FM and DAB+ coverage in tunnels
Entering a tunnel means losing radio signal: reinforced concrete blocks FM and DAB+ waves from outside. This is not just a comfort issue — in an emergency, continuous radio communication to vehicles in transit is essential for evacuation and emergency coordination.
Why tunnels block radio signals
Reinforced concrete and metallic structures form a shield that strongly attenuates incoming RF signals. For FM (87.5–108 MHz) and DAB+ (174–240 MHz), penetration from outside is practically zero: without an internal distribution system, a tunnel is a dead zone from the first meter.
How to bring the signal into a tunnel
The coverage system consists of three elements:
- External receiving antenna — captures FM and DAB+ signals available in the area outside the tunnel.
- Amplifier — receives, amplifies, and distributes the signal toward the interior while maintaining quality.
- Radiating cable (leaky feeder) — carries the signal along the full length of the tunnel and radiates it uniformly through controlled openings in the shielding. It provides continuous coverage for any tunnel length, with no dead zones.
The radiating cable is installed at the top of the walls or the vault: protected from traffic, accessible for maintenance, and compatible with other tunnel systems (lighting, ventilation).
Emergency break-in: the VBI system
In modern tunnels, the FM/DAB+ system does more than redistribute ordinary programming: it can interrupt it to broadcast emergency messages to vehicles in transit (Voice Break-In, VBI).
Top-priority messages are activated automatically by tunnel sensors (fire detection, traffic anomalies) or sent manually by control center operators. The system overrides any ongoing transmission. For international tunnels, multilingual messages can be configured.
Message types:
- traffic alerts (queue at exit, alternative routes)
- safety warnings (roadworks, stationary vehicles, adverse weather conditions)
- emergencies (fires, accidents, evacuation instructions)
The TP-DAB solution
TP-DAB is the Teleproject system for FM/DAB+ rebroadcast inside tunnels with break-in capability for emergency messages to vehicles. It integrates with the tunnel management system for automatic alarm activation and meets the regulatory requirements for tunnels on the trans-European network.
Regulation
Directive 2004/54/EC, transposed in Italy by Legislative Decree 264/2006, governs safety in tunnels on the trans-European network. For tunnels longer than 1,000 m with traffic exceeding 2,000 vehicles per lane, radio rebroadcast for emergency services is mandatory. Requirements also include user information systems, periodic maintenance plans, and documented functional tests.
Maintenance and reliability
A tunnel radio coverage system requires:
- periodic inspections of component integrity
- signal level checks along the full tunnel length
- content and VBI logic updates for digital systems
- scheduled maintenance of the radiating cable and antennas
Frequently asked questions
What is the difference between an FM and a DAB+ system in a tunnel?
Both use a radiating cable, but DAB+ transmits digitally a multiplex carrying multiple stations and data services (traffic, weather) on a single 1.536 MHz frequency block. FM is analog and occupies approximately 200 kHz per station. The two systems often coexist on the same radiating cable.
Does the VBI system work even where there is no cellular coverage?
Yes. VBI acts on the FM/DAB+ signal redistributed internally, independently of the mobile network. Emergency messages reach vehicles even in tunnels without cellular coverage.
From what length is a radio system mandatory in a tunnel?
Directive 2004/54/EC (Legislative Decree 264/2006) requires radio rebroadcast for emergency services in trans-European network tunnels longer than 1,000 m with traffic exceeding 2,000 vehicles per lane. Shorter tunnels or those outside the TEN-T network may still be subject to requirements from the competent authorities.
Can the radiating cable carry services other than FM and DAB+?
Yes. The same radiating cable can simultaneously carry FM, DAB+, TETRA, DMR, GSM-R, and cellular networks, provided the frequencies fall within the passband of the cable and installed amplifiers.