Validation of Photometric Modeling Techniques for Retroreflective Traffic Signs

Abstract

Recent research in the United States has focused on the development of minimum retroreflectivity levels for traffic signs. The engineered approach uses a supply luminance versus demand luminance concept to achieve a metric that can be used to set minimum retroreflectivity levels. The supply luminance depends on reliable modeled estimates of headlamp illuminance reaching traffic signs and the resultant luminance directed back toward the driver. While the science behind the photometric modeling process is fundamentally sound, a small pocket of literature shows inconsistent relationships between measured and modeled illuminance and luminance levels. A systematic validation study of photometric modeling was done with field measurements made under conditions identical to those that were modeled. A set of headlamps were photometrically measured along with three different samples of retroreflective sign sheeting materials. Illuminance and luminance levels were then modeled for nine sign positions and three vehicle types. Then field illuminance and luminance were measured using the same scenarios, headlamps, and retroreflective sign sheeting materials. The field measurements were recorded once on a concrete pavement and once on an asphalt pavement. Both sets of field measurements also included baffled and unbaffled measurements so the impact of pavement reflection could be considered. Photometric modeling techniques of nighttime traffic sign performance compared well with field measurements made under identical conditions. The maximum illuminance difference was 16% and was attributed to the pavement reflection properties. The maximum luminance difference was only 2.4%. The retroreflective performance of traffic signs redirects most of the pavement reflection back toward the pavement. Therefore, estimates and measurements of luminance are less affected than illuminance.