — wet asphalt reflects warm sodium lighting under a curved concrete vault, two jet fans hang from the ceiling, a green emergency exit sign glows above a steel refuge door and an emergency telephone on the right wall, a stopped lorry far down the bore trails grey smoke that flattens along the ceiling, and a tunnel safety engineer in a hi-vis jacket and white hard hat stands by a service pickup in the foreground reading an event-tree diagram and a falling risk curve on a tablet

Fire Risk Assessment

Road Tunnel Fire Risk: The QRA Born From Mont Blanc 1999

The Mont Blanc tunnel fire killed 39 people in 1999. Here come the event trees, design fires, critical velocity and F-N lines that now govern tunnel QRA.

September 8, 2026 · 18 min read