Researchers studying the role of illumination in traffic safety make the case for adaptive control systems.

Roadway lighting offers significant safety benefits but also represents a substantial share of the operating budgets of agencies tasked with maintaining the lighting infrastructure. Standard practice requires that roadway lighting systems, when installed, provide illumination for the maximum vehicle and pedestrian volume, regardless of roadway conditions or real-time roadway usage. Typical practice also requires maintaining a single, specific lighting level over time, which is difficult to do, because as a light source is used, it diminishes in output due to aging of the source and dirt that accumulates on the lens. To overcome the expected reduction in output, engineers tend to overcompensate in the initial lighting design by specifying the use of more intense luminaires to reach the correct lighting level.

These traditional approaches to lighting design result in significant over-lighting of roadways and excessive energy usage. Adaptive lighting, that is, adjusting illumination levels based on the needs of roadway users, offers an approach to overcome these challenges.

Driven by the development of new lighting technologies and a nationwide push to reduce energy use and environmental impacts, adaptive lighting is a growing trend in the roadway industry. It entails the use of a design methodology in which the light output of a system adjusts as traffic conditions change. More specifically, the level of lighting can be reduced or dimmed when traffic on highways or sidewalks lessens. Here’s a look at how adaptive lighting may be implemented while maintaining the safety of road users and how two agencies are deploying this technology on their roadways.

Implementing Adaptive Lighting

Although transportation agencies have begun to introduce adaptive lighting into their roadway projects, the techniques for doing so have yet to be standardized. In 2011, the Federal Highway Administration’s Office of Safety Research and Development launched the Strategic Initiative for the Evaluation of Reduced Lighting on Roadways to investigate the issues associated with the application of adaptive lighting to the roadway environment and to develop recommended practices for implementing those systems.

In July 2014, the FHWA Office of Safety Research and Development released a report titled Design Criteria for Adaptive Roadway Lighting (FHWA-HRT-14-051). This report describes an indepth effort to assess the effect of adaptive lighting on the overall safety performance of roadways. Researchers working with FHWA conducted studies to determine optimal times, conditions, and suitable approaches for reducing lighting; appropriate lighting levels for various roads and features; energy savings and reductions in greenhouse gases resulting from reduced lighting; and potential legal issues.

The resulting design methodology provides a process that transportation agencies can use to determine whether adaptive lighting is appropriate for a given roadway. The researchers proposed a set of criteria to assist jurisdictions in making sound, safety-based decisions when considering adaptive lighting approaches. In addition, the study’s evaluation of real-world lighting data can serve as the foundation for future analyses related to roadway lighting.

Roadway Light, Visibility, and Safety

For part of the research resulting in the 2014 report, the FHWA Office of Safety Research and Development partnered with the Virginia Tech Transportation Institute to investigate how lighting levels affect safety on the road, and to develop an approach for selecting appropriate lighting levels for various roads and features. Previous research had shown that roadway lighting can affect crash risk. For this study, the researchers analyzed the relationship between lighting levels and quality and crash rates. Establishing such a relationship can help agencies determine the optimal lighting level for roadways under various traffic conditions.

Four criteria to consider in the design of roadway lighting are horizontal and vertical illuminance, luminance, and uniformity. Horizontal roadway illuminance is the amount of light falling on the roadway surface. Vertical illuminance is the amount of light falling on a vertical surface, such as a pedestrian. Luminance is the amount of light perceived by the road user, and uniformity is the ratio of illuminance or luminance values, such as maximum to average, average to minimum, or maximum to minimum.

Researchers used detectors mounted on top of a vehicle to collect data to measure each of these criteria on thousands of miles of roadway in seven States (California, Delaware, Minnesota, North Carolina, Vermont, Virginia, and Washington). After taking measurements, the researchers compared the varying lighting levels, roadway characteristics, and traffic volumes with night-to-day crash rate ratios on segments of road. This ratio reflects the relative magnitude of nighttime crash risk compared with daytime crash risk.

