Discussion of Results

There are several very large questions surrounding lighting calculations for streets and roads. Here are a few worth our consideration.

How has the Luminance standard changed lighting calculations?

The old way of calculating illuminance (incident light) was easy. Place points everywhere of concern and evaluate statistics. Curves are not special and slopes are not either if the software can handle them. Luminance on the other hand, is light reflected back towards the eye of an observer. This required substantial research on observer position as well as empirical measurements for standardized pavement types (the R-Tables). The result is a calculation system that can provide results more in tune with human visibility resulting in superior lighting installations. However, it is problematic. The R-Tables are limited in terms of angular data which limits their application outside of the typical section of road. They also do not represent all modern pavement types. What is needed moving forward is a comprehensive format that can be used to report on any material that might serve as a road surface. This would enable the luminance system for real-world roads with curves and hills, think 3D models. AGi32 already allows users to customize any roadway standard and change observer characteristics, point locations and even mix calculations from other standards.

What is a user to do for real-world projects with curves and hills?

The IES has been rather restrained on this but it has recently come to light that the only way to adequately handle real-world projects (other than straight roads) is to calculate illuminance. The illuminance criterion for roads (and streets) is published in IES RP-8-00, but not explicitly in RP-8-14. The topic was clarified in RP-8-18. Specifically, page 11-9 provides conversion factors for different pavement types (R1-R4) from luminance to illuminance.

The generally accepted method is to develop pole spacing and arrangement to meet the luminance criteria, calculate luminance where you can (straight sections), and then calculate illuminance for everything else to meet the RP-8-18 criteria. Intersections specifically are to be illuminance based.

What are the other standards?

There are several other international standards, all of which are supported by AGi32. Many other parts of the world will use CIE document 140. The Australians use AS-1158, the British standard is BSEN 13201 and the Brazilians use ABNT NBR5101. Each of these standards is unique (although similar in some cases) and different from the IES standard. Each has its merits. If interested you can see exactly how the other standards are set up in AGi32’s Roadway Standards dialog from the Tools menu. If so inclined, you can modify a standard and save it as a new one unique to your study. You can then use it in Roadway Optimizer or the Roadway calculations command we have seen in this course.

IMPORTANT: If non-standard calculations are performed, it is the designer or engineer’s responsibility to make it clear what assumptions were made. As an example, a statement like this might be included with a non-standard calculation grid: “Because of the particular circumstances of this project, for the purpose of calculating pavement Luminance, the distance from the Observer to the calculation grid points is 60 meters instead of the standard 83.07 m.”