Currently, national guidelines regarding the nighttime visibility of signs are limited to the stipulation in the Manual on Uniform Traffic Control for Streets and Highways (MUTCD) that all warning and regulatory signs be illuminated or reflectorized to show the same color and shape by day or night. There are no objective measures that can be used to determine when a sign has reached the end of its service life and needs to be replaced. This study seeks to fill that need by establishing minimum retroreflectivity requirements for traffic signs. Given the wide range of visual, cognitive, and psychomotor capabilities of the driving population and the complexity of the relationships between the driver, the vehicle, the sign, and the roadway, a mathematical modeling approach was selected. The model determines the distance at which a driver needs to see a sign, uses this distance to determine the luminance required, and then calculates the coefficient of retroreflection at standard measurement angles. This model is called Computer Analysis of Retroflectance of Traffic Signs (CARTS). The CARTS model was executed for each sign in the MUTCD at various vehicle speeds, sign sizes and sign placements. The results are summarized and presented in a format that can be implemented by practitioners. Retroflectivity values are given for both yellow and orange warning signs, white on red regulatory signs, white regulatory signs, and white on green guide signs.
Abstract