Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 39

Fig. 3.27 A set of objects
typical of pedestrian clothing
( top ), seen with road lighting
only ( middle ) and with the
combination of road lighting
and a driver's car lights that
make the objects brighter
( bottom )

road lighting only

road lighting + car lights

darker background (in positive contrast). Here, the higher the vertical illuminance
on the road and the lower the road-surface luminance, the better the visibility of any
objects on the road. Fixed road lighting and car headlights thus influence each other
negatively. With dipped, or low-beam, halogen car headlights the negative influence
is only noticeable up to some 60 m from the car. With low-beam high-intensity
discharge car headlights the negative influence may reach as far as some 80 m. With
main, or high-beam car lights, however, the negative influence is noticeable much
further. The principle of the effect is shown in Fig. 3.27 , where the middle part shows
the effect of “road lighting only” on the visibility of a set of objects, and the bottom
part the effect when the objects all receive the same amount of extra light from the
car headlights.
Figure 3.28 shows the reflectance-factor range that is invisible 100 m in front of a
car in a situation with only fixed road lighting (car headlights off) and with fixed road
lighting and own headlights on. The example is given for a road-lighting installation
of 1 cd/m 2 average road-surface luminance. With headlights switched on, the range
of invisible objects shifts to those with lower reflectance factors that in practice do
in fact occur on the road more often, as also indicated in the figure.
The overall conclusion when comparing the situations with car headlights only and
car headlights with fixed road lighting is that the visibility of most objects is improved
by introducing road lighting, especially of those objects at greater distances where
headlights (dipped-beam or low-beam headlights especially) gradually become less
effective.

3.7.2 Glare from Oncoming Cars

On unlighted roads, glare from oncoming cars, especially those with badly-aligned
headlights, has a pronounced negative effect on visual performance. Glare is then
often experienced, at best, as being very annoying, and at worst as threatening. On