Evaluating the Retroreflectivity of Pavement Marking Materials under Continuous Wetting Conditions

Wet weather performance of pavement markings has received increased emphasis with the availability of new products designed specifically to improve nighttime visibility in wet weather conditions. Procedures for measuring the performance of these and standard marking materials are found in ASTM E1710, E2176, and E2177, which describe retroreflectivity measurement in dry, continuous wetting, and recovery conditions, respectively. Research comparing the retroreflectivity values measured for 18 different marking samples in 14 measurement conditions is described. Each sample was measured according to the three ASTM procedures, plus an additional 11 rates of continuous wetting. In addition, analysis was done of 20 years of Texas rainfall data to gain an understanding of the relationships between actual rainfall rates and the rates specified in E2176. Findings indicate that while the allowable rainfall rate in E2176 ranges from 6 to 14 in./h, 88% of rainfall events produced maximum rates less than 0.75 in./h. A wide variability was found in the retroreflectivity values of materials as a function of the wet condition. For most markings, the retroreflectivity level decreases as the rainfall rate increases, but changes in retroreflectivity were not consistent for the different samples. Also, an analysis of the effect of cross slope on wet retroreflectivity measurements indicated that it has a major impact on the measured values. Findings suggest that the range of conditions permitted by E2176 brings into question the ability to use this procedure to compare material retroreflectivity in a standardized manner.