What the DarkSky International Study Found

DarkSky International's analysis of Washington DC's outdoor lighting measured actual roadway illuminance across the city's eight wards and compared it to AASHTO minimum recommended levels for roadway lighting. AASHTO minimums represent the floor — the lowest level considered adequate for safe roadway visibility under standard conditions. Illuminance significantly above the minimum is not safer; it is brighter than necessary, and it produces the glare, light trespass, sky glow, and health impacts that excess outdoor lighting generates.

The study found that roadway illuminance exceeded AASHTO minimums citywide by an average of 181%. In Wards 7 and 8 — the two wards east of the Anacostia River, which have historically received less infrastructure investment than wealthier western wards — the exceedance was 238%. The over-illumination in the most over-lit wards is 57 percentage points above the already-excessive citywide average.

Why Over-Illumination Is Not a Safety Benefit

The intuitive assumption is that more light equals more safety. This assumption is wrong — and the AASHTO standard is one of the clearest expressions of why. AASHTO minimums define the illuminance floor for safe roadway visibility. Above that floor, additional illuminance does not improve safety outcomes. What it does produce is:

• Glare: Excessive brightness at the source reduces visibility by overwhelming the visual system — a driver looking toward an over-lit roadway has reduced ability to see pedestrians, cyclists, and obstacles in the darker zones between fixtures
• Adaptation impairment: Eyes adapted to a very bright lit zone lose sensitivity in adjacent darker areas — the contrast between lit and unlit zones is actually more dangerous than uniform lower-level illumination
• Melatonin suppression: Blue-rich LED output at excessive levels suppresses melatonin production in residents exposed to light through windows — the health impact that DarkSky's study specifically cites in connection with the Ward 7 and 8 findings
• Sky glow contribution: Over-illuminated streetscapes scatter more light upward than appropriately illuminated ones — contributing disproportionately to the sky glow that the PNAS Nexus study found cities with mitigation policies measurably reduced

What DarkSky Recommends — and What It Means in Specification Terms

DarkSky International's recommendations for Washington DC include three specific actions that translate directly into outdoor lighting specification decisions:

The Broader Lesson: LED Conversion Is Not the End of the Process

Washington DC's experience — a major LED conversion that improved energy efficiency while leaving over-illumination unaddressed — is not unique to the District. It is one of the most common outcomes of first-generation municipal LED conversions nationwide. Municipalities that converted streetlights from HPS to LED in the 2015–2022 window frequently maintained or increased lumen output during the conversion, because LED sources are available at high lumen outputs and because the initial driver was energy cost reduction rather than lighting quality improvement.

The second generation of municipal LED optimization — what DarkSky's DC study is calling for — requires going back to completed conversions and asking a different question: not "are these LEDs more efficient than the HPS they replaced?" but "are these LEDs producing the right amount of light, at the right color temperature, with the right shielding, for the illuminance requirements of this specific location?" The DC Wards 7 and 8 data is the clearest demonstration in this library of what happens when that second question is not asked.

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