What are the specific risks associated with tunnels?

This section only refers to risks to users travelling through tunnels. It does not refer to risks during construction.

The first type of risk is related to the instability of the tunnel and the equipment attached to the ceiling. Regular monitoring and maintenance provide complete protection against these risks. Malfunctioning lighting or ventilation systems can affect user comfort and even cause discomfort, due to poor visibility or toxic air. Risk-generating events such as breakdowns, accidents and fires can have serious consequences due to the confined nature of tunnels. The vast majority of fires are caused by spontaneous combustion of vehicles due to technical failure, but all of the rare fires that have resulted in fatalities have been the result of an accident, with the exception of the 1999 fire in the Mont Blanc tunnel.

Fires involving heavy goods vehicles are the most feared event in tunnels. It is on this scenario that designers and operators focus all their attention.

The effects of a fire occur in the following order:

• Occurrence of smoke, which is very opaque, incapacitating and considerably hinders the evacuation of users,

• Users who are unable to evacuate due to poor visibility are inconvenienced or even suffocated by smoke, which is becomes increasingly toxic.

• The heat generated by the fire causes high temperatures.

As shown in the figure opposite, the risk factors are the vehicles and their loads, the behaviour of users, the characteristics of the infrastructure, and the operator’s ability to make proper use of the equipment at their disposal.

Local authorities have no control over vehicle design, but they can prohibit or regulate the passage of certain vehicles, such as those transporting dangerous goods.

The term “infrastructure” refers to the geometric characteristics and structural elements of the tunnel, including emergency exits, and all technical equipment (power supply, lighting, ventilation, incident detection systems, radio communications, signalling, etc.).

Operating procedures vary greatly according to the type of tunnel. They range from no monitoring to highly sophisticated supervision systems where the operator, thanks to the data provided (notably through automatic incident detection), is able to act very quickly. The operator can then:

• inform users and encourage them to adopt the most appropriate behaviour,
• alert emergency services (with a particular focus on firefighters, whose speed of response determines the chances of controlling and extinguishing the fire),
• take action using the available equipment (closure of the tunnel, activation of smoke extraction systems, etc.), and trigger the on-site intervention of specialized operating teams.

It is important to emphasize the essential role played by user behaviour. Before the emergency and rescue services have time to arrive on the scene, users are alone in the tunnel. Their behaviour, and in particular their responsiveness in the first few minutes after the initial event, is a determining factor in safety. This explains the efforts that are undertaken to provide equipment that can guide users in difficulty.