Tunnel operators have two means of directly communicating with users:
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: