Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 287

In the event of an accident in the tunnel that also affects the proper operation of
the normal tunnel lighting, emergency lighting is needed. This lighting should give
guidance and visibility to enable motorised traffic to leave the tunnel as quickly and as
safely as possible. It should also enable people leaving their vehicles to evacuate the
tunnel on foot through the tunnel exit or through emergency exits. Emergency lighting
for this purpose is often subject to explicit regional and national legal requirements
which should be followed. Therefore, only some general guidelines will be given
here, which are mainly based on a CIE Guide (CIE 2010 ). Of course, as with stand-by
lighting, the emergency lighting should be uninterruptible. For guidance of motorised
traffic to the exit, the same lighting as described above under stand-by lighting
conditions is required in case of a sudden power failure. To make emergency lay-bys
easily detectable their horizontal lighting level should always be a factor three higher
than that on the adjacent carriageway. Having the lay-bys lighted with a visually-
different type of lighting system, helps in their detection. In that case a lighting
level of only twice that on the adjacent road is needed. For fleeing pedestrians it is
important that the stand-by emergency lighting as described above is supplemented
with evacuation-route guidance lights. These should be positioned at both sides of
the tunnel at a height of less than 1 m to preserve visibility in the case of smoke,
and spaced at a distance of less than some 10 m apart. The emergency exits and their
immediate surrounds (up to some 2 m from the borders of the exit) should stand out
clearly and need therefore a lighting level three to five times the average illuminance
level of the adjacent walls. This lighting should also be on during normal operation
of the tunnel so as to familiarize all users of the tunnel with the location and situation
of the emergency exits. Only in the case of an emergency should green emergency
marker lights (preferably flashing), be on, around the exit doors.

20.10 Short Tunnels and Underpasses

20.10.1 Short Tunnels for Motorised Traffic

A tunnel is generally not in need of daytime lighting if the bright exit occupies a
quite large part of the field of view when seen from a distance equal to the stopping
distance of the motorist (Fig. 20.21 ). Obstacles in such a tunnel or underpass are
silhouetted against this bright exit. If, however, the dark frame round the bright exit
is too large, obstacles will become invisible in the absence of extra daytime lighting.
Whether or not obstacles are silhouetted clearly enough against the bright exit
depends on the following factors:

• the geometry of the tunnel or underpass, especially the length of the tunnel and

whether there are curves and or dips in the tunnel or on the tunnel approach road,

• the distance from which the tunnel is viewed; the greater that distance, is the

smaller the dark frame,