Discussion of Street and Roadway Lighting Principals

In North America, roadway lighting criteria are generally set by the Illuminating Engineering Society’s RP-8-14 document and include luminance, illuminance, and veiling luminance. Roadways and Streets are differentiated in RP-8-18 by degree of pedestrian conflict. The calculation principals are the same and this document may often use the terms interchangeably. In this lesson we will be technically be working with a "street". More on this to follow.

AGi32 can also calculate roadway metrics according to criteria set by CIE 140-2000, BSEN 13201-3.2003, and Australia and New Zealand 1158.2-2005. Roadway grids in AGi32 respond to any luminaires in the environment that may affect them. In this class, we will focus on the RP-8-18 metrics.

Luminance

Roadway Luminance is defined as the luminance of the road surface at a specific point, in cd/m2 as seen from a specific observer position. The luminance of a roadway surface depends on three factors: the direction of the light incident on the surface, the directional and quantitative reflective characteristics of the pavement, and the location of the observer (driver). In the IES method, the observer position is 1.45m above the pavement surface and 83.07m back from each computation point along a longitudinal line parallel to the direction of travel. Observer line of sight is 1 degree below horizontal. The observer position changes relative to the curb line to align with each row of computation points.

The IES has developed four categories that describe the majority of road surface types in North America. Their reflection characteristics are described by “R-Tables,” which AGi32 accesses to calculate roadway luminance, as well as veiling luminance and visibility level.

Properties of Luminance

Roadway Luminance grids are created in linear segments. The Roadway Luminance grid represents the calculation points for a single direction of flow of traffic. To consider the entire roadway, a calculation grid should be created for each direction of traffic flow. The directional flow of traffic is determined by the order that the curb points defining the length of the roadway are specified in. The direction of traffic flow is from the first point to the second point. AGi32 will set the spacing of the calculation points according to RP-8-18 (or other standard selected).

In AGi32, Roadway Luminance operates differently with the Direct-Only Calculation method than with the Full Radiosity Calculation method.

Direct-Only Calculation Method: The Roadway Luminance calculation takes obstructive but not reflective entities around or within the luminance grid into account. This is also true for the associated illuminance and veiling luminance calculations.

Full Radiosity Calculation Method: The Roadway Luminance calculation takes reflective and obstructive entities around and within the luminance grid into account, as long as the surfaces are defined as Roadway Contributors and face toward the Roadway Luminance grid. In addition, a Roadway Pavement surface type is available for visualizing the pavement in Render Mode using the assigned R-table reflectance value (derived from Q0). This is also true for the associated illuminance and veiling luminance calculations.

Roadway Luminance grids are created in linear segments, a straight section of road. Luminance per IES RP-8-18 (and others) using the current system is not capable of accurately computing curved of sloped sections of roadway.

Veiling Luminance

Veiling luminance (Lv) is a measure of disability glare. IES defines veiling luminance (candelas/sq.meter) as a luminance superimposed on the retinal image that produces contrast. It is this veiling effect, produced by bright sources or areas in the visual field, which results in decreased visual performance and visibility.

Properties of Veiling Luminance

Veiling luminance is computed at the same points as the pavement luminance, with the observer 83.07 meters back from each computation point. The observer’s line of sight is assumed to be a line parallel to the centerline of the roadway but downward at one-degree angle with the observer at a height of 1.45 meters above the roadway surface.

Luminaires shielded from the view of an observer, as by a vehicle’s windshield, do not contribute to glare. However, RP-8-18 does not make any allowance for shielding, and all luminaires are included in veiling luminance measurements and calculations. As a practical matter, since roadway luminaires do not generally generate their maximum candela values within 45 degrees of nadir, this has worked adequately.

Veiling luminance changes with each observer position and with each change in line of sight as the observer moves along the roadway and glances at different objects. In order to simplify the situation for roadway calculations, the specific restrictions described above are used.

Veiling Luminance Ratio

The IES defines this as the maximum veiling luminance (Lv-max) divided by the average pavement luminance (Lavg). Note that these values come from two different calculation grids. The Veiling Luminance Ratio is a measure of the disability glare produced by a lighting system.

Illuminance

Because of the way it’s defined, luminance isn’t appropriate for a curvy or hilly road; for these kinds of projects, illuminance may be used. Roadway illuminance is the density of luminous flux (light) incident on a surface from the roadway lighting system. Illuminance is measured in footcandles (fc) or lux. Illuminance is not dependent on an observer’s location or direction of view.

Properties of Illuminance

Because the appearance of the road as seen by the driver is dependent on the amount of light that reflects from the pavement toward the driver, and because different pavements exhibit varied reflectance characteristics, different illuminance levels are needed for each pavement type.

If the Roadway Calculation dialog is used, AGi32 will set the spacing of the calculation points according to RP-8-18 (or other standard selected). If one of the other grid specification methods is used (2-Point, 3-Point, Polygon, etc.), the user may set the point spacing.