Civil Engineering Reference
In-Depth Information
Chapter 3
Visual Performance for Motorists
Abstract
A motorist's visual performance can be assessed using a number of dif-
ferent performance criteria. The visibility of static objects (obstacles) and dynamic
objects (other road users, including pedestrians) and the visibility of changes in the
visual scene and the detection of relative movement are examples of important crite-
ria. Detection just on the threshold of visibility is often not enough for safe driving:
good supra-threshold visibility increases the chance of a motorist reacting in time.
Another aspect, important for decreasing reaction time, is peripheral vision: noticing
something out of the corner of the eye, “off” the line of sight, that needs attention. As
will be explained in this chapter, peripheral vision is different in the mesopic vision
range as compared to the photopic vision range.
In order to be able to draw conclusions as to what lighting quality is needed under
different situations, the individual relationships between photometric lighting para-
meters and the different performance criteria, such as revealing power, total revealing
power, visibility level, small target visibility and relative visual performance, have to
be studied. Fortunately, there are many research results on the relationships between
the most important performance criteria and the various lighting parameters. These
will be dealt with here. The impact of adverse weather conditions and of the effect
of vehicle lighting on the visual quality obtained from fixed road lighting, are other
items dealt with here. Finally, the neurological influence that lighting could possibly
have in helping to keep the motorist alert will be discussed.
3.1
Object Contrast
3.1.1
Contrast and Contrast Threshold
At the relatively low lighting levels common in road lighting, colour vision is poor
and visual detection is made possible more by the difference in luminance between
an object and its background (viz. the luminance contrast), than by any difference in
colour. An object has a luminance contrast (C) defined by:
L
o
−
L
b
C
=
L
b
in which
L
o
is the luminance of the object itself and
L
b
the luminance of the back-
ground against which it is seen. If an object is darker than its background it will be
Search WWH ::
Custom Search