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Adaptive Lighting in RP-8-25: When Dimming Is Appropriate and When It Isn’t

Why Should I Care About Dimming?

A dimming schedule is not a safety plan, even when it comes in a nice PDF.

I’m launching this monthly series because the conversation around solar powered street lighting is getting louder, while clarity is not keeping pace. In this space, adaptive lighting and dimming are increasingly discussed as if they are automatically appropriate simply because controls exist and because RP-8-25 references adaptive operation. In practice, responsible application depends on conditions, minimum criteria, and operational safeguards that often go unmentioned in everyday product messaging. When those details are missing, owners and project teams can be left making safety-related decisions with incomplete guidance.

This series is focused specifically on solar powered street lighting, where energy constraints, autonomy targets, battery protection logic, and seasonal variability can strongly influence dimming behavior and performance. As a roadway lighting specifier and designer, I’m in a position to translate RP-8-25 into practical expectations that support good specifications and defensible outcomes. The goal is not to criticize any company or product; the goal is to provide clear interpretation in my own words while pointing readers to the relevant RP-8-25 sections for verification.

This series does not address grid connected roadway lighting except where comparison is necessary to explain solar-specific constraints.

What RP-8-25 means by Adaptive

Adaptive lighting means matching the roadway conditions not running a dimming profile, hoping it works, and calling it a strategy.

In solar roadway lighting, dimming is often presented as a simple win: lower power, smaller panels, longer autonomy. Some manufacturers elect to support that pitch by saying, in effect, that "RP-8-25 allows dimming." What is usually missing is the part that matters most: the conditions that make a reduction appropriate, and the minimums that still have to be met.

RP-8-25 does discuss adaptive lighting. But it frames adaptive lighting as a criteria driven design and operations practice - not a universal dimming profile that can be applied everywhere by default.

If you are specifying or reviewing a system, one question forces clarity: At the lowest dimmed level, what roadway lighting basis is still being satisfied, and what documentation demonstrates that it is being met?

RP-8-25 describes adaptive lighting as varying lighting levels during off-peak periods using controls such as time schedules and, where suitable, occupancy-based control. The practical implication is straightforward: each operating level should be defensible for the conditions it is intended to serve. Adaptive is about matching light to need and not lowering light simply because a controller makes it possible.

The Floor You Can’t Dim Past

RP-8 isn’t a dimmer switch, it’s a guardrail that applies to every type of roadway differently.

Collector and major (arterial) streets

For collector and major streets, RP-8 establishes that reductions should not extend below the lowest applicable criteria for that roadway classification (based on the selected activity assumptions). Adaptive controls may support movement between higher and lower activity conditions, but the lowest applicable criteria remains a floor.

Freeways and highways

For freeways and highways, RP-8 is more conservative. It cautions against reducing illumination below recommended levels without additional research and assessment. Proposals that rely on deep reductions in high speed environments should be treated as exceptional and should be justified with clear inputs, risk review, and owner acceptance.

Local roads and low-speed contexts

RP-8 acknowledges that off-peak reductions may be considered in certain low speed contexts. It also makes clear that reductions should be evaluated and accepted by the owning jurisdiction. In other words: possible does not mean automatic. The burden is on the project team to show that the reduced state is appropriate for local conditions and expectations.

Annex K: What Responsible Adaptive Lighting Looks Like

Think of Annex K as the recipe card for adaptive lighting. It tells you what inputs matter and how to adjust responsibly, so you’re not just turning the dial and hoping the roadway still behaves the same.

Part 2, Annex K provides a structured way to evaluate whether a change in operating level is warranted. It links roadway and environmental characteristics to a selected lighting class using defined inputs and weighting factors. Adaptive operation is then achieved by changing those inputs as conditions change (for example, changes in traffic volume, speed, or pedestrian presence).

Annex K also calls out operational realities that are often absent from generic dimming schedules: coordination at conflict areas (such as intersections and crosswalks), consideration of how adjacent segments work together, and caution during adverse weather. These points matter because the driver and pedestrian experience is continuous; a corridor does not behave like isolated lighting islands.