Because daytime and nighttime crashes shared the same road design features and traffic control features, the crash rate ratio directly reflects the factors that only differ by day and night, with visibility level being the primary one. Therefore, the night-to-day crash rate ratio indicates the effect of lighting and light levels while controlling for the effects of roadway design, traffic control features, and other roadway characteristics. The researchers considered the night-to-day crash rate ratio to be the primary metric in evaluating the effect of roadway lighting on safety. (Note that weather conditions were not included in this analysis and therefore represent a basis for future research.)

The researchers analyzed more than 88,000 crashes that had occurred from 2004 to 2010. Of those, more than 64,000 crashes had occurred during the day, while nearly 24,000 had occurred at night. The National Oceanic and Atmospheric Administration’s recorded sunrise and sunset times were used to classify the natural lighting into daytime and nighttime conditions.

Horizontal Illuminance

Next, the researchers analyzed roadway horizontal illuminance, or the amount of light falling on the roadway surface, as one of four roadway lighting criteria in the study. They calculated the relationship between the horizontal illuminance of roadway segments and the night-to-day crash rate ratio of those segments.

reduce standard lighting levels by as much as 50 percent.

The results of their analysis show a significant decrease in the night-to-day crash rate ratio with an increase of average horizontal lighting levels, which are measured in foot-candles, or lux. In the study, the night-to-day crash rate ratios for lighting levels 0 and 0.1 foot-candle (0 and 1 lux) are significantly higher than for other levels. However, there is no statistically significant difference for lighting levels from 0.19 to 0.65 foot-candle (2 to 7 lux), and an increase in the lighting level from 0.46 foot-candle (5 lux) to higher levels did not appear to affect the crash rate ratio. There appears to be a further reduction in the crash rate at approximately 1.49 foot-candles (16lux); however, because data on only 28 miles (45 kilometers) of roadway are represented in this category, the result is not statistically significant. These findings indicate that although lighting will benefit road safety, increasing the lighting level does not necessarily always lead to a safer road.

Maintaining Roadway Safety With Less Light

After conducting similar analyses for each of the other lighting metrics, the researchers concluded that the possibility exists for reducing standard lighting levels on roadways during periods of less traffic while maintaining the overall level of roadway safety.

In one case, the data revealed the potential for reducing standard lighting levels on urban interstates by as much as 50 percent. The researchers calculated the minimum illuminance requirement for various road classifications and compared those requirements with roadway lighting guidelines issued by the Illuminating Engineering Society of North America (IES). The research team calculated that the lighting level required for urban interstates is 0.37 foot-candle (4 lux), which is significantly less than the 0.84 foot-candle (9 lux) that IES recommends. The results for other roadway classifications show that the existing IES recommendations are suitable; the researchers’ calculations resulted in lighting levels that fall within the range of IES minimum and maximum standards.

Selecting Lighting Levels

Selecting the appropriate lighting level for various roads and features is a critical aspect of adapting the lighting system. The International Commission on Illumination, a lighting standards organization, devised a system that provides both a methodology for selecting the lighting design level and a method for adapting the lighting level based on specified criteria for individual roadways. FHWA and the Virginia Tech Transportation Institute researchers based their design methodology on this system to comply with the international standard, but the team also proposes a more extensive classification system to take advantage of the benefits of adaptive lighting for roadways. Additional metrics include links to IES requirements and consideration of traffic volume, geometric design, and pedestrian volume.

The proposed system starts with the IES characterization of the facility to be lighted. The IES separates lighting design criteria by its application to roadways, streets, and residential or pedestrian facilities. First, roadway lighting criteria are provided for freeways, expressways, limited access roadways, and roads on which pedestrians, cyclists, and parked vehicles are generally not present. Second, street lighting criteria are provided for major, collector, and local roads where pedestrians and cyclists are generally present. Criteria for residential/pedestrian area lighting are provided primarily for the safety and security of pedestrians and not specifically for drivers. Once a lighting designer selects the facility type, he or she uses the characteristics of the facility as weighting functions to determine the requirements of the lighting system.

