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Internal vs. External Sign Illumination Revisited: Best Methods for Traffic Safety

Which lighting method is best for drivers who are seeing and reading the signs at night?

By Richard B. Crawford, USSC Legislative Consultant

See what the research done by the Larsen Transportation Institute at Penn State determined.

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  • Over ten years ago, the USSC began a series of four (4) industry funded on-premise sign lighting studies designed to help both sign companies and regulators understand how on-premise signs function at night when illuminated, and to address questions on the best type of lighting at night for the motorist and traffic safety - including best lighting level or luminance at night and best type of sign construction in regard to illumination. Sign illumination has both a constitutionally protected function in providing lighting for communication, and a traffic safety function by insuring that motorists can adequately see and read on-premise sign messages at night.

    In the last of the series of lighting studies, USSC returned to the question of comparing the actual performance of internally and externally lit signs, or rather: which lighting method was best for drivers who are seeing and reading the signs at night? This new study was truly historic, because it dealt with sign illumination in a "real world" environment, and not a test track, and the findings reaffirmed results of a previous comparative USSC study on sign illumination and traffic safety.

    The research was conducted for the United States Sign Council Foundation by the Larsen Transportation Institute at Penn State, titled: Internal vs. External On-Premise Sign Lighting: Visibility and Safety in the Real World (2009). The study conclusively demonstrated that the best and safest method of sign illumination for drivers at night is internal illumination, and provides USSC members with a tremendous resource in the areas of traffic safety and sign illumination design.

    Internal vs External Sign Illumination study examples

    The reason for this fourth study in 2009 was that there had been continuing concern in the industry about the so-called "Dark Sky" initiatives for lighting regulation and sign illumination. USSC had seen increasing scrutiny of all types of nighttime lighting, and on-premise signs were often lumped in together with general outdoor lighting restrictions. This was not correct, and the same is true today, for a number of reasons: on-premise signs provide a critical identification and wayfinding function at night; there are traffic safety implications if signs cannot be properly seen at night, particularly for older drivers and in inclement weather; on-premise signs have certain constitutional protections.

    In addition, Dark Sky based restrictions on sign illumination appear to have an aesthetic component, which raises the very valid question: can aesthetic interests ever supersede traffic safety interests when it comes to sign illumination and lighting? Sign internal illumination "bans", in fact, have already been implemented in towns in the US, both before and after the study.

    As many readers may be aware, the citizens of Earth celebrate "Earth Hour" on the last Saturday in March, from 8:30 PM to 9:30 PM, as a symbol of their commitment to the planet. Earth Hour is a worldwide movement for the planet organized by the World Wide Fund for Nature (WWF), and individuals, communities, and businesses are encouraged to turn off their lights for one hour, in the interests of the environment, lighting reduction and global warming. Sadly, the participants in Earth Hour have shown a propensity to want to turn off electrical signs as well. As one commentator analyzed it, "The Earth is fine. It's a person who sits in the dark to "save the Earth" who worries me."

    The USSC Research Description
    The Penn State researchers identified six (6) properties along standard 2 lane and 4 lane roadways where externally illuminated signs were located. They obtained the sign owner's agreement to participate in the study. USSC surveyed and designed exact duplicates or replicas of the externally illuminated signs: same colors, sizes, fonts, and design. USSC member companies volunteered to fabricate the replicas of the externally illuminated signs but with internal illumination.

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    Using this method, Penn State could obtain direct information about the efficacy of sign illumination type, because there was a direct before-and-after comparison between the two lighting methods (external vs. internal). When participants viewed the signs, the only difference between the test signs was the illumination method - all other factors were the same, including location and mounting conditions.

    Photo documentation of the signs was provided by Penn State (see photos). The externally illuminated signs used by Penn State were chosen because they were indeed representative of normal lighting methods and conditions seen in the Real World. The signs were not altered or changed in any way. The quality of the nighttime lighting should not be surprising, particularly to anyone in the sign business. The signs in fact were not unique, but average in design and construction.

    The Facts

      a. All the internally illuminated signs outperformed the externally illuminated signs - - they were easier to see from a greater distance.

      b. The benchmark for the test was distance and time; test participants could see the internally illuminated signs from a greater distance, and the externally illuminated signs from a shorter distance.

      c. These differences translate into a critical difference in time that the driver has to see and read a sign. In some cases, the internal illumination gave the driver more than twice the amount of distance (and time) to see the sign in question. This translates directly to traffic safety: giving a driver adequate time to see, read, and react to a sign message inherently promotes the interests of traffic safety. Why would one work to make signs harder for drivers to read?

      Internal vs External Sign Illumination comparison study d. Conversely, the externally illuminated signs did not provide adequate time and distance for drivers to see and read signs at speeds of 30-40 MPH. One cannot argue, based on these test results (and test results obtained in the previous USSC test track study on comparative sign illumination - The Relative Visibility of internally and Externally Illuminated On-Premise Signs, 2004) that external illumination is "just fine" for lighting at night along roads with posted speeds over 30 MPH. The facts and the research just don't support that opinion, and it in fact runs counter to the interest of traffic safety.

      e. The chart draws a comparison between internal and external illumination and speed of traffic. Based on the research, if one is travelling at 45 MPH down a typical suburban commercial roadway and one can see and read the internally illuminated sign, one cannot see and read the externally illuminated sign at the same time and distance; in order to see and read the externally illuminated sign with adequate time and distance, the driver has to reduce speed to 25 MPH. That type of extreme driving maneuver alone is a behavior that should be discouraged in the interests of public safety. Allowing for internal sign illumination solves the problem.

    These test results continue to be significant for the sign industry today. The four complete USSC studies on sign illumination can give anyone interested in traffic safety and good lighting design a wealth of facts and information. In the push and pull of local sign regulation, the information and studies offered by the USSC on sign lighting can help sign companies champion the interests of the 21st Century over the interests of the 19th Century, and USSC stands as a resource that you can count on when Earth Hour becomes 24 hours a day.

    Copies of this research are available at no charge to USSC members, and on the USSC website. For more information on this or any other USSC Foundation sign study, you may contact Richard Crawford at

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