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Abstract

IntroductionTunnel evacuation modelling1 has shown that in the event of an emergency, members of the public are generally reluctant to leave the perceived safety of their vehicles until it is too late. The EU tunnel directive2 recognises this by stating ‘Where there is a tunnel control centre, it must be possible to interrupt radio re-broadcasting of channels intended for tunnel users, if available, in order to give emergency messages.’ Put simply, if a user has their car radio switched on, an emergency message can be given. Atkins has implemented a number of systems combining both radio and loudspeaker public address, the most recent of which has been installed in the Rotherhithe Tunnel in East London.

ObjectivesWhilst the EU directive requires voice break-in for radio broadcast, best practice is to integrate this with a comprehensive loudspeaker public address system. Coupled with emergency signing and mobile ‘phone re-broadcast, this provides the facilities required for effi cient communication with users.

Speech intelligibility is the key to providing clear messages to tunnel users in an emergency. The purpose of the radio message is primarily to get drivers out of their vehicles and to follow emergency signs. They need to clearly understand the messages and be able to act on them immediately.

It is important that the emergency broadcast is heard simultaneously on all radio channels. It has to alternate with the loudspeaker public address so that the two systems are not heard at the same time as more detailed messages can be put over the loudspeaker system. The loudspeaker system is also zoned so that different messages can be given to users either side of an incident but the radio system cannot easily be zoned.

Car radio public addressIn order to encourage tunnel users to keep their car radios switched on, the normal off-air broadcasts are rebroadcast into the tunnel for a number of radio stations that, according to published listening fi gures, attract the majority of the public. In an urban environment this would typically be 15 VHF FM stations, four MW AM stations, two LW AM stations and fi ve Digital Audio Broadcasting (DAB) multiplexes, each of which carry multiple stations. In the event of an emergency, many car radios will be on and able to receive the emergency message. In addition, the use of the Radio Data System (RDS) on FM channels enables users’ tapes and CDs to be interrupted. Each of the DAB multiplexes carries many stations that can vary at any time. Whilst the uptake of DAB has been relatively slow, car manufacturers are beginning to install them in their new models. This will become the norm in due course and may well replace AM or even FM. The UK Government is keen to free up radio airspace, as they are doing with digital television.

The received signal passes through the switch most of the time and is interrupted with the emergency message from the PA system when required. The AM channels are carried out in a similar manner.

Tunnel evacuation modelling has shown that the public are generally reluctant to leave the perceived safety of their vehicles. Radio communications in road tunnels for emergency services have been well defi ned for many years but the ability of an operator to speak directly with tunnel users has traditionally required the use of emergency telephones.

Public address systems that use both radio break-in and loudspeakers have been installed in a number of tunnels in recent years. This paper considers an integrated solution that exceeds the requirements of the EU tunnel directive and provides an effi cient response in the event of incidents.

Intelligent Transport Systems

David SteningPrincipal Engineer

Radio and loudspeaker public address for road tunnels

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However, this is discounted due to cost and because of the dynamic nature of channel allocations within each multiplex which would require regular manual updates.

The fourth option to consider is software encoding which inserts a digitised emergency message into each multiplex stream. This is the preferred method and was selected for the Rotherhithe Tunnel which is the fi rst of its type in the world. It does however require a signifi cant amount of computing power to process each multiplex in real time.

Loudspeaker public addressThe advantage that loudspeaker public address has over radio is that it can be zoned so that specifi c information can be given to people in different areas. It also does not rely on drivers having their car radios switched on.

Public address is not common in tunnels mainly due to the high level of ambient noise and reverberation, both of which make speech intelligibility very diffi cult to achieve. Speaker design and the use of delay lines can minimise these effects. The sound from the nearest speaker is delayed electronically for a few milliseconds until the sound from the previous speaker arrives, ensuring all speakers in a zone are heard at the same time.

Rotherhithe tunnel speech trialsWhilst undertaking the Rotherhithe Tunnel project, site trials were carried out prior to full installation to determine the best speaker confi guration and to check speech intelligibility.

The hard tiled walls gave a reverberation time of about eight seconds, which was a signifi cant challenge for the system.