The designer then subtracts the sum of these weighting values from a base value. The base value changes based on the facility type. For roadways, the base value is 5. When the sum of the weighting values is subtracted from the base value, the result is called an “H-class.” If the result of this calculation is not a whole number, the next lower positive whole number is used (for example, H3.5 would use the H3 lighting level class). Negative numbers would result in applying the highest lighting level, or H1 class. Similarly, if the calculated number is higher than the highest class number, the lowest lighting level, or H4 class, is used.

Once the lighting designer calculates the H-class, he or she determines the design criteria for the roadway. For each class, the lighting levels are specified in terms of average luminance, uniformity, and veiling luminance. The average luminance is the average lighting level on the roadway. The uniformity represents the ratio of the average to minimum light level and the average to maximum light level. These uniformity ratios control the range of light and dark on the roadway. The veiling luminance is a measure of glare and limits the amount of light that is projected by a luminaire towards a driver.

For an adaptive lighting system in which the lighting level changes based on the conditions of the roadway, the weighting functions change as the roadway conditions change. This determines a different lighting class and therefore a different required design level.

Timing Lighting Adjustments

Some conditions that can change throughout the night and influence the lighting level required to maintain safety include traffic volume, pedestrian and bicycle presence, parked vehicles, ambient conditions, and pedestrian safety and security. As these conditions change, an adaptive lighting system will adjust luminance levels accordingly. (Weather conditions, such as fog, rain, and snow, are also factors that influence lighting level, but they were not included in this analysis.)

Two approaches typically used to trigger lighting adjustments in an adaptive system include curfews, in which the lighting system changes at a predetermined time, and roadway monitoring. Curfews are used to adapt lighting systems during defined time periods. Operators establish curfew times based on an evaluation of parameters of interest. For example, operators could evaluate average traffic and pedestrian volumes on an hourly basis to determine the timing of adaptive changes. Operators need to be able to override the adaptive cycle, as needed, for special events.

Actively monitoring the roadway through pedestrian and vehicle counts is an alternative to curfews. Active monitoring requires vehicle detectors or the review of roadway video to determine when to adjust lighting levels. The resource requirements for a monitoring system can be significant, although they might become less demanding once connected vehicle and connected infrastructure technologies provide a new source of data on traffic and pedestrian volumes.

Controlling Lighting Adjustments

The recommended technological approach to adaptive lighting is dimming. In the past, reduced lighting on roadways was typically accomplished through switching or “half-code” lighting, in which every other luminaire or the luminaires on one side of the roadway are turned off or removed. Although this method is cost effective, conserves energy, and is relatively easy to implement, half-code lighting makes it impossible to meet design criteria for uniformity and glare control. This approach presents legal issues because the inability to meet design criteria might affect the safety of roadway users.

In contrast, dimming a luminaire facilitates adjusting the light level without upsetting the other design criteria. Dimming luminaires are typically capable of dimming from 100-percent output to 10 percent of maximum output, depending on the technology of the light source. Although this method does not conserve as much energy as light-extinguishing methods, its ability to maintain lighting uniformity might be the best solution for conserving energy while minimizing the likelihood of negatively affecting driver or pedestrian safety.

Light-dimming methods have other advantages. Dimming lights instead of leaving them fully on at night reduces the effect of sky glow (a form of light pollution) and reduces disruptions to organisms’ circadian rhythms--the physical, mental, and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in the environment. Disruptions to circadian rhythms can impact sleeping patterns and general health and well-being.

Dimming works particularly well with solid-state lighting, which uses semiconductor LEDs as a source of illumination rather than electric arc lighting and, by its nature, dims more smoothly. In addition, solid-state lighting conserves energy, typically yielding a 50-percent reduction in energy use over traditional lighting. Using an adaptive lighting design with solid-state luminaires can further reduce energy usage. Converting to solid-state luminaires also might reduce maintenance costs because they last longer than traditional light sources.

