The safety benefits of outdoor lighting are well-established. On roads, it prevents crashes. In neighborhoods, it deters crime. In parks and other public spaces, it encourages recreation and social interaction, creating safe, usable places for people to be after dark.
It’s more or less common knowledge at this point that lighting an environment makes it safer, and few people—least of all us!—would argue its presence should be reduced. And yet, that’s precisely what adaptive lighting asks us to do: reduce light levels during off-peak periods or, as the FHWA puts it, “use lighting only when it is required and at the appropriate level to provide [safety].”
For agencies who equate more light with more safety, this can be an unsettling proposition, particularly for high-risk applications like roadways. But adaptive lighting is neither new nor radical, and with the release of updated guidelines from the Illuminating Engineering Society (ANSI/IES-RP-8-22), there’s more support than ever for implementing it—and reaping the benefits.
Adaptive lighting might be best understood by what it isn’t: static. Unlike a time clock, which switches lights on and off at predefined times each night, or a photocell, which performs the same function based on the amount of light it senses, adaptive lighting uses a control system (either centralized or localized on each luminaire) to raise or lower the light level in response to changes in the environment.
These changes can be all sorts of things, from traffic volume to weather conditions to pedestrian usage—all of which can impact the amount of lighting required. For example, on residential streets, pedestrian volumes tend to follow a similar, predictable pattern, peaking early in the evening before dropping off around 10 or 11 p.m. In retail districts, activity generally mirrors business hours. With adaptive lighting, light can be provided when it’s needed and dialed back when it’s not.
Example lighting schedule from the City of San Jose’s Streetlight Design Guide. The city was one of the first in the country to recognize the benefits of adoptive lighting and formalize its implementation.
For agencies that use traditional AC-powered lighting, the impetus to try adaptive lighting is obvious: less energy consumed, less strain on an overtaxed grid, and fewer digits on their utility bill. But there are reasons why we, a solar lighting manufacturer (whose products are already off-the-grid and free to power), are excited about adaptive lighting too.
For starters, though we might not be concerned with reducing energy consumption for financial reasons, we’re still very interested in lowering the amount of energy required to power a system. Why? Because a lower energy requirement—we call it a load—means we can use fewer and smaller solar panels and batteries, which brings down both system size and cost. (Smaller systems have a lower weight and EPA, meaning cheaper poles can be used; batteries and solar panels add weight and expense, so the smaller, the better!)
Reducing system load also means we can make solar lighting work in locations that may have been difficult if a constant output were required. Whereas a solar streetlight in a place like Kalispell, Montana, might not be able to collect enough energy to provide 12,000 lumens from dusk ‘til dawn in the depths of winter, dimming the light to, say, 50% between midnight and 5 a.m., could make it a sustainable solution. (Rest assured—we always do the math to ensure our systems are properly sized.)
Reducing system load using adaptive lighting extends the reach of solar lighting to high-latitude locations that receive less solar radiation.
Finally, we’d be remiss not to mention the environmental benefits of adaptive lighting. Reducing a fixture’s output, even for just a few hours, can significantly cut down on light pollution, which has been found to mimic, mask, or confuse natural light signals, for both animals and humans. Adaptive lighting minimizes the negative impact of light on our ecosystem, ensuring nighttime illumination that is low-level, controlled, and useful (which just so happen to be three of the IES’ Five Lighting Principles for Responsible Outdoor Lighting!)
Now that we’ve sold you on the benefits of adaptive lighting, let’s return to RP-8-22, which contains the most current recommendations on where and how to implement it in the United States (for our international readers, the Transportation Association of Canada and the Institution of Lighting Professionals in the UK publish similar guidelines).
RP-8-22 uses two criteria to determine the amount of light required for a street or sidewalk: the classification of the street and the level of pedestrian activity. The classification is based on the number of lanes and traffic volume and is defined by the City (many cities have maps showing their roads’ classifications). The pedestrian activity level is usually an estimate of the number of pedestrians on a single block of sidewalk during the hour of highest nighttime pedestrian conflict (some cities have this too, though obtaining quality pedestrian data is notoriously difficult).
In the table above, the pedestrian activity classifications are as follows: A volume greater than 100 pedestrians is high, a volume between 10 and 100 is medium, and a volume of 10 or fewer is low.
For adaptive lighting, we’re most interested in the times pedestrian volumes drop from one level to another. For example, if pedestrian volumes on a local road declined from 120 per hour at 6 p.m. to 30 at 10 p.m., the average luminance (lighting level) could be reduced using RP-8’s recommendations. When pedestrian presence increases in the morning, the light levels could be brought back up, before eventually shutting off after sunrise.
A growing number of municipalities and utilities are now following these updated RP-8 recommendations when implementing new solar lighting projects since they address previous liability concerns. (Cities, counties, and utilities have been sued in the past for their failure to provide adequate street lighting; however, they are found not legally liable in most cases.)
While the RP-8 says reducing light levels based on pedestrian activity “should be strongly considered” for local, collector, and major (arterial) streets, as well as sidewalks, walkways, and alleyways, it cautions against taking a blanket approach. Scenarios where it should not be implemented include:
Figure 4: Adaptive lighting is not recommended for a number of roadway applications, including roundabouts. For locations that require dusk-to-dawn operation, Sol’s EverGen lights offer a reliable, durable solar-powered option.
Even if a location is a candidate for adaptive lighting, it’s important to take a measured approach that considers local conditions and user needs. Pedestrian data can be difficult to access, and activity patterns complex (for example, the streets around a stadium are likely to be crowded on game nights but deserted on others, making broad changes to the light levels unwise and unsafe). As RP-8 explains, “It is the responsibility of the lighting designer to apply sound rationale and logic in determining pedestrian activity and schedules for the levels.”
For locations where wholesale schedule changes aren’t an option, due to pedestrian usage being unpredictable or ambiguous or where the consequences of not providing enough light are high, there’s an alternative: motion detection. Sensors come in a variety of flavors—radar, infrared, hybrid—and allow the state of the light to be changed when motion is detected (e.g. to turn a light on or to change it from a dimmed to a fully on state).
Motion detection can be useful in various scenarios, including parking lots, industrial areas, parks, and campus walkways. According to RP-8, “The police often prefer motion sensitive lights in alleyways so that activity can be noticed by neighbors or from adjoining streets, as the lights’ turning on will indicate activity.”
Wherever they’re used, motion detectors confer many of the same benefits as adaptive lighting, providing energy (or solar sizing) savings without sacrificing safety. And if full output is required (say, at a roundabout or multi-use path with high nighttime use) know that Sol’s system are up to the task, providing reliable dusk-to-dawn wherever it’s required.
While agencies in the past may have been reluctant to reduce lighting for fear of decreasing safety, current IES guidelines allow and even encourage adaptability. Several cities, including San Jose, have already adopted adaptive lighting guidelines, and federal grant programs like Safe Streets and Roads for All have specifically named it as an eligible use.
This move toward adaptive lighting is exciting for us in the solar world because it means we can deliver compact and affordable systems to an even wider range of locations and customers—something we’re keen to do! If you’re interested in learning more about adaptive lighting, or if you have a project you think could benefit from it, don’t hesitate to reach out.
