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.
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: