Road Tunnels Manual

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8.3. Communication and alert systems

It is important for the operator to be able to communicate with the user. It should be possible for this communication to take place in both directions - operator to user and user to operator. These exchanges should be possible in all operational situations: normal, degraded or critical.

Several devices allow ensuring this communication function (the alert is considered as a particular form of communication). They do not all offer the same functionalities: some of them allow establishing a transmission from user to operator (alarm pushbuttons, automatic alarm while using certain evacuation systems ...) while others allow a transmission from operator to user (messages broadcast on FM frequencies, loudspeakers). Only one of them allows a full exchange (emergency telephones).

8.3.1. Emergency telephones

Emergency telephones allow a user, victim of an accident in tunnel, to contact the control-command centre in charge of the tunnel. In addition to establishing a voice link, the use of an emergency telephone by a user also gives his precise location.

These emergency telephones are installed at fixed intervals on boxes or in emergency stations of different types. The distance between two emergency telephones is often specified by regulations and therefore varies from one country to another.

The structure of this device is quite simple. The emergency telephones in the tunnel are connected to a centre that receives the calls made from the tunnel. Usually, this centre is located in the control-command centre of the tunnel and sometimes in the premises of police services under whose jurisdiction the tunnel is placed.

8.3.2. Alarm pushbuttons

Alarm pushbuttons allow a user to send an alarm to the control-command centre in case of accident in tunnel. Being not very expensive equipment, it can be installed at frequent intervals.

This device is not much used because to a certain extent, it duplicates the emergency telephone and moreover, it does not allow a two-way communication between the user and the control-command centre.

8.3.3. Automatic alarm when emergency systems meant for users are used

As mentioned above, the user has access to several devices that he can use in tunnel, particularly in emergencies: emergency telephones and sometimes alarm pushbuttons. He also has fire extinguishers and, in most tunnels, emergency exits.

It is essential for the operator to be informed as early as possible when a user operates one of these devices, in order to take adequate actions. This is not difficult when emergency telephone and the alarm pushbuttons are installed because, very often, the control-command centre receives the call or the alarm information. When the emergency telephones are terminated in a place other than the control-command centre, procedures have to be set up so that the service receiving the call informs the control-command centre at once.

In the case of extinguishers and emergency exits, sensors are very often installed for detecting a change of status and communicating this information to the control-command centre by using the SCADA system. The operator is then informed that a user in tunnel is requesting assistance.

For the fire extinguishers, the information taken into account is often the action of removing the equipment from its support or opening the door in the emergency station etc. For the exits, the information taken into account may be the opening of the door or the detection of a presence in the exit or both of these.

8.3.4. Automatic incident detection

When a tunnel is equipped with a video-surveillance system (refer Section Systems for surveillance and control of traffic), the images coming from the tunnel and its vicinity are shown by displays installed in the control-command centre. It is difficult for the operator to monitor more than a few displays simultaneously with a constant alertness during several hours.

For remedying this difficulty, operators are increasingly using automatic systems for detecting an incident. In certain countries, the use of such equipment is even obligatory for specific tunnels.

Type and Function of Automatic Incident Detection

Automatic incident detection (AID) is normally based on computer-based analysis of video image streams generated from cameras set up to view tunnel traffic. A number of algorithms are available which can detect a range of incidents, including:

  • stopped vehicles
  • vehicles moving in the wrong direction
  • speed drop
  • slow vehicle
  • pedestrians
  • debris in road tunnel
  • smoke
  • flames
  • entry into restricted zones

Since serious vehicle fires normally develop after traffic has stopped (e.g. following an accident), it follows that a 'stopped vehicle' alarm from an AID system can be expected to precede alarms triggered by other systems, such as temperature and smoke detectors. This early warning provided by AID allows time for tunnel operators to confirm the nature and location of the incident, and to allow effective intervention., This may be through the choice of an optimal choice of ventilation configuration, prevention of secondary accidents through operational measures, rapid warning to motorists upstream of the incident. It also gives opportunity to call the emergency services, closing of access, messages on variable message signs and on the radio, call to breakdown lorry, advice to exit the tunnel, etc.

Video smoke detection systems are described in Section 6.3.3 "Currently Used Methods" of report 05.16.B 2006.

Video-based AID systems can provide real-time information on the traffic flow, volume and speed. They can record pictures at the origin of the incident and can interact with other systems such as the Supervisory Control and Data Acquisition (SCADA) system. Video-based AID systems normally include cameras, a video image processing system processing images from one or several cameras, Internet Protocol (IP) video encoders and decoders on IP to return images to monitors or computer displays. Furthermore a video management system composed of one or two redundant servers providing video and other functions (recording of video mass and AID incident, collecting and storing real-time traffic data and traffic events, interfacing with the tunnel SCADA system), network equipment and communications lines(optical fibres, coaxial and Unshielded Twisted Pair cables).

