22 August 2006
provides excellent peripheral detection compared to high-pressure
Reflected Glare: Wet surfaces often provide a surface that has the
potential for reflected glare. Select and locate luminaires to minimize this
as much as possible.
Shadows: Locate luminaires to eliminate shadows that could hide
Source / Task / Eye Geometry: Consider the angles between the driver,
the luminaires, and the tasks. The tasks may be just the roadway or it
may be pedestrians in a crosswalk or other cars at an intersection.
Vertical Illuminance: Vertical illuminance serves to light objects that may
be hazards as well as pedestrians that may be near a roadway or in an
intersection. See IESNA RP-8-00.
Small Target Visibility (STV): Varies, based on application. See IESNA
Luminance: Varies, based on application. See IESNA RP-8-00.
The visual environment needs along the roadway can be described in terms of
pavement illuminance and luminance, uniformity and direct glare produced by the light
sources. (Lighting Handbook page 22-9).
There are three calculation methods available for roadway design: small target visibility,
luminance and illuminance. Since Roadway lighting is a specialized design area, refer
to IESNA RP-8-00 for specific design criteria.
Small target visibility (STV) is the preferred method since it best accounts for identifying
an object or pedestrian crossing the roadway. In order to achieve a high STV, luminaire
placement and arrangement is critical. For example, median mounted poles or poles
lined up opposite each other produce the highest STV values. A staggered pole
arrangement produces the lowest STV values. If safety is a concern, then the preferred
method is small target visibility.
Luminance method calculates the pavement brightness as seen by a motorist. The
glare potential from the roadway luminaires is taken into account with the veiling
luminance calculation. The Luminance method is an excellent alternative method since
it realistically models the pavement brightness and its uniformity.
The Illuminance method is not recommended since it usually produces poor STV
results. Traditionally, the illuminance method was used in roadway calculations since it
is simple to calculate and obtain field measurements.
The recommendation is to use STV first and luminance method as a final check for
roadway lighting quality.
For roadway lighting applications where peripheral vision is important such as detecting
pedestrians or potential off axis activity, white light as produced by a metal halide,
fluorescent, or induction lamp is recommended. In addition to providing better visibility,