Effective illumination of streets and roadways requires consideration of many factors.
These include: sufficient light levels to allow easy identification of other vehicles, people and objects; control of light distribution to optimize for the target area and minimize stray light; uniformity of light; adequate contrast; and minimal glare.
In addition, other factors such as efficacy of the light source, maintenance requirements and cost will enter into the decisionmaking for street and roadway lighting installations.
This post will focus on one of these factors – light distribution.
Street lights, mounted on poles, can be located on one or both sides of the road, in the center of the road or a combination of locations depending on the application. The light distribution pattern needs to accommodate the width of the road, the spacing between light poles, the height of the light poles and the shape of the illumination so that it is optimized for the target area.
The Illuminating Engineering Society of North America (IESNA) has developed a classification system that provides a simple method of selecting fixtures that provide a light distribution pattern generally suitable for the location of the light fixture relative to the roadway.
Roadway Light Distribution Classification
Lateral distribution (approximately 15° wide) on two sides of the fixture covers the entire width of the road. This distribution pattern is suitable for fixtures located in the center of the street (or walkway). Type I distribution applies primarily to smaller applications.
Lateral distribution (approximately 25° wide) is used for relatively narrow roadways when the fixture is located on one side of the street. It provides a long, narrow pattern useful for side streets and wide walkways.
Lateral distribution (approximately 40° wide) applies primarily to roadways requiring a wider, larger illumination pattern than either Type I or Type II. This distribution is used with fixtures located to the side of the roadway with light projecting outward to fill the entire target area.
Lateral distribution (approximately 60° wide) produces an oval light pattern and is generally used for wide roadways with the fixtures mounted on along the side of the road.
A circular distribution pattern with uniform light density throughout, Type V distribution is meant for fixtures mounted in the center of an intersection or in the median strip of a wide roadway.
Manufacturers of roadway / street / area light fixtures should provide, in the specification sheet, the light distribution types available for a particular fixture. This applies to both traditional HID light source fixtures as well as LED.
While many other considerations will enter into the final decision regarding fixture selection, the light distribution pattern required for a street lighting application, whether new or retrofit, must be determined at the outset.
For best practice guidelines on all aspects of roadway lighting, see ANSI/IES publication RP-8-14
Distribution diagrams above, from IESNA.
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