The Questions that Decide Whether Dimming is Responsible

Dimming profiles are like diets, they sound great until you ask what the rules actually are. These questions separate responsible adaptive lighting from “turn it down and hope it works.”

Some manufacturers elect to treat "dimming allowed" as if it is a blanket permission. In practice, adaptive lighting only holds up when the inputs, constraints, and operational rules are defined up front and when the dimming logic is coordinated with the whole roadway system, not just a single luminaire. Before any dimming profile is accepted, these questions should be answered clearly and in writing:

1. What criteria is each dim step designed to satisfy (by roadway segment and conflict area)?
2. What is the minimum operating level - and how does it relate to the lowest applicable criteria for that roadway type?
3. What triggers the change, and how is the trigger validated (time schedule, occupancy/activity sensing, speed, volume, special events)?
4. Is the dimming plan coordinated across adjacent segments and conflict areas, so the corridor behaves coherently?
5. What happens during adverse weather - is dimming suspended, limited, or reverted, and what triggers that response?
6. Will the owner actually realize savings (for example, does the tariff or metering approach recognize reduced consumption)?
7. Where is the operational data logged, and who owns it (states, overrides, exceptions, faults, maintenance actions)?

What to Require in a Specification (especially for solar)

Solar is where good intentions meet physics. If the dimming profile is really just the battery’s survival mode, it’s not adaptive lighting, it’s an under-sized system.

You can keep the conversation neutral and enforceable by requiring the following deliverables. This avoids naming vendors while still forcing the proposal to show its engineering basis:

• A dimming narrative tied to roadway classification, activity assumptions, and corridor context (including conflict areas).
• Declared minimum operating floors aligned to the lowest applicable RP-8 criteria for the roadway type.
• Defined inputs and rules (what is measured or sampled, thresholds, and decision logic).
• Transition behavior (ramp rates and coordination so changes are not abrupt or inconsistent).
• Weather and exception handling (lockouts, overrides, special events logic, and fail-safe behavior).
• A tariff/metering plan if energy cost savings are being claimed.
• Audit logs and reporting (operating states, overrides, failures, and maintenance history).

Closing thought

This is a safety system, not a mood setting.

Roadway lighting is a safety system, not just an energy line item. Adaptive lighting can be a strong tool when it is implemented as RP-8 frames it: contextual, criteria-driven, and bounded by roadway type and risk. The simplest test is still the most effective: at the lowest dimmed level, what is being met and how will you prove it?

References and where to verify in RP-8-25

• ANSI/IES RP-8-25 (2025), Part 1: Sections 5.2.9-5.2.10 (context and definitions related to control systems and adaptive lighting).
• ANSI/IES RP-8-25 (2025), Part 1: Section 6.10.2 (adaptive lighting design considerations and cautions).
• ANSI/IES RP-8-25 (2025), Part 1: Sections 6.10.2.2.2-6.10.2.2.3 (roadway-type boundaries; collector/arterial floor; freeway/highway cautions).
• ANSI/IES RP-8-25 (2025), Part 1: Sections 6.10.3 and 6.10.3.7-6.10.3.7.1 (operations, metering, and tariff considerations).
• ANSI/IES RP-8-25 (2025), Part 1: Section 6.10.1.2.4 (data and system capabilities relevant to verification and accountability).
• ANSI/IES RP-8-25 (2025), Part 2: Annex K (especially K.2, K.2.2, K.5.2, and K.5.3) for adaptive methodology, scheduling considerations, conflict-area coordination, and adverse-weather cautions.
• National Highway Traffic Safety Administration (NHTSA), Traffic Safety Facts: Pedestrians (2023) - fatality distributions by light condition (for general context on night-time risk).
• Federal Highway Administration (FHWA), Design Criteria for Adaptive Roadway Lighting (FHWA-HRT-14-051) - background research and methodology context.

Piotr Mikus is a roadway lighting designer and specifier focused on solar powered street lighting and controls.