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Page 37
Suggested Citation:"Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. Guidelines for Nighttime Visibility of Overhead Signs. Washington, DC: The National Academies Press. doi: 10.17226/23512.
Page 37

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37 Abbreviations AASHTO American Association of State Highway and Transportation Officials ASTM American Society for Testing and Materials CCT Correlated color temperature DOT Department of transportation FHWA Federal Highway Administration GLCM Gray-level co-occurrence matrix HPS High-pressure sodium IESNA Illuminating Engineering Society of North America LED Light-emitting diode LOOCV Leave-one-out cross validation MLR Multiple linear regression MUTCD Manual on Uniform Traffic Control Devices TCD Traffic control device Terms Conspicuity The property of being detected among other objects Edge ratio Number of pixels at the edges of objects divided by total pixels in an image Entropy A statistical measure of randomness in an image Illuminance Intensity of light incident on a surface Legibility The ability to read a message without previously knowing the message Luminaire A lighting unit Luminance Intensity of light directed at a viewer Photometer Device for measuring light intensity Readability The ability to read a message with either a task of legibility or recognition Recognition The ability to read a message with prior knowledge of its message Retroreflectivity The reflecting of light back to a source Visual complexity Clutter in a scene that inhibits the ability to see a target Glossary

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TRB's National Cooperative Highway Research Program (NCHRP) Report 828: Guidelines for Nighttime Visibility of Overhead Signs explores legibility distances for drivers in controlled conditions and the effects of sign luminance and visual complexity on the distance at which a driver can read overhead signs and street signs. While the Manual on Uniform Traffic Control Devices (MUTCD) provides minimum retroreflectivity standards for overhead signs, there are few guidelines that agencies can reference to decide how to provide sufficient nighttime performance of overhead signs in site-specific situations. The report presents proposed guidelines for nighttime overhead sign visibility, formatted as a potential replacement for the current Chapter 10, Roadway Sign Lighting, in the 2005 AASHTO Roadway Lighting Design Guide.


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