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

Document Type

Journal Article

Publication Date


Subject Area

operations - performance, organisation - performance, mode - rail


Wheel rail interaction, Tire pavement interface, Test sections, Surface course (Pavements), Smooth tires, Rolling contact, Road surfaces, Ribbed tires, Reproducibility, Repeatability, Precision, Performance measurement, Locked wheel testers, Friction number, Friction measurement, Friction, Florida Department of Transportation, Field studies, Data collection, Data acquisition, Blanket course, Asphalt pavements, Accuracy


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 (FN40R) and smooth tire (FN40S). 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.