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Depending on where you live in the world, you may have seen them already. Some look like small computer monitors in shop windows showing opening times. Others are larger-than-life roadside billboards that seem like oversized televisions. They all use electronic displays to convey information, and at night they emit light. While these illuminated signs have different trade names, they are displacing earlier means of communicating messages at night. What does this mean for light pollution? Can these signs actually be good for the night?
Digital signs: advertising of the future
There is no doubt that illuminated signs are in demand by advertisers and other users. They allow broadcasting messages, commercial or otherwise, at all hours of the day and night. To make messages legible at night, signs use various types of illumination. "Conventional" illuminated signs often involve upward floodlighting that can spill light past the sign edges and into the night sky. While a better approach lights from the top down and uses shielded luminaires, light spill and trespass can be a problem. Even light fully captured by the sign surface ends up, in part, lost to the sky due to reflection and scattering.
Digital signs make the sign surface itself luminous, usually through the controlled use of light-emitting diodes (LEDs). The light is not only bright and colorful, but also dynamic. The brightness of the message can vary to remain legible in conditions ranging from full daylight to total darkness. The displays can synthesize any color and rapidly change the assignment of light to individual pixels. The sign face can reconfigure to show a new message within a fraction of a second. Some displays can even show full-motion video.
These displays are big business. Market Research Future estimates that the digital billboard market will be worth almost $42 billion worldwide by 2030. Grand View Research expects that market value to increase by about 8% a year through the 2020s. LED technology is driving this growth. As equipment prices fall, more advertisers adopt LED for its configurability and high energy efficiency. It enables them to serve more ads at a lower cost of operation. In short: the future of outdoor advertising is digital. It's a future that consists of more lighted signs at night.
Your results may vary
Like LED technology generally, this could be good or bad depending on the details. For this lighting application, as for others, LED has a lot of desirable characteristics. Its light is very directional, meaning that it doesn't tend to spread out much as it travels. Owners of displays can thus target the light in space to reach more of its intended audience with less waste. Further, the brightness, timing, and color are controllable, serving advertisers' needs with attention-catching visuals. This can mean using less light to convey messages compared to conventional sign lighting techniques.
But, like in many area lighting applications, LED is often deployed with little concern for light waste. Digital signs often have very wide angular fields of view in the name of maximizing visibility. They're often over-bright given the strong directional nature of LED. Results of a 2010 study suggest that owners of digital signs operate them at brightnesses several times higher than those of most conventional illuminated signs.  Viewers can perceive them as harsh, yielding a lot of glare that makes it difficult to read the message.
Digital signs broadcast their light sideways at angles that are bad for skyglow. Unlike other kinds of lighting, shielding isn't a convenient option for limiting this impact. Shielding would be so large to impose unacceptable wind loads on structures. Lack of vertical controls is the big problem. Designs direct much of the surface light upward and not toward advertisers' potential customers.
We don't have much in the way of hard data on these signs' contributions to skyglow or light pollution on the ground. But we can make some educated guesses based on what we know about them. While it is arguable that they may become an important new source of light pollution, the jury remains out on their exact effect.
A problem for nighttime safety?
There are concerns about the impact these signs may have on public safety, given their typical brightnesses. Data suggest that digital signs are more effective than conventional ones in drawing drivers' attention away from the road.  When messages displayed on digital signs change very rapidly, drivers may be further distracted.  Yet we still do not have definitive evidence that implicates digital signs in raising the incidence of vehicle crashes. 
It is important to distinguish the source of any presumed distraction. Is it the illumination, or is it the (changing) messages? Again, we don't know. The situation resembles that of road safety involving the use of mobile telephones. While hands-free equipment was first thought to be a safer approach, studies have not shown this to be always true. [5,6] It may be that talking on the phone while driving, rather than the manner in which one uses the phone, is most distracting.
