Guidelines for Nighttime Visibility of Overhead Signs

This 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. The research for the project was conducted in two phases. In the first phase, a closed-course study was conducted at the Virginia Smart Road at the Virginia Tech Transportation Institute. The goal of this study was to determine legibility distances for drivers in controlled conditions for three different sign legend and background configurations lit by either high pressure sodium or light emitting diode systems or they were unlit. In the second phase, an open-road study was conducted by the Texas AM San Antonio, Texas; and Orlando, Florida. The goal of the open-road study was to determine the effects of sign luminance and visual complexity on the distance at which a driver can read overhead signs and street signs. Both studies employed cohorts of recruited drivers evenly split in terms of gender and age. The closed-course study found that sign lighting does not significantly impact legibility distance of signs in rural and dark areas, suggesting that sufficient illumination for visibility was provided by headlamps. Using software developed for the open-road study, digital images were analyzed to quantitatively measure visual complexity in urban and suburban areas. Visual complexity of sign surroundings was found to reduce the distance at which a driver can correctly recognize information from signs, however, this effect can be countered by increases in the legend luminance. This report fully documents the research and includes four appendixes: Incremental Effects of Light Sources and Sign Sheeting on Legend Luminance for Overhead Guide Signs; Assessment of Background Complexity Using Digital Images of Roadway Scenes by Image Processing; Open-Road Study Details; and Guidelines for Nighttime Overhead Sign Visibility.