Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 32

visibility metrics such as small-target visibility, total revealing power, and now also
relative visual performance can be important as a comparison or fine-tuning tool for
road-lighting installations that have been designed on the basis of pure photometric
criteria but that can benefit from further optimization Another example of the use of
these tools is analysing lighting effects at locations, such as pedestrian crossings and
road intersections, where clearly-defined, specific visibility tasks are key (Rea et al.
2010 ).

3.4 Visibility on Road Surrounds

One of the many vision-related tasks of a driver concerns the detection of objects that
are not on the road but in the adjacent areas of the road. He must be able to decide
whether or not these are moving towards or into his line of travel. Road lighting can,
and should, facilitate this task.
An example of the work carried out concerning this aspect of road lighting is
the study conducted at Philips 2nd generation open-air road-lighting laboratory
(Fig. 3.20 ) (Van Bommel and Tekelenburg 1986 ). Each of ten drivers was asked
to drive along the laboratory's test road, where they were called upon to react to
the presence of pedestrians by actuating a micro-switch on the steering wheel. The
pedestrians were instructed to appear at random positions, not on the traffic lane but
three metres from its border on the adjacent surroundings of the road (road verge).
This test thus involves peripheral or off-line vision. Plastic cones were used to make
the narrow driving lane slightly less than straight so that the drivers were obliged
to keep their eyes on the traffic lane ahead. The detection of the pedestrians on the
surrounds was therefore indeed “off” the line of vision. From the point where the
drivers reacted to the presence of a pedestrian, the remaining distance to the pedes-
trian was determined. The tests were carried out for different luminances of the road
and for two different situations of the lighting of the road verge: one where the verge
was lit to 40 % of the illuminance on the traffic lane itself (“bright surrounds”) and
another one with no verge lighting (“dark surrounds”)—see Fig. 3.21 . The results are
shown in Fig. 3.22 where the average remaining distance at the moment of reaction,
d remain , is given as a function of the average luminance of the traffic lane, L av .For
the situation with the bright road-side surrounds a clear trend is evident: the higher
the average luminance of the traffic lane, the greater the remaining distance to the
pedestrians at the moment the drivers reacted.
For the situations with dark road-side surroundings, increasing the lighting level
on the traffic lane itself has no positive effect on the detection distance of the pedestri-
ans. As will be shown later in the section on Lighting Quality Parameters (Chap. 8),
this is why many standards and recommendations require a minimum amount of
light on a stretch of some 3-5 m adjacent to the carriageway itself.
One series of tests was carried out without any fixed road lighting but with dipped
car headlights only. This result is also shown in Fig. 3.22 . It can be seen that poor
road lighting (0.3 cd/m 2 or 1 cd/m 2 with the dark surroundings) is no better than the