During the trial a number of small speakers similar to those in the Dartford Tunnel were installed at 3.5 metre intervals. Two larger speakers specifi cally designed for tunnels were set on stands 50 metres apart (Figure 5). The horn shape projects the sound along the tunnel roof. The trial took STI measurements as well as subjective listening tests.

VHF FM

Digital Audio BroadcastingDigital Audio Broadcasting (DAB) is handled quite differently. There are currently up to fi ve multiplexes in any area, each carrying many stations. The digital stream is extracted from each ensemble. The emergency messages are digitised and then inserted into the audio sections of the digital stream. The timing of the interrupt signal has to be synchronised with the broadcast message otherwise the security system built into car radios suspects that the broadcast could be a pirate station and mutes the radio.

DAB is still relatively new (approximately 15 years) and not all car radio manufacturers implement the alarm features available in the same way.

A number of methods of carrying out the DAB voice break-in have been considered and the broadcasters and car radio manufacturers have been consulted.

The fi rst option to consider is to switch off DAB broadcasts and allow car radios to revert back to the FM broadcast. This relies on the software implementation of the DAB standards to search for an alternative station but is not implemented by all manufacturers. Most car radios switch over to FM but this can take up to 30 seconds.

The second option is to ‘spoof’ the radios to tune to one multiplex or to use ‘local windows’. This is more expensive and has similar drawbacks to the fi rst option, so is not worth further consideration.

The third option is to resolve all channels to audio and back again to RF.

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Figure 1 - A simplifi ed block diagram of the radio voice break-in system for FM

Figure 2 - A simplifi ed block diagram of the radio voice break-in system for DAB.

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While the ambient noise level was low, both types of speakers gave good results but when the fans were turned on and the ambient noise rose to over 87db, the small speakers merely added to the noise and it became very diffi cult to distinguish the words. Even with the nearest speaker switched off, and at a distance of 100 metres, sound from the horn speakers was still crisp and clear and every word could be heard both in and out of vehicles.

When considering the speaker gain settings, we considered that 6db above ambient noise gave the best all-round results. However automatic gain control is required to adjust the system volume depending on ambient noise levels. Typical ambient noise levels in tunnels are around 87db so an additional 6db gives a system level of approximately 93db.

IntegrationIt is important that the two parts of a public address system are fully integrated to avoid confl icting messages and to ensure that the radio and speakers to do not operate at the same time. A common user interface should also be provided. The public address system should also be integrated with other Intelligent Transport Systems (ITS). Static and variable message signs need to be co-ordinated with the announcement zones to avoid confl icting messages.

Three types of messages are to be provided, automatic messages triggered after operator confi rmation of a fi re alarm. These messages are location specifi c and are intended to direct the tunnel users away from the incident. In addition a selectable menu of messages is available, for example “Stay in vehicle and wait for instructions”, “Leave vehicle and follow emergency signs” or “Please switch off engines - there is a delay ahead”.

Messages use a short delay followed by chimes to attract the user’s attention. Once activated the system does not revert to normal off-air broadcasts until the incident is cleared. Pre-recorded messages need to be clear and professionally recorded. To cater for unexpected situations a microphone is to be provided on the operator’s desk.

ConclusionThe use of purpose designed systems can provide a suitable tool for tunnel evacuation communications in the event of a fi re or similar incident. The combination of radio rebroadcast and loudspeaker public address, along with other ITS solutions, enables the operator to communicate effectively with tunnel users, provided they are carefully integrated and user-friendly.

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Figure 3 - A simplifi ed layout of four tunnel zones that align with the fi re ventilation plans plus one zone on each approach.

Figure 4 - A trial section of speakers in the Rotherhithe Tunnel.

Figure 5 - A purpose built tunnel horn speaker being used in the trial.

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ReferencesReport of the Task Force for Technical Investigation of the 24 March 1999 Fire in the Mont Blanc Vehicular Tunnel 1. http://www.atmb.net

EU Directive 2004/54/EC Annex I section 2.16.2 2. http://ec.europa.eu/transport/road/roadsafety/roadinfra/tunnels/index_en.htm

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