Legal Implications of Adaptive Lighting

The parties most concerned with the legal implications of an adaptive lighting system are the owners of such systems and their designers. The owner will likely be the government agency responsible for drafting the regulations defining the system and for implementing it. The designers include the engineers and design professionals working for the owner or agency.

The legal concerns involved range from the agency’s justification for implementing such a system to the legal liability of the owners and designers in the event of a personal injury lawsuit attributed to the adaptive lighting system.

For the agency implementing an adaptive lighting system, demonstrating the basis for the application of its expert discretion is crucial when defending its decision to implement the system. Supporting the decision with empirical data will strengthen the case for needing the system, as will adherence to industry-accepted guidelines in the system’s design. A narrative explanation of the agency’s reasons for the decision to implement the system--written or endorsed by the engineers with the particular expertise--will leave little doubt as to the basis for the new system. A well-supported agency decision will likely receive substantial deference from a reviewing court.

Adaptive Lighting Systems in Use Today

San José, CA, has implemented a widespread adaptive lighting system. Since 2008, the city of San José has gradually upgraded its 62,000 yellow sodium vapor streetlights to use LEDs and paired them with a remote monitoring and adaptive control system. This approach enables the city to boost the efficiency and life expectancy of its streetlights, obtain timely and accurate information on the performance of its lights, and modulate lighting levels to provide only the amount of light needed.

The control system for the streetlight network provides real-time reporting of energy usage and nonoperating streetlights for improved response. Converting to LED lighting reduced streetlight energy costs by 40 percent to 60 percent, while improving lighting quality and visibility and enhancing safety. The city is earning ongoing savings by extending the maintenance cycle for bulb replacement, and it receives credit from Pacific Gas and Electric (PG&E) for dimming its streetlights in the late evening hours.

“In 2011, San Jose supported the California Street Light Association in negotiations with PG&E for a pilot program to reduce the energy bill for controllable luminaires for a few streetlight customers,” says Gregory Jobe, an associate engineer who works for the city. “By late 2015, we will have implemented a network-controlled dimmable streetlight pilot program open to all PG&E’s streetlight customers at a reduced administrative cost. After an introductory period of a few months, the dimming schedule may be adjusted on an annual basis, and the tariff may be evaluated for potential modifications.”

By switching to more energy-efficient lights and modulating its lighting levels in relation to changing activity levels, San José will save 1,885,000 kilowatt-hours annually, avoiding emissions equivalent to approximately 1,433 tons (1,300 metric tonnes) of carbon dioxide. That savings is equivalent to the volume of greenhouse gases emitted annually by 274 passenger vehicles.

San José aims to replace 100 percent of the city’s streetlights with LED lighting equipped with an adaptive control system by 2022. The city expects to convert more than 20,000 lights by the end of 2015. So far, the city has concentrated on heavily used major roadways and a contiguous area in the southeastern portion of San José, including lights on a number of corridors identified as having a high percentage of pedestrian and bicyclist injuries and fatalities. Other priority locations include 20 areas identified by the local police department as gang activity hot spots, as well as areas with a high rate of streetlight wire theft. The streetlight control system provides real-time notification of circuit malfunctions, enabling the city to intervene and deter wire theft.

“Overall, residents have reported that they feel safer after dark because they can see better,” says Amy Olay, planning and sustainability division manager with San José. “Others are impressed by the LEDs’ lighting quality, although we have received some complaints. One complaint is that the lights are too bright, shining into houses. In those cases, we adjusted the tilt of the LED fixture to correct the problem.”

Adaptive Lighting In Cambridge, MA

The city of Cambridge, MA, is replacing about 7,000 lights (4,900 streetlights and 2,100 in specialty and park fixtures) with LEDs and installing an adaptive control system that enables fine-tuning of light output, reduction of energy use, and energy usage tracking capabilities.

