David M. Burns

Metrology of fluorescent retroreflective materials and its relationship to their daytime visibility

Abstract This paper describes aspects of fluorescence that relate to high-visibility retroreflective materials used for visual signaling and markings. Full colorimetric characterization of these materials requires separation of the fluorescent and non-fluorescent components. Quantification of the individual components allows for accurate prediction of performance under the various conditions of illumination and viewing encountered in practical applications. When measuring retroreflective materials, it has been found that measurement geometry has a significant influence on the reflected spectral radiance factors. For fluorescent retroreflective materials, the reflected component is more sensitive than the fluorescent component to geometric restraints. Therefore, geometric specifications and tolerances are critical in the measurement of these materials. Assessment of the relative visibility and conspicuity of fluorescent materials requires an understanding of the relationship between the spectral illumination and the total luminance factor of the specimen. Precise calculation of the total luminance factor under the varying conditions of illumination encountered in outdoor signing and marking applications is possible using the complete bispectral radiance factor array. The relationship between calculated results and field measurements is described. For routine quality control and material specification a more abridged metrology making use of the fluorescence luminance factor (YF) has been found useful. YF for CIE illuminant D65 can be used to assess the fluorescent content and durability of fluorescent materials used in signing applications. YF has also been found to correlate with the increased visibility performance of fluorescent signs under poor daytime visibility conditions. The introduction of YF into materials specifications is beginning to take place.

Nighttime Visibility of Prototype Work Zone Markings Under Dry, Wet-Recovery, and Rain Conditions

This paper presents the findings of a dynamic pavement marking visibility study conducted as part of the FHWA Highways for LIFE Technology Partnerships Program Temporary Wet-Weather Pavement Markings for Work Zones project. Three prototype optics-on-paint marking systems employing high refractive index dual-optics drop-on elements were evaluated at night under dry, wet-recovery (immediately after rainfall), and rain conditions. These three prototypes were chosen based on their durability from an initial pool of 22 experimental systems evaluated in a test deck. This dual-optic system was designed to provide good visibility in both dry and wet weather conditions. Two commercially available marking systems were also evaluated as industry benchmarks (one glass beads-on-paint system, one wet-reflective removable tape). Thirty participants driving through simulated work zones on a closed course viewed all of the marking types at night under all three weather conditions. Each drivers task was to identify the direction of work zone lane shift tapers delineated by the markings. In wet recovery, all three prototype marking systems and the wet-reflective tape sustained 60% to 80% of their dry average detection distances, and in rain, they sustained 50% to 70% of their dry average detection distances. In contrast, the average wet-recovery and rain detection distances for the conventional glass beads-on-paint benchmark system dropped to 28% and 17% of the dry detection distance, respectively. In addition, participants failed to detect the conventional glass beads-on-paint benchmark system in nearly half of the observations in the rain condition.

Modern Pavement Marking Systems: Relationship Between Optics and Nighttime Visibility

Road marking systems have retroreflective properties to assure adequate nighttime visibility. Modern systems can also provide night visibility under rain and wet conditions. The FHWA is currently working to establish requirements for minimum maintained retroreflectivity of pavement markings on all U.S. roads. Its task is complicated by the fact that conventional dry retroreflectivity measurements do not tell the entire story about pavement marking visibility at night. To that end, there is ongoing research into the relationship between a pavement marking systems retroreflective properties and the level of on-road visibility the system provides. A pavement marking system is composed of three basic components: the optics, the structure, and the binder. Each component influences the retroreflective efficiency of the marking, its visibility at night, and the reliability of predictive correlations between the two. Such correlations are required to set practicable minimum maintained pavement marking visibility levels. A Comité Européen de Normalisation 30-m coefficient of retroreflected luminance (RL) value is commonly assumed to be a good predictor of the on-road visibility of pavement marking. While useful for quality assurance, this standardized RL value is not a measure of visibility. This paper describes choices for pavement marking components and their impact on night visibility. Data on the fundamental relationship between optics and retroreflective efficiency related to road marking visibility are presented. Dry and wet conditions are considered, as are flat and structured markings. Field measurements of pavement marking luminance are presented to illustrate how the optical design of a pavement marking system controls nighttime visibility.

Brightness and Color of Fluorescent Yellow and Fluorescent Yellow Green Retroreflective Signs: Comparison of Laboratory and Field Measurements

There is a growing body of traffic engineering research indicating that fluorescent-retroreflective signing can promote safer driver behavior on the road. However, missing from these studies are quantitative evaluations of the field performance of fluorescent-retroreflective materials on the roadway. How do the properties of fluorescent signs differ from those with ordinary colors? Information on the photometric performance of fluorescent-retroreflective signs under daytime and nighttime driving conditions relative to conventional signing is of interest to both researchers and practitioners. Laboratory measurements of the photometric and colorimetric properties of signing materials are commonly used in standard material specifications as surrogates for the field performance properties of traffic signs. Yet there are few data in the literature illustrating the relationship between laboratory testing of sign materials using standardized test methods and the photometric performance of signs on the road. Consequently, measurements were made of the photometric properties of a series of fluorescent and ordinary (nonfluorescent) retroreflective signs in both the laboratory and the field. The results show that laboratory testing does correlate to some extent with field measurements of the same properties. However, the data point toward a need for improvements in the current laboratory test methods to make them correspond more directly to what is observed by the driver.

Pavement Marking Photometric Performance and Visibility Under Dry, Wet, and Rainy Conditions: Pilot Field Study

Pavement markings are a fundamental component of the roadway safety infrastructure. They play an especially important role at night, when there are fewer visual cues for the driver. Although progress is being made in pavement marking management practices, advances in materials are still needed to fulfill driver needs for nighttime visibility and guidance under all weather conditions, especially wet and rainy conditions. This study was conducted to investigate the relationship between the photometric properties of pavement markings and their visual performance under dry, wet, and rainy conditions. For this pilot study four levels of material performance were created by a systematic reduction of the retroreflective efficiency of a single wet retroreflective tape construction. The nighttime visibility of the pavement marking materials was evaluated under dry, wet (just after rainfall), and rainy conditions. Eight 4-in. skip lines were viewed on the road in a standard 10-ft skip line and 30-ft gap pattern. Th...