Alliance for Lighting Information

Recurring Errors in Ordinances: Site Uniformity and Property Line Limitations

by David M. Keith

The application of uniformity ratios in ordinances is an attempt to mandate a certain quality of outdoor lighting. However the authors of the some uniformity standards either do not understand lighting systems or can not do arithmatic.

The problem is in the combination of criteria as applied in ordinances. A reasonable uniformity ratio for a parking lot alone - say 20-to-1 maximum-to-minimum - is revised - "improved" as one planner put it - to a 10-to-1 ratio over the entire site. This extension of area-based uniformity ratios to the entire site demonstrates significant ignorance about the reasonable application of uniformity ratios, since among other things it requires lighting some areas that could otherwise be left dark. Even such "improvement" would be workable - terrifically expensive, but workable - except the same ordinances typically allow a maximum lighting level at the property line of say 0.2 fc (or even 0.1 fc in some cases). Therefore, to meet both the property line limit and the uniformity requirement, the site has to be lighted from one end to the other at between 0.2 and 2.0 fc (or even 0.1 to 1.0 fc). These values are maximum, and so would correspond to "initial" conditions. The corresponding maintained illuminance values (using a LLF of 0.7) are all below the IESNA recommended practices for all walkways (DG-5-94), all parking lots (RP-20-98) and almost all roadways (RP-8-00). Even though the same ordinances include extensive lists of maximum allowable illuminances - all over 2.0 fc - the restrictions of the property line and uniformity will apply and prohibit any higher levels, necessary to meet the current recommended practices of the IESNA.

It seems reasonable to expect that an ordinance that "allows" a stated maximum illuminance should not make it impossible to provide even that restricted level because of additional constraints.

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