Sensitivity Analysis of Sign Luminance as Function of Input Factors

The luminance, or brightness, produced by a traffic sign at night is a function of several different factors including the performance of the retroreflective sheeting, the light output of the headlamp, the geometry between the headlamps and driver, and the position of the sign relative to the vehicle. In this study, the author calculated luminance for 27 different combinations of the factors affecting sign luminance. The result is eight figures comparing the sign luminance for the analyzed conditions to the luminance required for a dark rural or suburban roadway environment. The results indicated that there can be a wide range in luminance performance depending on the input variables, many of which are beyond the control of transportation agencies. The final comparison found that the difference between the best- and worst-case combinations of input variables resulted in a luminance difference of over 2,000%. Such a wide range of performance measurement means that traffic sign sheeting that is selected based on performance for a specific set of circumstances may perform in a vastly different manner because of other factors such as the type of vehicle, the headlamps on the vehicle, the roadway geometry, or the position of a sign relative to the roadway and vehicle.