Recommended values for illuminance, luminance, and small-target visibility are published as Tables 2, 3, and 4 in ANSI/IESNA RP-8-00, American National Standard Practice for Roadway Lighting, www. iesna. org. These lighting criteria for the three recognized calculation methods are typically adopted by AASHTO and published as a part of the AASHTO Guideline for Roadway Lighting. The Federal Highway Administration, in turn, historically has accepted the RP-8 criteria as adopted by AASHTO for lighting projects partially funded with federal funds. Refer to the latest edition of the AASHTO Roadway Lighting Design Guide for current recommended criteria. Each lighting designer should have access to the latest versions of these documents, which sometimes contain substantial modifications.
In using the AASHTO tables, there may be conditions for which different luminance and illuminance levels are desirable or necessary. The lighting designer should use all available pertinent information in reaching a decision regarding the level to be used for any specific street or highway.
There are many locations where very high levels of luminance or illuminance are provided for streets in the central city business district. This is usually a commercial consideration directed toward making the downtown business area more appealing to shoppers. Levels considerably higher than the levels in the table must be justified on some basis other than solely for the safe and efficient flow of traffic. If higher than recommended levels are desired, the lighting designer should consider using a white light source, such as metal halide, rather than a monochromatic source, such as high- pressure sodium (HPS). Visibility tests have shown there is a lack of contrast with high levels of HPS. The lack of contrast reduces the ability to distinguish an object from its background, the details of an object, or the color of the object.
