Intersection Lighting: Adopted Standard Versus Dark-Sky Ordinance
An intersection of a two-lane state route and a county road is being reconstructed. The state route is 45 mph with 8,000 vehicles per day; the county road is 30 mph with 2,500 vpd. A new elementary school and a transit stop are within 600 ft, creating nighttime pedestrian crossings. The only existing lighting is a single 100-W HPS post-top on the far corner. The project manual references the locally adopted IES RP-8 and requires intersection lighting, but the city’s dark-sky ordinance caps average maintained illuminance at 0.6 fc and requires full-cutoff luminaires. The designer proposes two 60-W LED luminaires on the existing corner pole, producing 0.55 fc average over the intersection but 8:1 max/min. The city wants no new poles; the state requires the adopted intersection standard to be met. The contractor has already ordered the two luminaires.
1. What design basis governs, and is the two-luminaire corner layout defensible? Show the reasoning you would use in a review comment.
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The governing basis is the adopted standard for the intersection as a conflict area, not a simple midblock average. First classify the state route and county road, then evaluate pedestrian conflict: the school and transit stop within 600 ft create nighttime crossings and usually raise the required lighting class. The design must address average illuminance, uniformity, and veiling luminance on the intersection and crosswalks. The proposed 0.55 fc average may sit under the 0.6 fc ordinance cap, but 8:1 max/min is not defensible for an intersection with pedestrians; two luminaires on one corner leave the other approaches and the crosswalk dark. The ordinance and the adopted standard conflict: the reviewer cannot silently apply the lower number. The defensible path is a written code-conflict memo, a photometric point-by-point for a compliant layout, and a variance or alternative-compliance approval if the ordinance cap cannot be met.
2. Recommend a layout and control strategy that can satisfy both the safety standard and the ordinance, and describe what must be documented.
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Use a four-corner layout, or two poles with back-to-back or dual arms, with lower-output full-cutoff luminaires aimed to keep light on the pavement and crosswalk. More, lower-output points improve uniformity and reduce glare compared with two larger corner units. If the average must stay at or below 0.6 fc, select optics and wattages that produce a uniform 0.5-0.6 fc over the conflict area, with max/min within the adopted uniformity criterion and controlled veiling luminance. Add adaptive controls: full output during crossing hours and a verified curfew dimming schedule only if the standard and local approval allow it. Document the classification, pedestrian counts, photometric calculations, control settings, and the variance or approval. The city’s no-new-poles preference cannot override the safety standard; if poles are truly impossible, obtain a signed alternative-compliance decision rather than installing a noncompliant layout.