Communicating with tunnel users

Tunnel operators have two means of directly communicating with users:

Messages broadcast on FM frequencies which are retransmitted inside the tunnel,
Messages broadcast over public address systems (e.g. loudspeakers) located in the part of the tunnel open to traffic.

Broadcasting messages on FM frequencies is obviously only possible if the tunnel is equipped with a radio communication system. When such a system exists, it enables a limited number of frequencies to be retransmitted and the user’s car radio must be switched on. One of the benefits of this system is that information directly reaches the user in his/ her vehicle.

Broadcasting messages through loudspeakers is also an efficient means of communication, as long as they are clearly audible. Adjusting the system to reach a satisfactory level of performance, and notably to prevent reverberation, can be quite tricky.

This chapter is split into two parts:

Radio BREAK-IN systemS

A tunnel is an enclosed structure that usually does not permit the natural transmission of radio waves to travel very far into the tunnel. Since vehicle radios are a useful means of allowing tunnel operators to broadcast emergency messages to tunnel users, we must therefore provide a means to propagate radio signals through the tunnel.

This re-broadcast of radio stations is provided through the radio re-broadcast system using transmission equipment and a coaxial cable running through the tunnel. The coaxial cable leaks radio waves and is referred to as the “leaky feeder”.

The radio system is primarily used to allow emergency services to use their radios in the tunnel. It is also possible to install specific equipment that allows the retransmission of the required local and national radio frequencies into the tunnel.

To be able to broadcast emergency messages to vehicles on the radio frequencies, operators must interrupt the normal broadcast of radio stations, break into the frequencies and broadcast their own emergency message. This is called Voice Break-In, VBI.

Fig 1: Voice break-in system used via vehicle radios

When needed during incidents, radio broadcasts are interrupted and safety messages are broadcast to tunnel users by the operator. These messages may be pre-recorded or live broadcasts. Text messages may also be transmitted to users’ radios using the Radio Data System.

These VBI messages will be transmitted in coordination with the public address system, if installed, such that PA messages are not broadcast at the same time as the VBI radio messages.

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

An antenna to receive radio stations “off-air”
Receivers and transmission units for AM, FM, DAB, Tetra, UHF, that allows broadcasting from the outside into the tunnel’s technical rooms
Voice Break-In equipment to allow emergency messages to be transmitted into the tunnel
Radiating devices in the tunnel (radiating “leaky feeder” cables or antennas)

Public Address Voice Alarm system (PAVA)

Figure 1: Loudspeaker installed in the Yamate tunnel (Japan)

Public address systems enable operators to inform tunnel users of an incident that requires evacuation. These systems include:

Loudspeakers installed at fixed intervals along the tunnel, which enable operators to address users directly to give them information or to ask them to behave in a particular manner.
Sirens installed at fixed intervals along the tunnel that emit a sound signal indicating a danger
Sound beacons installed near emergency exits that help to indicate the location of the exit and inform users that they should use it to evacuate.

These systems are explained in more detail in PIARC report 2016R06EN: “Improving safety in road tunnels through real time communication with users” and in the appendix to this report which provides the results of an international survey on the use of loud speaker public address systems.