Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 286

Fig. 20.20 Grouping of
luminaires in the interior zone
avoids flicker problems

80 km/h 5.6 Hz 4 m

100 km/h 7.0 Hz

120 km/h 8.3 Hz s/h = 1

80 km/h 1.9 Hz

12 m

100 km/h 2.3 Hz

120 km/h 2.8 Hz

s/h = 3

20.8 Night-Time Lighting

As has already been mentioned several times, a tunnel is more dangerous than an
open road because in it there is less room available to manoeuvre. Also, motorists
often feel less confident when approaching and driving through the tunnel. At night,
therefore, the luminance level in the tunnel (both on the road surface and on the lower
two metres of the walls) should be no less than that required on the adjacent sections
of open road: preferably one step higher (see Sect. 9.1.1 of Part 1). In cases where
the adjacent open roads have no lighting, the lighting in the tunnel should be at least
1 cd/m 2 . At night, the danger of the black-hole effect occurring at the tunnel exit is
present if the open-road leading from the tunnel exit is not lighted. In order to avoid
this problem and facilitate adaptation, transition lighting along the open stretch of
the road is required. The lighting should cover a stretch of 200 to 300 m and should,
in the case of lighting levels in the tunnel greater than 2 cd/m 2 , gradually decrease
in level, in steps of not greater than 3:1.
Of course, the night-time tunnel lighting should also meet the same uniformity,
glare and flicker-restriction requirements as those recommended for the daytime
tunnel lighting.

20.9 Emergency Lighting

Stand-by lighting is needed in the event of a sudden technical failure in the electrical
installation normally providing the tunnel lighting. Here the task of the emergency
lighting is to enable motorised traffic to safely drive to the exit of the tunnel. For
this purpose a stand-by, no-break power supply is required that automatically pro-
vides uninterrupted electrical power from banks of batteries. This emergency power
provision should supply power to a small number of the luminaires providing the
normal night-time lighting so as to achieve at least 10 lx average illuminance on the
road surface, with a minimum of 2 lx at each individual point on the surface. The
duration required of the stand-by emergency supply is dependent upon the length of
the tunnel. It goes without saying that proper signalling before the tunnel (to stop
traffic entering) and in the tunnel should accompany this emergency lighting.