Recommended Laboratory Test for Predicting the Initial Retroreflectivity of Pavement Markings from Glass Bead Quality

Pavement markings contribute to motorist safety by providing much-needed guidance along the roadway in both daytime and nighttime conditions. Basic characteristics of pavement marking, such as color, width, and placement, are defined clearly in the current Manual on Uniform Traffic Control Devices; however, nighttime visibility is not. Roadway agencies are left to decide the minimum retroreflectivity thresholds with which they are comfortable—if retroreflectivity is monitored at all—and how they will achieve these thresholds within the ever-expanding market of choices. This paper summarizes the results of an NCHRP project that developed a recommended laboratory test to predict the initial retroreflectivity of pavement markings in the field on the basis of the quality of the glass beads. This effort consisted of two key components: a laboratory test and a field verification. The study considered only AASHTO M 247 Type I glass beads in 15-mil waterborne paint. The research team developed a drawdown laboratory test to determine the potential retroreflectivity in the lab. The test was calibrated and validated with a field test. The drawdown is rapid (preparation and testing takes 24 h), repeatable and reproducible, cost-effective, and easy to use.