Signposting is one of the means available with the operator for communicating with the user.
For a given type of road, one can see in a tunnel the same signposting as in open air:
The different safety devices available to the users in tunnel (emergency telephones, extinguishers, emergency exits...) require in addition a specific safety signposting.
The principal problem faced for signposting in tunnel is the location. In fact, the geometrical characteristics of an underground tunnel are optimised and increasing the transversal section will lead to significant surplus costs. In practice, a compromise must be found between the need for good visibility of the signs (therefore, panels sufficiently large) and the space available.