Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 288

Fig. 20.21 A short tunnel with the typical dark frame round the bright centre

• the degree of daylight penetration into the tunnel. Tunnels in open areas with no
tall buildings or trees surrounding the tunnel exit have good daylight penetration,
whereas exits facing the north have poor daylight penetration compared to other
directions. High-reflectance tunnel walls improve daylight penetration,
• the type of possible obstacles; larger cars are, of course, more easily silhouetted
against the bright exit than are smaller pedestrians and cyclists. The latter may
become easily invisible against the dark frame.
A measure for the geometric and observation distance aspect is the so-called Look-
Through Percentage, LTP, defined as the perspective area of the bright exit relative
to that of the entrance (Schreuder 1998 ). In a formula (Fig. 20.22 ):

area EFGH
area ABCD
The ceiling area is not taken into account, since obstacles are normally not large
enough to be seen against the ceiling as background. Daylight penetrating the tunnel
may brighten a small stretch of road surface and walls at the entrance and exit. The
depth of the dark frame is therefore shortened: at the entrance with some 5 m and at

LTP = 100 ∗