Equipment: Road Surfaces - Road Lighting: Fundamentals, Technology and Application - page 173

Fig. 12.10 Specular factor S1
and average coefficient Q 0 for
a road surface in its dry and
standard wet condition. The
dashed lines indicate the
condition where there is no
difference between the dry
and the wet condition. Based
on Danish road surfaces
(Sørensen and Nielsen 1974 )

16

S1 wet

12

8

4

S1 wet = S1 dry

0

0 0.5 1.0 1.5 2.0

S1 dry

0.3

Q 0 wet

0.2

Q 0 wet = Q 0 dry

0.1

0

0 0.05 0.10 0.15 0.20

Q 0 dry

for the W classes and the properties of the W standard tables are given in Table 12.3 .
The W classification system with its W1-W4 standard R-tables makes it possible to
make luminance calculations not only for the dry condition but also for the (standard)
wet condition. Comparing the class limit of the WI class with Fig. 12.10 shows that
most road surfaces of class WI correspond to road surfaces that belong to class RI in
the dry condition.
Because of the fact that, as we have seen, in the wet condition both the S1 and Q 0
value of a road surface changes, Frederiksen and Sørensen ( 1976 ) defined a specific
specular factor for the wet condition. This they called the corrected specular factor
S1'. It is defined on the basis of both the S1 wet and Q 0 wet value as follows:
log S 1
0 . 147

log S 1 wet

0 . 147

=

1 − Q 0 wet / 0 . 687

Table 12.3 W-class limits and class-standards for wet road surfaces
Class name Class limits Class standard Standard Q 0

WI S1 < 4.5 W1 0.114

WII 4.5 ≤ S1 < 7.2 W2 0.150

WIII 7.2 ≤ S1 < 9.8 W3 0.196

WIV 9.8 ≤ S1 < 12.0 W4 0.247