The requirement of adequate visibility is essential for safe traffic operations during both day and night operation. Visibility can be separated into at least three classifications when applied to highway driving: perception, recognition, and decision making [5]. Perception involves the condition of our eyes, the quantity and the direction of the available light, size of the object being viewed, contrast of the object against its background, and the time available for viewing the object. Effective roadway lighting can aid in these tasks by providing the quality of light required by the human eye to increase its visual acuity.
The practice of roadway lighting in the United States is governed by tenets published in the ANSI/IESNA RP-8, American National Standard Practice for Roadway Lighting. In all the editions of RP-8 published from its inception in 1928 through 1983, the criteria for roadway lighting design were based on illuminance (horizontal footcandles). Illuminance is a measure of the amount of light that falls upon a roadway surface. In the 1983 version of the document, alternative criteria were used—one in terms of illuminance (footcandles or lux) and the other in terms of pavement luminance measured in candelas per square meter (cd/m2). The preferred method was luminance since it more accurately described that which is perceived by the human eye. Further research into visibility has led to a new concept and provides alternative design criteria that may be used. This alternative set of criteria is based on the concept of providing an adaptation level on and adjacent to the roadway that aids in recognition of low-contrast objects.
The visibility of a stationary object on the roadway of a fixed size and uniform luminance is a function of the following:
1. The contrast between the luminance of the object and its immediate visual background
2. The general level of adaptation of that portion of the retina of the eye concerned with the object
3. The amount of veiling luminance (disability glare) entering the eye
4. The difference in eye adaptation between successive eye movements (transient adaptation)
5. The size, shape, and color of the object
6. The background complexity and the dynamics of motion
7. Visual capability of the roadway user
Visibility level (VL) is a metric used to combine mathematically the varying effects of the several factors listed above on the visibility of a standard observer. VL for an object at a particular location on the roadway viewed from a specified point and direction is the amount above the visibility threshold as seen by the observer. Visibility level is a ratio and has no units. The VL as commonly used is based on detection of a “small target” that is flat and 7 in (18 cm) on each side. Small-target visibility (STV) is the weighted average VL for an array of targets as calculated by the visibility mode. A full and complete discussion of the STV method is included in Annex F of RP-8-00 published by the Illuminating Engineering Society of North America, New York (www. iesna. org).
