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

Abstract

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 in any measurement process, it also must be appropriately quantified or assessed. Therefore, the Florida Department of Transportation (FDOT) initiated a field study to assess the level of precision of its own locked wheel testers for field measurements. Friction measurements were acquired with 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 were first analyzed to determine the friction characteristics at each test location in terms of a friction number at 40 mph with a standard ribbed (FN_40R) and smooth tire (FN_40S). The results were then used as the basis for an evaluation of the repeatability (within-unit precision) and reproducibility (between-unit precision) of the friction units.