Roadway LEDs Address Unintended Effects on Flora and Fauna

Roadway LEDs Address Unintended Effects on Flora and Fauna

This piece was originally published in the February 2018 issue of electroindustry.

Matthew Palmer, Senior Research Associate, Center for Infrastructure-Based Safety Systems, Virginia Tech Transportation Institute. Ronald Gibbons, PhD, Director, Center for Infrastructure-Based Safety Systems, Virginia Tech Transportation Institute. Mr. Palmer has researched many aspects of roadway lighting technology. Dr. Gibbons has researched and written policy and guidelines for roadway lighting.

Caption: Immature soybean growth can be seen behind each HPS roadway lighting fixture. Photo courtesy of Virginia Tech Transportation Institute

The transition to light-emitting diodes (LEDs) reminds us that roadway lighting often influences flora and fauna in unforeseen and unintentional ways. Given the measurable safety benefits of lighting, especially where vehicles and other roadway users come into conflict, there is a clear need to balance lighting’s positive and negative effects. Fortunately, a variety of tools are now available that roadway lighting designers can use to minimize adverse influences on flora and fauna.


In the U.S., soybeans are an important part of the crop rotation needed to fix nitrogen in the soil. Soybeans are an annual plant; at the end of the season, they recycle all of the energy used to grow by pushing nutrients and sugars into the seeds and dying.

Soy plants grow taller in the vicinity of roadway lighting but never mature and produce fewer and lower-value beans. The effect of roadway lighting on soybeans can be seen driving down any lighted road in the Midwest during August and September. Light—more specifically, the lack of light caused by the shortening of the day—triggers the plant to know it is time to finish the seeds.

As the majority of a field starts turning yellow, and later brown, semicircles of green plants remain near streetlights due to house-side light trespass. High-pressure sodium (HPS) lighting can cause the plants to grow up to 50 percent taller than normal and can reduce production by measurable amounts, depending on illuminance levels.[1]

Farmers cannot include immature seeds in their harvests because they will receive a lower price per bushel from buyers. To top it off, the green plants often clog the combines used for harvest. Research is underway by the Virginia Tech Transportation Institute to determine if LED luminaires are better for soybeans since the primary sensitivity of soy is in the blue and red wavelengths.

Rice plants also rely on changes in the duration of light to trigger the production of the seed. Based on a study between 2009 and 2013,  Yamaguchi University determined the aspects of roadway lighting that result in a 20 percent reduction in rice production. This led to the creation of an LED luminaire to minimize the effect. The luminaire combines a blue LED with a yellow-green LED and pulses the lighting at a frequency not visible to humans. With it, the harvesting of crops illuminated with as much as 10 lux of horizontal illuminance was delayed by only three days compared to nonilluminated crops.


Animals are also adversely affected by roadway lighting. The Route 462 bridge over the Susquehanna River between Columbia and Wrightsville, Pennsylvania, was recently upgraded with luminaires that are replicas of the originals installed in 1930 but with LEDs. Adult mayflies are drawn to the new lights, where reproduction occurs. The female, however, is not able to navigate back to the water’s surface to lay her eggs before dying.[2] Most of the mayfly’s lifespan is spent in the larval stage, and these larvae control algae growth, which keeps rivers clear and provides food for fish, so a reduction in the number of mayfly larvae can have a cascading disturbance to a river ecosystem.

Artificial lighting has a devastating effect on sea turtle hatchlings. To a hatchling’s eyes, short or full spectrum lighting resembles the moon and causes confusion—it is believed that turtles rely on the moon to navigate from their sandy nests to the ocean. Many municipalities have laws preventing excess lighting near the shore, and turtle watchers use red lights to avoid disturbing hatchlings. Several municipalities have specified lighting with lower correlated color temperature (CCT) to reduce the effects on turtle hatchlings.

Birds that migrate at night use the stars for navigation. Uplighting from luminaires in urban areas confuses them, as does illumination on antennas, power lines, ship masts, and lighthouse beacons. Navigation confusion can trap birds in urban environments, potentially resulting in fatigue, exposure to the elements, and increased vulnerability to predators.

The optics of LED luminaires allow much tighter control of the beam pattern and cutoff to reduce uplight while providing larger lighted areas than traditional light sources. This can mitigate the negative effects on migratory birds and sea turtles if full cutoff or cutoff less than 90° luminaires is chosen. Full cutoff luminaires can reduce the impact on insects as well by limiting the distance over which the light source can be seen.

Minimizing Impact

Modern lighting designers have many tools to control the unintended consequences of lighting. LED luminaires are now available in a wide range of spectral power distributions (SPDs), with CCTs ranging from 2100K to 6500K. For LED luminaires with continuous SPDs, lower CCT values roughly correlate with increased long wavelength content in the SPD, which can reduce the effects on taxa.

There is some evidence that 4000K LED lighting may allow humans to detect objects farther than other CCTs. Detection is the first step that needs to occur to avoid conflicts with vehicles, pedestrians, cyclists, and other hazards. Therefore, the choice of CCT has to be weighed against the safety benefits for each design.

The dimmability of LED lights when combined with wireless controls and sensors allows lighting to be reduced or removed on a curfew or when traffic volume is low. This can abate the overall consequences on a variety of flora and fauna. The National Cooperative Highway Research Program has established guidelines for adapting the lighting level to roadway type, traffic speed, median presence, intersection/interchange density, traffic volume, pedestrian traffic, and ambient lighting.[3] These guidelines enable lighting to be changed as frequently as every hour.


Humans depend on flora and fauna, and we need to use the tools available with modern lighting to reduce the unintended consequences of roadway lighting while maintaining safety. LED lighting can achieve a broad range of spectral selections that minimize impacts to specific vulnerable taxa, while dimming and instant on-off capabilities minimize impacts to all taxa.

Adaptive technologies, such as modern lighting controls, sensors, and control software, can provide lighting only when needed, allowing the darkness needed for flora to mature. Spectral selection of HPS, ceramic metal halide, and LED products can produce longer wavelength content to reduce the effect on fauna or shorter wavelength content to reduce the effect on flora. Finally, limiting lighting outside of the roadway with improved beam control and house-side shielding benefits all flora and fauna.

These suggestions are not all-inclusive. Roadway lighting tools can be applied intelligently for each job, project, roadway, and ecosystem.

[1] Matthew Palmer, Ronald Gibbons, Rajaram Bhagavathula, David Holshouser, and Daniel Davidson, “Roadway Lighting’s Impact on Altering Soybean Growth: Volume 1,” Illinois Center for Transportation Series, no. 17-014, June 2017

[2] Ryan Robinson, “WATCH: ‘Blizzard’ of mayflies swarm Route 462 bridge,” LancasterOnline, June 14, 2015

[3] Ronald Gibbons, Feng Guo, Alejandra Medina, Travis Terry, Jianhe Du, Paul Lutkevich, David Corkum, and Peter Vetere, Guidelines for The Implementation of Reduced Lighting on Roadways, FHWA, June 2014

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