The city developed a system of classification to determine appropriate lighting levels for each street. Using detailed information from the city’s geographic information systems database, as well as onsite evaluations, staff assessed streets for width, light pole spacing, and vehicular and pedestrian activity, and assigned each to categories corresponding to lighting criteria in accordance with industry guidelines, including the IES Standard Practice for Roadway Lighting (RP-8-14) and Technical Memorandum on Light Trespass (TM-11).

After comprehensive analysis, the city assigned the streets to a range of categories that address lighting requirements for both roadways and sidewalks, and limit glare and light trespass onto abutting properties. A wireless control system enables operators to dim the streetlights to 70 percent of their initial brightness. Later in the evening, the lights dim even further to about 35 percent of their initial brightness in response to low pedestrian volumes at that time of night. The new streetlight system consumes less than 25percent of the energy of the existing streetlights, saving the city an estimated $500,000 per year in electricity costs.

“Cambridge’s initial reasons for using adaptive controls were energy savings and asset management,” says Glenn Heinmiller, a design consultant involved in the project. “But we soon realized that significantly reducing light trespass by dimming later in the evening was a huge benefit, especially in our dense residential neighborhoods.”

The new streetlights distribute light in a pattern similar to the old streetlights, but the amount of light crossing property lines from the public way will typically be half as much as with the existing lights, and even lower late at night. The new streetlights make colors look brighter and more faithful to the natural color. Trees look green instead of brown, a blue car looks blue instead of grey. Because of this improved color rendering, everything appears brighter and sharper under the new streetlights, even when the amount of light is less than with the old lights.

Lowered light levels are sometimes perceived as unsafe to roadway users and pedestrians, but residents in Cambridge have not raised concerns about the new lighting. “Based on our experience in Cambridge,” Heinmiller says, “I think that fears that lowered light levels will be perceived as unsafe are unfounded. In our city of 100,000 residents, we’ve had no comments about lower levels. No one has complained or even seemed to notice when the light levels are cut in half.”

The Future of Adaptive Lighting Systems

Adaptive lighting systems effectively reduce the cost and extent of the undesirable effects of roadway lighting while maintaining safety and usability. Integrating these systems with tools that are capable of sensing and communicating near real-time electrical and lighting data from the field will make it possible to control lighting levels more effectively--and even on demand. Vehicle-to-infrastructure communication systems that send real-time information to operating systems could control lighting in response to just a single vehicle.

The Virginia Tech Transportation Institute has created demonstration prototypes of an on-demand lighting system. On-demand lighting systems also could help first responders by flashing light where assistance is needed or along evacuation routes during disasters. Although these technologies are still in the early stages of development, adaptive systems are already shining light on today’s increasingly dynamic roadway environment.

Ronald B. Gibbons, Ph.D., is director of the Center for Infrastructure-Based Safety Systems and is the lead lighting research scientist at the Virginia Tech Transportation Institute. He also is the director of Division 4 of the International Commission on Illumination and a past president of the IES. Gibbons earned his Ph.D. in systems design engineering from the University of Waterloo, Canada.

Joseph Cheung is a civil engineer in the FHWA Office of Safety, where he helps develop safety technologies. He has B.S. and M.S. degrees in civil engineering, with an emphasis on traffic and transportation, from the University of Maryland. Cheung is a registered professional engineer in Maryland.

Paul Lutkevich, P.E., has more than 30 years of experience in the design and research of exterior lighting systems. He is the past chair of the IES Roadway Lighting committee and Technical Review Council. He also is a member of the International Commission on Illumination. Lutkevich has a B.S. degree in electrical engineering from the University of Massachusetts Dartmouth.

For more information, see www.fhwa.dot.gov/publications/research/safety/14051/14051.pdf or contact Ron Gibbons at 540–231–1581 or rgibbons@vtti.vt.edu, Joe Cheung at 202–366–6994 or joseph.cheung@dot.gov, or Paul Lutkevich at 617–960–4903 or lutkevich@pbworld.com.