High-Mast Interchange Aiming, Glare, and Maintenance Access
A new high-mast lighting system at a rural interchange uses four 40 m poles, each with six 600 W LED luminaires at 4000 K. The design photometric plan predicted average 0.7 fc and uniformity of 4:1, with luminaires aimed 30 degrees from nadir. After energization, a farmhouse 180 m away reports glare into windows, and drivers report a bright pool at the ramp gore and dark pavement 15 m beyond. The maintenance contractor finds three luminaires aimed 10 degrees from nadir and two at 45 degrees; one pole's lowering-device cable is kinked and cannot be operated. No as-built aiming record exists. The owner wants to add more luminaires to fix the dark spots. The system is on a 240 V circuit with a photocell and contactor; no dimming is installed.
1. Are the reported glare and dark spots likely design failures or construction/aiming problems? Explain.
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Adding luminaires is not the first response. The as-built aiming is inconsistent with the design: 10 degrees from nadir concentrates light near the pole, creating the bright pool; 45 degrees throws light outward toward the farmhouse and beyond the intended area. Both reduce uniformity and increase glare and spill. The dark pavement 15 m beyond may be an aiming or optics issue, not a lack of luminaires. Six 600 W LED luminaires on a 40 m pole are high output; adding more could increase disability glare and skyglow. The kinked lowering cable is a maintenance safety defect. First compare as-built aiming, luminaire model, drive current, and photocell schedule with the design. Only after correction should the owner consider supplemental lighting.
2. What records and field measurements should be obtained before changing the design?
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Require an as-built aiming record for every luminaire, including azimuth, tilt, and pole ID; luminaire serial and model; drive current; and the design photometric file. Measure nighttime illuminance on a grid covering the ramps, gore, and mainline, plus vertical illuminance and glare or spill at the farmhouse. Photograph the aiming pattern and check for uplight. Verify the lowering device with the manufacturer's procedure under lockout/tagout. Compare measurements to the design and to the applicable roadway lighting criteria. If the owner has no criteria, establish them before adding equipment. Also check voltage and control operation at each pole.
3. Recommend corrective actions and state what residual risk remains.
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Corrective actions: repair or replace the lowering cable and verify all poles can be lowered safely; re-aim all luminaires to the approved design or to a revised aiming plan; replace misapplied optics if needed; add shields or louvers for house-side spill; reduce output or add dimming and curfew if overlighting is confirmed; and re-measure. If uniformity still fails after aiming, a new photometric design may justify additional or relocated luminaires, but not before. Document before and after measurements and retain as-built records. Residual risk: high-mast glare at 180 m may require shielding or reduced intensity even after aiming.