Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 22

Fig. 3.10 Cumulative
percentage of occurrence of
reflectance factors of
pedestrian's clothing and of
objects dropped on express
ways. Solid lines: clothing of
Hansen and Larsen and
objects of Hirakawa et al.
Broken line: clothing of
Smith

100

Cumulative
frequency
(%)

clothing

80

objects

60

40

20

0

0 0.1 0.2 0.3 0.4 0.5

ˁ

the darkest position on the road (local minimum road-surface luminance) have been
calculated for different combinations of L av ,U o and TI. The basis for the calculations
are the contrast-threshold curves calculated from Adrian's formulae (Appendix B)
for a 30-year-old person and an object seen under an angle of 7 min (observation
time 0.2 s). The angle of 7 min corresponds to an object of 20

20 cm seen from
a distance of 100 m, which in turn corresponds to the stopping distance of a car at
a driving speed of some 100 km/h to 120 km/h. A field factor of 3 has been applied
(see Sect. 3.1.1.1). A common road lighting installation with a TI value of 10 and
30 % will produce a vertical illuminance value that is numerically equal to about
respectively 5 and 7.5 times the value of the average luminance, respectively. The
vertical illuminance values following from this relationship have therefore been used
in the calculations.
Figure 3.12 shows the results for a 30-year-old person for both the cloth (Hansen
and Larson) and the brighter objects (Hirakawa et al.) and for a good overall unifor-
mity of U o =

×

10. With this good uniformity
and glare restriction, the revealing power RP is only 50-60% at a lighting level of
L av =

0.4 and a good glare restriction of TI =

0.5 cd/m 2 . It increases to 70-80 % at a lighting level of 1 cd/m 2 and to 80-90 %
at 2 cd/m 2 . This shows that at the lower end of the range 0.5 cd/m 2 to 2 cd/m 2 (shaded
area in the figure), revealing power is not good enough and that at the higher end of
that same range, revealing power scarcely improves further. Good road lighting, for
high speed roads, requires lighting levels of between 1 and 2 cd/m 2 for 30-year-old
motorists. The fact that in this range, the revealing power of the brighter objects is
lower than that of the darker cloth, once again illustrates the silhouette concept of
road lighting. In both cases the majority of the objects are seen in silhouette (negative
contrast). In the case of the brighter objects more of these shift into the invisible area
between negative and positive contrasts, leading to a loss of negative-contrast vision.