Retroreflective Requirements for Traffic Signs - A STOP Sign Case Study
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Retroreflective Requirements for Traffic Signs - A STOP Sign Case Study

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    Final Report
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    Poor sign reflectivity is a contributor to the high proportion of nighttime accidents. At night, signs must have enough brightness to allow a driver to recognize and react to the intended message of the sign in a safe manner. Failure to recognize a regulatory sign, such as a STOP sign, could result in a severe accident. The Manual on Uniform Traffic Control Devices (MUTCD) contains no standards on minimum initial or maintained retroreflectivity for traffic signs. However, the Federal Highway Administration, through the Federal Register, is considering comments to determine whether to establish minimum retroreflectivity standards for in-service traffic signs. This study determined the performance of STOP signs based on their retroreflective properties. Thirty-five 30-inch STOP signs, including engineering grade and high intensity, were measured in the laboratory to determine their retroreflective properties. A subset of ten signs was taken to the field to determine how far they could be recognized by paid subjects. Mathematical relationships between the various retroreflective properties and recognition distance were developed. The overall specific intensity per unit area (SIA) was found to be a good measure for estimating the recognition distance of STOP signs. By computing the minimum sign viewing distance for various approach speeds, the required integrated SIA of STOP signs was found. Findings from this study will (1) aid in establishing minimum in-service levels of retroreflectivity for STOP signs, (2) assist field personnel to determine whether a particular STOP sign is providing the desired recognition distance of whether it should be replaced, and (3) give an insight to the feasibility of creating a minimum in-service retroreflective standards for other types of traffic signs.
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