Design and Commissioning of AID Systems

The design of AID systems in tunnels should be undertaken with due account of the following issues:

  • Choice of incidents to be detected
  • Detection accuracy (i.e. minimisation of 'false negatives' in detection incidents)
  • Minimisation of false alarms (i.e. minimisation of 'false positives')
  • Location of existing cameras in the tunnel
  • Geometric features of the tunnel
  • Access for maintenance staff
  • Bright sunlight 'bleaching' near portals
  • Sunlight movements in the vicinity of portals
  • Changes caused by the passage of vehicles in the tunnel (lights, occlusion by high vehicles)
  • Change of lighting regime in the tunnel (lighting on/off)
  • Reflections in the tunnel
  • In case of an AID system integrating IP video stream, the capacity of the existing IP network should be checked to ensure there is sufficient bandwidth available

The 2009 Routes/Roads article "Fire Detection Systems in Road Tunnels - Lessons Learnt from the International Research Project" concluded that "to deal with obstructions, most manufacturers of field of view detectors recommend two detectors covering the same area from different angles, such as from both directions within a tunnel". Multiple cameras may also be required for redundancy purposes, in case of camera failure. Typically, the camera fields of view are designed to overlap, such that failure of any one camera can be compensated through the images from neighbouring cameras.

Section IV.2.1. "Incident Detection Devices" of report 05.15.B 2004 suggests that camera locations can vary from 30 to 150 meters if they are used for automatic incident detection.

The performance of an AID system performance depends to a great extent on successful commissioning and calibration, prior to deployment. Experience from tunnel installations indicates that such commissioning and calibration can take several months to undertake.

8.3.5. Fire/smoke detection : Purpose of fire and smoke detection

Fire and smoke detectors are always an integral part of a control loop which is set up by sensors, alarm triggering equipment, transmission cabling, evaluation units, etc., and which taken together are generally referred to as a fire alarm system.

Fire and smoke alarm systems in road tunnels are designed to detect fires and smoke production as fast as possible so that safety equipment and procedures can be activated without delay. Their main objectives should be:

  • informing tunnel users at the earliest opportunity so to enable them to organize their self-evacuation and self-rescuing;
  • passing on all the possible fire parameters to the operational tunnel staff in order to enable them to change ongoing tunnel operations (traffic control and ventilation systems) according to the emergency procedures (so-called fire mode), and to call in the rescue services, medical staff, fire brigade, police, etc.
  • to identify the locations of the fire or incident, in order to direct rescue service resources to the appropriate places to assist motorists, for example.

Principles of fire detection

Basically, the fire detection principles are based on the perceived parameters determined by the fire i.e. heat, smoke, radiation and production of typical chemical substances. Therefore, the fire detecting sensors can be classified as :

  • Heat detectors : all material the characteristics of which are sensitive to an increase of heat energy whenever this implies a temperature rise. Examples are sensors which measure temperature differences with a reference temperature or rate of temperature rise, glass-fibre cables where its light transmission characteristics are a function of temperature, linear sensor cable with built in electronic circuit, etc.;
  • Flame detectors based on their sensitivity of infrared and/or ultraviolet wavelength spectrum;
  • Smoke detectors which measure the extinction of a infrared light beam through CO and CO2 ionisation areas;
  • Detectors which combine different types of sensors.

Each of these detectors has their own specific application domain, related to its response time, robustness, reliability, etc.

Recently video AID systems have proven to be very efficient and fast in detecting fires. In fact, they detect incident any object or vehicle which does not conform to the normal expected traffic stream. The cameras can be automatically turned towards the incident scene, which enables the operator to discover the very early start of a fire.

Fire/smoke detection systems are described in Section 6.3 "Fire detection" of the report 2006 05.16.B.

Requirements for Fire detection system

In a general way, fire detectors in road tunnels must be designed to withstand the following environmental conditions: air velocities up to 10 m/s, reduced visibility resulting from diesel exhaust fumes and abrasive wear stemming from tires and the road surface, increased and short-term fluctuating concentrations of pollutants (carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides and hydrocarbons), changing headlight intensities, engine heat and hot fumes vehicle exhaust gases, electromagnetic interferences, mixed vehicular traffic (i.e., cars, small lorries, heavy load lorries, buses and tankers) that will result in varying degrees of tunnel cross section obstruction.

