Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 41

Fig. 3.29 Visibility level of
an object (0.20 0.20 m and

reflectance factor 0.20) 40 m
in front of a car with dipped
halogen headlight in different
situations in the presence of
an oncoming car 100 m away
(with dipped halogen
headlight). Road lighting
installation: L av 2.45 cd/m2,
U o 0.6, Ul 0.7 and TI 5 %.
(Based on Bacelar 2004 )

×

no road lighting, dipped beam

no road lighting, dipped beam, oncoming car

road lighting, dipped beam, oncoming car

0

30

10 20

Visibility level

the adjacent parts of the road may further strengthen the signalling effect. For this to
be effective, the light transition should be sharp: that is to say, the extra illumination
should be confined to a narrow band around the crossing area. This also helps to
encourage pedestrians to cross at the crossing point provided. To ensure that the
illuminated crossing stands out against the other road parts, the difference in lighting
level should be at least a factor of 2.
The additional lighting also has an influence on the contrast of the pedestrian seen
against the road surface behind the crossing, as seen from the motorist's viewpoint.
If the additional lights are placed behind the crossing, as seen from the motorist's
viewpoint (Fig. 3.30 top-left and top horizontal bar), the contrast changes into a larger
negative contrast (silhouette vision). However, when the motorist comes closer than
some 80 m to the crossing there comes a point where, because of his own car lights,
the contrast changes polarity. Around the point of polarity change there is a short
stretch where the pedestrian may be invisible. To avoid this change of polarity so close
to the crossing, the additional lighting is best placed before the crossing so that the
extra light is not only radiated towards the horizontal surface of the crossing but also
illuminates the side of the pedestrians seen from the approaching driver (Fig. 3.30
top-right and bottom horizontal bar). In this way the contrast is shifted over the whole
approach towards a more positive contrast. If the effect in the latter case is only weak,
the pedestrians may still become invisible. It is therefore important in this situation
to take care that the vertical (or semi-cylindrical) illuminance is significantly higher
than the horizontal illuminance.
In a field test, Gibbons (Gibbons and Hankey 2006 ; Gibbons 2007 ; FHWA 2008 )
studied the detectability of pedestrians wearing black, denim or white clothing at a
pedestrian crossing. The crossing was on a major road with fixed road lighting and
additional crossing lighting delivering an extra vertical illuminance of 5 lx, 20 lx, 40
lx or 60 lx. The tests were performed by 16 elderly persons (average age 69.5 years)
seated in a stationery car with low beam headlights from a distance of 83 m from the