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 system's 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 (R_L) value is commonly assumed to be a good predictor of the on-road visibility of pavement marking. While useful for quality assurance, this standardized R_L 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.