Standards and Recommendations - Road Lighting: Fundamentals, Technology and Application - page 295

the calculation of TI is slightly different from the standard formula:

95

L v
L av 1,05

TI =

21.1.6 Restriction of Flicker

According to the CIE recommendation, the flicker frequency range between 4 and 11
Hz should be avoided when its duration is longer than 20 s. Comparing this require-
ment with Fig. 20.19 of Chap. 20 would seem to show that this CIE requirement is
not strict enough.

21.1.7 Switching Steps

The adjustment of the lighting levels in the tunnel to the changing outdoor lighting
situation can be done through continuous dimming or by switching in steps. The
difference between consecutive switching steps should, according to CIE be less
than a factor 3.

21.1.8 Night-Time Lighting

If the tunnel is on a section of illuminated road, the quality of the night-time tunnel
lighting should be at least the same, in terms of level, uniformity and glare restriction,
as that of the road lighting on the open road. The uniformity of the night-time tunnel
lighting should never be less than that during daytime.
If the tunnel is on a section of unlit road, the average road-surface luminance in
the tunnel should be at least 1 cd/m 2 , the overall uniformity U o at least 0.4 and the
longitudinal uniformity Ul l at least 0.6.

21.1.9 Short Tunnels

CIE requires that all tunnels longer than 125 m should be treated as long tunnels. For
tunnels shorter than 125 m the lighting requirements are dependent on:

• the length of the tunnel,

• whether or not the exit is fully visible from the stopping distance in front of the

tunnel,

• whether daylight penetration is good or poor,