Intersection Retrofit with Reduced Mounting Height and Glare Complaints
A city is replacing four 400 W HPS luminaires at a signalized intersection of a four-lane divided arterial and a two-lane collector. Existing poles are 10.7 m high, set 1.5 m behind the curb at each corner, with 2.4 m arms. The proposed LED luminaires are 240 W, 4000 K, mounted at 9.1 m on the same poles with 1.2 m arms. The arterial approach speed is 55 km/h. A two-story residence is 12 m from the northeast pole, and bedroom windows face the intersection. The photometric sheet claims average 1.0 fc and max/min 3:1, but it uses a 0.5 maintenance factor and shows no glare, spill, or vertical illuminance. The utility will supply 120/240 V single-phase. Residents already complained about glare from the HPS. The contractor wants approval to order poles and luminaires.
1. Should the engineer approve the proposed luminaire and pole arrangement? Identify the specific deficiencies and explain how each affects intersection lighting performance.
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Do not approve. The proposal changes the optical geometry, not just the source. Lowering the mounting height from 10.7 m to 9.1 m while using a high-output 240 W LED increases the potential for disability glare and reduces the distance over which the luminaire can distribute light across the intersection. Shortening the arm from 2.4 m to 1.2 m pulls the luminaire back from the traveled way and can leave the crosswalk and stop-bar area underlit while putting more light toward the nearby residence. The photometric sheet is inadequate: a 0.5 maintenance factor may be optimistic, and average fc plus max/min does not prove intersection uniformity, glare control, or spill control. For an intersection, review should include the whole conflict area, crosswalk vertical illuminance, uniformity including max/avg, veiling luminance or glare, and light trespass at the bedroom windows. The HPS complaint history shows the community is sensitive to glare. Approval should wait for a complete layout using actual LED photometry and site geometry.
2. What photometric and field information should be required before approval, and what acceptance measurements would you specify after installation?
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Require a complete photometric calculation using the selected luminaire's IES file, actual mounting height, arm length, tilt, maintenance factor with documented lumen and dirt depreciation, and a grid covering all approaches, crosswalks, and the residence facade. Ask for calculated average and minimum illuminance, max/min and max/avg uniformity, vertical illuminance at crosswalks, veiling luminance or a glare metric, BUG rating, and spill values at the windows. After installation, verify aiming and tilt, measure illuminance on the same grid at night with a calibrated meter, check voltage at the first and last pole, and measure spill at the residence. Acceptance should require as-built records, not just a catalog cut sheet. If criteria are not met, the contractor should adjust aim, shielding, or output before final acceptance.
3. If the poles must stay in the same locations, propose practical mitigation and state the residual risk.
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If poles cannot move, first re-aim and shield. Use house-side shields and backlight control, verify cutoff optics, and consider reducing drive current or dimming during late-night hours. Increasing mounting height or arm length may help only if the foundation and pole can accept the load; a structural check is required. Relocating the northeast pole a few meters away from the residence or shifting it to the far side of the intersection may be more effective than adding luminaires. Do not add output to fix uniformity; that usually worsens glare and spill. Residual risk: lower mounting height and fixed pole locations may still produce spill. Document the compromise, obtain owner and resident acceptance, and re-measure after changes.