It cannot be stressed enough that they must have a high degree of fail-safe operation and be able to locate the fire as close as possible. It is advisable that the systems of fire detection possess certain level of intelligence to avoid false alarms, because false alarms could entail significant expense to rectify and even worse, may discourage the operators after a while run from paying attention to the alarms.

Furthermore, it is imperative that the fire detection/alarm installation is reasonably priced, has low operating costs and simple to maintain: refer Section 6.3 "Fire detection" of the report 2006 05.16.B.

Parameters dictated by Codes and Standards

The following parameters for automatic fire detectors are specified in national and international codes and standards : maximum time for a fire to be detected, determination of the fire site location, minimum fire load to be detected, approved detection methods, assembly points for fire alarms, details pertaining to which tunnels should be provided with automatic fire alarm installations (e.g. length of tunnel, tunnels with mechanical ventilation, tunnels that are not permanently monitored by personnel, short tunnels with particularly high traffic densities).

A list of detailed reference material regarding fire detector parameters are described in codes and can be found in Section 10 "References" of the report 2006 05.16.B.

Fire/smoke detectors currently in use

The efficiency of fire detection is not only based on the type of devices (temperature, light beam extinction, ionisation, etc.), but also on the detection strategy which and been developed, which includes the number of sensors and their level of surveillance in the tunnel.

Automatic incidents detection, analysis of video images including AID systems, closed-circuit television (CCTV) observation, equipment such as fire extinguishers which activate alarms by the removal, as well as the emergency telephones are generally good means to raise an alarm.

Many detectors in use are based on heat and on the rate of temperature rise. When well calibrated, this type of system generates only few false alarms, but may have a slow reaction rate. Detectors based on smoke obscuration give early signals but have suffer more false alarms because of smoke exhaust from diesel vehicles: refer to Section VI.3.1 "Fire detection" of report 05.05.B 1999.

The 2009 Routes/Roads article "Fire Detection Systems in Road Tunnels - Lessons Learnt from the International Research Project" deals with fire/smoke systems of road tunnels such as linear detection of the heat, optical detection of flames, detection by video imaging, punctual detection of heat and smoke detection by air sampling system. It concludes that the system of air sampling gives a good performance in terms of response time and ability to accurately locate and monitor a fire and the effect on road environment, when taking into account overall performance including false alarms, maintenance and fire detection. The information from this study can be used to determine the most appropriate technology for tunnel fire detection.

8.3.6. Radio-retransmission of public FM broadcasts, frequencies of operators and emergency services

A tunnel is a closed and confined space that very often does not allow the propagation of radio waves from broadcasters outside the tunnel. For re-establishing this propagation, it is necessary to install the equipment that allows the retransmission of the needed frequencies. Several types of services can be retransmitted:

  • Rescue services (fire brigade, police...)
  • Operator (patrols, maintenance crews, taxis, bus companies, ...)
  • Public FM broadcasts
  • Public DAB broadcasts (Digital Audio Broadcasting)
  • Cell phones.

There are a very large number of services whose frequencies can be retransmitted but they are not all of them covered, because of problems of cost, not to mention feasibility. As a general rule, one can find certain frequencies used by the rescue services, frequencies used by the operator, a few FM and or DAB frequencies and frequencies of cell phone operators.

When one or more Radio frequencies are retransmitted, a device is installed that allows inserting pre-recorded messages. In case of need, these radio broadcasters are interrupted and messages regarding the tunnel are broadcasted for the attention of users, in order to give them indications regarding the steps the operator wants them to follow.

A radio-retransmission installation in tunnel is essentially composed of:

  • An antenna
  • A transmission/reception unit that allows transmitting from the outside into the tunnels technical room that needs to be cooled.
  • A transmission/reception unit that allows transmitting from the tunnel to the outside (not for public broadcasters but for emergency services etc.)
  • A radiating unit in tunnel (radiating cables or antennas).

8.3.7. Loudspeakers

There are not many devices that allow addressing the user directly for giving information or asking the tunnel users to behave in a particular manner. For solving this problem, some tunnels are equipped with loudspeakers. In practice, depending on how they are used, the loudspeakers offer different functionalities. The following points can be mentioned in particular:

  • Loudspeakers installed at fixed intervals in the tunnel, to give information and instructions to users whose vehicle has stopped inside the tunnel
  • Loudspeakers (or sirens) installed at fixed intervals inside the tunnel that emit a sound signal indicating a danger
  • Loudspeakers (or sound beacons) installed near emergency exits that providing information to users about to use an exit and where it is located.

These devices are not however widely used at present. Their use must be studied for each case and often, they are suited for very specific tunnels (very dense traffic, length, etc.).

Reference sources

No reference sources found.