The way forward
Success in dealing with these concerns is a combination of technological innovation and common-sense regulation. Advertisers have an interest in ensuring that viewers see and read their messages. This can contribute in a positive way to reducing light pollution from digital signs. At the same time, viewers will not read messages on very bright signs. In fact, such signs may annoy them, causing them to form a negative opinion of advertisers.
There are some signs that sign manufacturers understand this. Some companies are testing technology that limits the range of angles in which digital signs are directly viewable. This is in part due to increased municipal regulation of digital signs in the U.S. An important Supreme Court decision in City of Austin, Texas v. Reagan National Advertising of Austin, LLC (2022) affirmed the right of cities and towns to regulate these signs. Regulation ensures adequate stakeholder input and public oversight. And further design innovation will make for more efficient sign operations, lowering costs.
There are emerging best practices for how to operate digital signs in ways that don't harm the night sky. In 2019, the International Dark-Sky Association released its "Guidance for Electronic
Message Centers (EMCs)". This document suggests ways to operate and regulate digital signs to reduce their impacts on the nighttime environment. In the same year, the Illuminating Engineering Society introduced similar standards in its RP-39-19 document.
A body of knowledge is also emerging around how to regulate digital signs in the U.S. municipal context. For example, Scenic Utah offers a "primer" for communities that seek to enact ordinances regulating these signs. The ideas are simple: Limit digital sign brightness and size; the product of these is the total light emission of the sign. Place strict constraints on allowed sign brightnesses. Impose curfews prohibiting sign illumination during overnight hours. And wherever possible, restrict digital displays to single-color messages on black backgrounds. These rules reduce light pollution from digital signs while ensuring that their messages are clearly legible.
The rising market dominance of digital signs is an indicator of progress. As with previous lighting technologies, they present a challenge: to balance user needs against the equally valid need to protect the night from light pollution. Dark-skies advocates are working with municipal officials and advertising representatives. They are finding solutions that work for all involved.
Many communities have no rules on the books governing the use of digital signs. It is preferable to put such rules in place before digital signs arrive in your city or town. Contact us today to find out how we can help craft durable ordinance language regulating digital signs.
 Luginbuhl, C., et al. (2010). Digital LED Billboard Luminance Recommendations How Bright Is Bright Enough? White paper. http://www.illinoislighting.org/resources/DigitalBillboardLuminanceRecommendation_ver7.pdf
 Dukic, T., Ahlstrom, C., Patten, C., Kettwich, C., & Kircher, K. (2013). Effects of Electronic Billboards on Driver Distraction. Traffic Injury Prevention, 14(5), 469-476. https://doi.org/10.1080/15389588.2012.731546
 Belyusar, D., Reimer, B., Mehler, B., & Coughlin, J. F. (2016). A field study on the effects of digital billboards on glance behavior during highway driving. Accident Analysis & Prevention, 88, 88-96. https://doi.org/10.1016/j.aap.2015.12.014
 Oviedo-Trespalacios, O., Truelove, V., Watson, B., & Hinton, J. A. (2019). The impact of road advertising signs on driver behaviour and implications for road safety: A critical systematic review. Transportation Research Part A: Policy and Practice, 122, 85-98. https://doi.org/10.1016/j.tra.2019.01.012
 Lipovac, K., Đerić, M., Tešić, M., Andrić, Z., & Marić, B. (2017). Mobile phone use while driving-literary review. Transportation Research Part F: Traffic Psychology and Behaviour, 47, 132-142. https://doi.org/10.1016/j.trf.2017.04.015
 Caird, J. K., Simmons, S. M., Wiley, K., Johnston, K. A., & Horrey, W. J. (2018). Does Talking on a Cell Phone, With a Passenger, or Dialing Affect Driving Performance? An Updated Systematic Review and Meta-Analysis of Experimental Studies. Human Factors: The Journal of the Human Factors and Ergonomics Society, 60(1), 101–133). https://doi.org/10.1177/0018720817748145
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