Structural elements

Motorists travelling through tunnels must be able to do so in complete safety, which means that the road layout must be well designed, consistent and easy to follow, and that the infrastructure must be equipped with all the necessary facilities to ensure the safety of people in the event of an incident.

The first aspect relates to technical provisions (overall geometry, treatment of verges, intersections, etc.) for road safety; the second aspect, specific to tunnels, deals with provisions for the safety of users in a confined environment. The technical directive (appendix 2 of the circular of 25thAugust 2000) focuses on the second point and details the following main aspects in turn:

• The carriageway, lay-bys and walkways
• Facilities for the evacuation and protection of users and access for emergency services
• Facilities for emergency vehicles
• Emergency call stations and water supply recesses

The overall geometry of the infrastructure

Firstly, it is clear that the general provisions and best practices applicable to the design of open-air road infrastructure apply. In tunnels, even more so than elsewhere, the aim is to achieve the most harmonious layout possible (curves, gradients, etc.) so as never to surprise motorists. Particular attention will be paid to visibility conditions so that, despite the side walls, any obstacle, object on the carriageway or stationary vehicle is visible early enough to allow for safe stopping. The most delicate issue to deal with often concerns possible junctions with entry or exit ramps. Anything that requires the driver to change lanes or choose a direction with the risk of hesitation can create accident-prone situations.

The carriageway and walkways

The main issue here is the width of the carriageway, which must allow access for emergency services even if vehicles are stopped in the tunnel. However, this condition is not required if the emergency services have other access routes available (from the outside or from a second tube).

The carriageway must also be wide enough to allow vehicles to pass safely around a stationary vehicle; if this is not possible, extra-wide sections (lay-bys) are provided in long tunnels.

Walkways also have a minimum width in order to better protect users in difficulty who are walking to reach emergency equipment.

Facilities for the evacuation and protection of users and emergency access (carriageways and walkways)

These are the most important elements in terms of safety. In the event of an incident or risk of a serious accident, users must be able to quickly find nearby exits that lead them to safe places. Emergency exits are spaced 400 metres apart in non-urban tunnels and 200 metres apart in urban tunnels (heavier traffic), or even less if there are a large number of lanes.

Depending on the configuration of the tunnel, exits can take several forms: direct connections to the outside (if the ground cover is low), connections between the two tubes (if the tunnel has two tubes), a safety gallery parallel to the tunnel, or finally, protected shelters connected to an evacuation route.

Very strict measures are in place to prevent fire or heat from spreading to these protected areas. Similarly, wheelchair users must be able to access these exits and pass through the airlock doors without difficulty

Facilities for emergency vehicles

These facilities are designed to enable emergency vehicles to pass quickly from one tube to another. The directive defines the minimum distances and dimensions to be provided.

When the tunnel has only one tube, widenings or short galleries are provided to allow these vehicles to turn around without difficulty.

Similarly, spaces are reserved outside, near the tunnel entrances, for emergency vehicles to park. Service lanes also allow direct passage from one carriageway to the other.

In general, structural elements are designed to facilitate access for emergency services and the passage of emergency vehicles (including fire engines) from one tube to another and from one direction to another.

Emergency call stations and water supply recesses

Although emergency stations, located approximately every 200 metres, contain various safety equipment (emergency call points, fire extinguishers, etc.) and are equipped with doors to limit noise during conversations between users and operators and to prevent dirt from entering, they are in no way intended to protect users from the effects of fire. This protection is provided by emergency exits, which should be clearly distinguished from the emergency call stations. These call stations must be accessible to all, particularly wheelchair users.

Water supply recesses, also located approximately every 200 metres, are useful for firefighters, who can find a water supply (hydrants or fire hydrants) with the recommended flow rate and pressure. The dimensions of these recesses are to be carefully considered, as it is important that they provide the necessary room for manoeuvre for firefighters to do their work.

The provisions outlined here are not exhaustive. They simply cover the essential points to which designers must pay particular attention in terms of safety.