Equipment: Road Surfaces - Road Lighting: Fundamentals, Technology and Application - page 168

Fig. 12.5 Road-surface class
limits of the R classification
system with the standard Q0
value for each class

Lightness

Q 0

0.10

R1

0.08

R4

0.07

R2 R3

Class I Class II Class III

Class IV

0 0.42 0.85 1.35

Specularity S1

be possible to determine the parameters of the model, and subsequently the whole
R-table by just a few luminance coefficients. Possibly, this method would also enable
us to determine from a model valid for an observation angle of 1 ⓦ , indicatrices, and
thus R-Tables, for observation angles other than 1 ⓦ . CIE has set up a new Technical
Committee to draft a report based on current road surfaces.

12.3 Classification System

A classification system of road surfaces has been developed with the purpose of
obtaining a reflection table without the need for measuring the actual road surface.
The requirement in developing the system, of course, was that luminance calculations
based on such a reflection table would give accurate enough results. Each type of
dry road surface is placed in one of four classes according to its S1 value and thus
according to its degree of specularity. Figure 12.5 shows the four classes with the
class limits of the R-classification system defined by CIE ( 1984 , 2001 ). The higher
the class, the larger the degree of specularity of the road surfaces in that class. By
accepting a certain inaccuracy it is possible to characterise each road surface in each
class by only one reflection table typical for that class. In other words, each class
can be assigned a standard reflection table with which luminance calculations for
surfaces falling in that class can be made. CIE has specified these standard reflection
tables, the so-called R1-R4 tables. The reflection table shown in Table 12.1 is in fact
the standard reflection table R3 for class 3. As the standard reflection tables are scaled
for a Q0 value of 1.0, the tables must be rescaled according to the actual average
luminance coefficient Q0 of the road surface in question for which the luminance
calculations are to be made. Often, however, the actual Q0 value of a road surface will
not be known. For these situations CIE defined standard Q0 values for the different