Precission of Smooth and Ribbed Tire Locked Wheel Testers for Measurement of Roadway Surface Friction Characteristics

The present state-of-the-art locked wheel testers for roadway surface friction evaluation are fully automated. As with any testing using subject-driven, instrumented devices, the major concerns of the end usefulness of the resulting data are accuracy and precision. Although a level of uncertainty is always inherent to any measurement process, it must also be appropriately quantified or assessed. Therefore, the Florida Department of Transportation (FDOT) initiated the present field study to assess the level of precision of its own locked-wheel testers for field measurements. Friction measurements were acquired using four friction locked-wheel testers concurrently on a number of asphalt section sites. These test sections were randomly selected to include both open and dense graded surface mixtures. The collected friction data was first analyzed to determine the friction characteristics at each test location, in terms of a friction number at 40 mph using a standard ribbed (FN40R) and smooth tire (FN40S). The results were then used as a basis for an evaluation of the repeatability (within-unit precision) and reproducibility (betweenunit precision) of the friction units. This report presents a description of the testing program, the data collection effort and the subsequent analyses and findings.